Transformer Type & Tutorial

Power Transformers - Switch Mode

What differentiates a power transformer and a switch mode power

transformer from other transformers? Power transformers (and inductors) are

essentially A.C. (alternating current) devices. They cannot sustain

transformer operation from a fixed D.C. (direct current) voltage source.

However they can sustain transformer operation in a transient condition(s)

that allows resetting or reversal of the transformer’s magnetic flux levels. An

A.C. voltage source keeps reversing the polarity of the voltage being applied

across the transformer. Consequently the magnetic fields keeps reversing.

Voltage reversal can also be accomplished with a D.C. source such as a

battery. The connections between the D.C. source and the transformers are

repeatedly switched, thereby reversing the voltage polarity across the

transformer, hence reversing the magnetic field. The transformer can also be

switched off from the D.C. source. In this case the magnetic field simply

collapses until it reaches its residual value (ideally equal to zero). This

collapse “resets” the transformer’s magnetic field. Switch mode power

transformers (and supplies) get their name from the switching action needed

to sustain transformer operation.

By controlling the amount of “on time” and “off time” of the switches, one

can also control the amount of power delivered to the transformer’s load (or

load circuit). The voltage can be fed to the switch mode power transformer

in voltage pulses. The pulse duration is a portion of an overall cycle time.

The cycle time is equal to the inverse of the operating frequency. The terms

“duty cycle” and “pulse width modulation” arise from the control of the

switching “on time” and “off time”.

Switch mode power transformers are used extensively in electronic

applications, usually within a switch mode power supply. A switch mode

power supply is usually powered from a D.C. source, such as a battery. The

switching mode power supply converts the input D.C. source to one or more

output D.C. sources. The power supplies are often referred to as “DC to DC”

converters. In similar fashion, the switch mode power transformers are often

referred to as “DC to DC” transformers (or “DC-DC” transformers). A

switch mode power transformer can have several secondary windings.

Consequently, the switch mode transformers permits multiple outputs which

can be electrically isolated from one another. Transformer action permits

one to “step up” or “step down” the voltage as needed via an appropriate

turns ratio. Pulse width modulation is used to provide voltage regulation.

Many electronic applications require some sort of power supply which

converts power from the conventional low frequency sinusoidal A.C. wall

socket (for example, 115V 60 Hz) to the necessary voltage, current, and/or

waveform required by the circuit. Typically the circuits need a well-

regulated D.C. voltage. Designers often choose either a rectifier type circuit

(to convert A.C. voltage to D.C. voltage), a switch mode power supply, or

both. For the “both” case, the A.C. voltage is first rectified to provide a D.C.

voltage. The D.C. voltage varies as the A.C. voltage varies, hence good

voltage regulation cannot be assured. One or more switching mode power

supplies follow the rectifying circuitry. The switching mode power supplies

provide a more tightly regulated output voltage. A.C. rectification is not a

necessity. Although tricky, it is possible, through switching actions, to

divide (“chop”) the A.C. waveform into a series of pulses, which are directly

fed into the switching mode power transformer. Pulse width modulation is

used to control the regulation.

Butler Winding can make (and has made) switching mode power

transformers (and /or inductors) for Buck, Flyback, and Boost applications

(discussed below) in a wide variety of shapes and sizes. This includes;

various standard types of “core with bobbin” structures (E, EP, EFD, PQ,

POT, U and others), toroids, and some custom designs. Our upper limits are

40 pounds of weight and 2 kilowatts of power. We have experience with foil

windings, litz wire windings, and perfect layering. For toroids, we can (and

have done) sector winding, progressive winding, bank winding, and

progressive bank winding.

Butler winding has a variety of winding machines, bobbin/tube and toroid.

That includes two programmable automated machines and a taping machine

for toroids. Butler Winding has vacuum chamber(s) for vacuum

impregnation and can also encapsulate. To ensure quality, Butler Winding

purchased two programmable automated testing machines. Most of our

production is 100% tested on these machines. For more information on

Butler Winding’s capabilities, click on our “capabilities” link.

Switching Mode Power Transformers, Basic Application Circuits

The design of a switch mode power transformer will differ depending upon

the type of circuit used. There are many variations of switching mode power

supplies, but they can be narrowed down to three basic circuit configurations

(each also has a mirrored configuration); “Buck”, “Boost”, and “Flyback”.

Be aware that the name for the “Buck” circuit varies from industry to

industry and from person to person. It may also be referred to as an

“inverter”, “D.C. converter”, “forward converter”, “feed forward”, and

others. There are also unipolar and bipolar (push-pull) versions.

The basic “Buck” circuit is illustrated in Figure 1A with an inductor and in

Figure 1B with both a switch mode power transformer and an inductor. A

push-pull version is shown in Figure 4. The basic “Flyback” circuit is

illustrated in Figure 2A with an inductor and in Figure 2B with a switch

mdoe power transformer. The basic boost circuit is illustrated in Figure 3A

with an inductor, Figure 3B and 3C with a transformer and in Figure 5 with

a push-pull forward converter type of switch mode power transformer. The

circuits shown in Figures 1A, 2A, and 3A, which have no switch mode

power transformers, are the simplest circuits. They are useful for explaining

the operating theory.

The Forward Converter (Buck) Circuit

The inductors in all of the buck circuits act as filtering elements to smooth

out the ripple and reduce peak currents. Since they must store energy for part

of a cycle they usually have a discrete air gap(s) or a distributed air gap in

the magnetic core path.

The switch mode power transformer in the Buck Circuit of Figure 1B

couples energy from the input side (primary) to the output side (secondary).

An ideal transformer does not store any energy and consequently does not

provide any ripple filtering. The inductor does the ripple filtering. Ideally, a

Buck circuit transformer couples energy without storing it (hence it meets

the true definition of a transformer). The transformer does not need to do

any ripple filtering.

The transformer should have minimal air gap. The “on time” on the

transistor (switch) controls how much energy is delivered to the capacitor

hence it regulates the output voltage. Note that for the inductor circuit of

Figure 1, the average capacitor voltage can never be more than the source

voltage even for ideal circuit components. Real life voltage drops (diode,

transistor, winding resistance) ensure that the average output voltage will be

less than the source voltage. The transformer in Figures 1B remove this

voltage limit and can also provide electrical isolation between input and

output.

The circuits of Figures 1A and 1B are unipolar applications of forward

converters. Push-pull versions, such as that shown in Figure 4, are bipolar

applications. Unipolar and bipolar applications are explained further below.

Click on the available link for more information about push-pull switching

mode power transformers.

Inductive Flyback (Kickback) in Switch Mode Power Transformers

Unlike the Buck transformer; the flyback inductor, flyback transformer,

boost inductor, and boost transformer intentionally store energy during the

“on time” (charging portion) of a cycle and then discharge energy during the

“off time” portion. (Technically, since they intentionally store energy, the

switch mode flyback and boost power transformers are not true

transformers.) They usually have a discrete air gap(s) or a distributed gap in

their core’s magnetic path. The transistor is turned on and current flows into

the inductor or transformer (which has inductance). When the transistor is

turned off, the input current that formed and maintained the core’s magnetic

field become zero. The magnetic field collapses causing a voltage reversal to

occur in the inductor or transformer. The collapsing magnetic field induces

sufficiently high voltage (known as inductive kickback voltage) to discharge

energy into the capacitor connected to the inductor or to the switch mode

power transformer secondary.

Inductive discharge into the capacitor continues until the magnetic field

completely dissipates or power is restored to the input. Restoring the power

starts the inductive charging cycle again. The use of inductive kickback

permit the output voltages of the inductor circuits of Figures 2A and 3A to

be either lower, equal, or greater than the input source voltage. A

transformer “step up” is not needed to achieve voltages higher than the

source voltage. Flyback transformers are usually preferred over flyback

inductors. The appropriate turns ratio can optimize current levels. The

transformer can provide voltage isolation between input and output, and

removes a polarity restriction that comes with a flyback inductor design.

Boost Inductor Circuits

You might ask what distinguishes the boost inductor application from the

flyback inductor application. One characteristic is the polarity reversal of the

output capacitor due to the placement of the circuit components. Compare

the circuits of Figures 2A and 3A. The diode in the flyback circuit, Figure

2A, completely blocks direct flow of current from the input source to the

capacitor regardless of the capacitor’s voltage value. The capacitor can only

be charged by the inductive kickback. The diode in the boost circuit, Figure

3A, permits current flow from the input source to the capacitor without the

use of inductive kickback if the capacitor voltage is sufficiently low.

Consequently it both “stores” energy and “passes through” energy during the

charging portion of a cycle. “Pass through” current flow stops whenever the

capacitor voltage approaches the value of the source voltage minus the diode

voltage drop. (Further increase requires the inductive kickback voltage.)

This may be a desirable feature for rapid power supply startup

Few designers are aware of the boost transformer circuit shown in Figure 3B

because the circuit is not very practical. With only half-wave rectification it

is either a forward (Buck) converter transformer application or a flyback

transformer application depending on choice of polarity. Full wave

rectification, as shown, permits it to duplicate the boost inductor actions

discussed in the prior paragraph; both storing energy and “passing through”

energy (by transformer coupling like a Buck transformer) during the

charging portion of a cycle if the secondary capacitor voltage is sufficiently

low. It acts likes a flyback transformer during the discharging portion of the

cycle. It is rarely used with the full wave rectification as shown. It has seen

some limited use as modified in the circuit shown in Figure 3C. The

transformer has two secondary windings. One is used as a

Forward (Buck) converter. The other is used as a flyback. It effectively

divides the full-wave rectification into two half-wave applications. A more

common boost inductor application is shown in Figure 5. A boost inductor is

used with a push-pull (Buck) transformer. “High power” power supplies

might use this type of circuit. In this application both switches are not open

at the same time. Both switches are closed to charge the inductor, otherwise

the switches are alternated on and off with “one closed and one open”.

Unipolar versus Bipolar

What is the difference? When a current flows through an inductor or a

transformer a magnetic field is created in its core. The value of the magnetic

field will be greater than zero and it will have a direction associated with it.

This direction is also referred to as the polarity of the field. If the value of

the current varies, then the value of the magnetic field will vary accordingly,

but the field polarity (direction) will remain the same as long as the current

direction does not reverse. When an inductor or transformer continually

operates with the same magnetic polarity it is a unipolar application. The

circuits shown in Figures 1 through 3, including A thru C versions, are all

unipolar applications.

Applications were the magnetic field polarity is continually reversing are

bipolar applications. A.C. applications are bipolar applications. Push-Pull

types of forward converters (Buck) are bipolar applications. “Push-pull”

transformers are often used in “inverter circuits” to create A.C. voltage from

a D.C. source. A “push-pull” center-tap application is shown in Figure 4.

There are several types of “push-pull’ applications. More information about

push-pull transformer applications is available on this website. Click on the

available link.

Need More Power TransformerTechnical Information?

More information about the theory of operation for flyback transformers is

available. Click on the available link for flyback transformer. Much of its

theory of operation also applies to the boost inductor. There is also a link for

forward (Buck) converters and links for other types of power transformers,

inductors, chokes, etc.

Also, feel free to contact Butler Winding and ask for technical or

engineering assistance.

Toroidal Transformer

Toroidal transformers are the high performers among transformers. They

offer the smallest size (by volume and weight), less leakage inductance, and

lower electromagnetic interference (EMI). Their windings cool better

because of the proportionally larger surface area. A 360 degree wound

toroidal transformer has a high degree of symmetry. Its geometry leads to

near complete magnetic field cancellation outside of its coil, hence the

toroidal transformer has less leakage inductance and less EMI when

compared against other transformers of equal power rating. Toroidal

transformers with a round core cross section are better performers than

toroidal transformers with a rectangular cross section.

The cancellation is more complete for the round cross section. The round

cross section also gives a shorter turn length per unit of cross sectional area,

hence lower winding resistances. The toroidal transformer also has better

winding to winding magnetic coupling because of its toroidal shape. The

coupling is dependent on the winding being wound a full 360 degrees

around the core and wound directly over the prior winding, hence sector

wound windings do not couple as well and have higher leakage inductance.

As winding turns are positioned further away from the core less complete

coupling will occur; hence toroidal transformers with multi-layered

windings will exhibit more leakage inductance.

Toroidal transformers can be used in any electronic transformer application

that can accommodate its shape. Although usable, toroidal transformers are

not always practical for some applications. Gapped toroidal transformers

usually require that the gap be filled with some type of insulating material to

facilitate the winding process. This is an extra expense. Split core current

transformers can be assembled directly on a conductor while toroids must be

passed over a disconnected end of the conductor. A toroid can be split in

two, but a suitable clamping mechanism (difficult and costly) is required.

Some printed circuit boards are space critical. Mounting a toroidal

transformer flat on the board may take up too much precious board area.

Some applications also have restricted height so the toroid cannot be

mounted vertically.

Generally speaking toroidal transformers are more expensive than bobbin or

tube wound transformers. Sufficient winding wire must first be wound

(loaded) onto the winding shuttle, then wound onto the toroidal

transformer’s core. After that, the best situation, from a cost perspective, is

no insulation required over the winding and the next winding uses the same

wire size. If the wire is different, then the leftover wire must be removed and

the wire for the next winding must be loaded. However, if the winding must

be insulated, then if must either be insulated (taped) by hand or the toroidal

transformer must be removed and taken to a separate taping machine, then

placed back on the toroid winding machine after taping. The shuttle must

then be loaded with the wire size and type for the toroidal transformer’s next

winding. A toroidal transformer with a single winding (auto-transformer,

current transformer) wound on a coated core will probably be cost

competitive with an equivalent bobbin or tube wound transformer since the

toroidal transformer will not require a bobbin or tube. The cost differential

will then depend on the method and cost of mounting the transformers.

Toroidal transformer cores are available in many materials: silicon steel,

nickel iron, moly-permalloy powder, iron powdered, amorphous, ferrites,

and others. Silicon steel and nickel iron are available as tape wound cores or

laminated pieces. Non-magnetic toroids are also available to make air core

toroidal transformers.

Butler Winding manufactures toroidal transformers in a wide variety of

materials and sizes. To ensure quality, Butler Winding purchased two

programmable automated testing machines. Most of our production is 100%

tested on these machines. For more information on Butler Winding’s

capabilities, click on our “capabilities” link.

Need More Technical Information about Electronic Transformers in

general ?

More information is available on other web pages included in this web site.

Saturation and the volt-second product are discussed in the “pulse

transformer” web page. An equivalent circuit for a transformer is included in

the “power transformers” web page. The “inverter transformer” and “push

pull” web pages include some discussion about magnetic “bipolar” and

“unipolar” operating modes. There are web pages for various types

(applications) of electronic transformers (and inductors). Click on one of the

available links.



Flyback Transformers - Kickback Transformers

A simple and low cost power supply is bound to be quite popular. The single

ended flyback circuit topology fits this description. The flyback transformer

utilizes the "flyback" action ( also known as "kickback" ) of an inductor or

flyback transformer to convert the input voltage and current to the desired

output voltage and current. Figures 1A and 1B show simple flyback

transformer schematics for an inductor and a flyback transformer. These

schematics do not show any parasitic effects ( such as leakage inductance

and winding capacitance ). Modern flyback transformer and circuit design

now permit use in excess of 300 watts of power, but most applications are

less than 50 watts.

By definition a transformer directly couples energy from one winding to

another winding. A flyback transformer does not act as a true transformer. A

flyback transformer first stores energy received from the input power supply

(charging portion of a cycle) and then transfers energy (discharge portion of

a cycle) to the output, usually a storage capacitor with a load connected

across its terminals. An application in which a complete discharge is

followed by a short period of inactivity (known as idle time) is defined to be

operating in a discontinuous mode. An application in which a partial

discharge is followed by charging is defined to be operating in the

continuous mode. See figures 2A and 2B for illustration.

Gapped core structures increase the magnetizing force needed to reach

saturation and lower the inductance of the flyback transformer (or inductor).

Consequently, a gapped flyback transformer (or inductor) can handle higher

peak current values, and thereby storing more energy, most of which is

stored in the magnetic field of the gap. For these reasons almost all flyback

transformers (or inductors) are gapped. The gap may be a discrete physical

gap, several smaller discrete physical gaps or a distributed gap. Distributed

gaps are inherently present in low permeability powdered cores. The bulk of

the stored energy is stored in the magnetic field of the gap(s). Most modern

flyback transformers are operated at high frequency hence gapped ferrite

core materials are typically used.

Butler winding can make (and has made) flyback transformers in a wide

variety of shapes and sizes. This includes; various standard types of “core

with bobbin” structures (E, EP, EFD, EC, ETD, PQ, POT, U and others),

toroids, and some custom designs. We have experience with foil windings,

litz wire windings, and perfect layering. For toroids, we can (and have done)

sector winding, progressive winding, bank winding, and progressive bank

winding. Butler winding has a variety of winding machines, bobbin/tube and

toroid. That includes two programmable automated machines and a taping

machine for toroids. To ensure quality, Butler Winding purchased two


tested on

these machines. For more information on our capabilities, click on our

"capabilities" link.

How does a flyback transformer ( or inductor ) work?

Flyback circuits repeat a cycle of two or three stages; a charging stage, a

discharging stage, and in some applications idle time following a complete

discharge. Charging creates a magnetic field. Discharging action results

from the collapse of the magnetic field. The typical flyback transformer

application is a “unipolar” application. The magnetic field flux density

varies up in down in value ( 0 or larger ) but keeps the same ( hence unipolar

) direction.

Charging Stage

The flyback transformer ( or inductor ) draws current from the power source.

The current increases over time. The current flow creates a magnetic field

flux that also increases over time. Energy is stored within the magnetic field.

The associated positive flux change over time induces a voltage in the

flyback transformer ( or inductor ) which opposes the source voltage.

Typically, a diode and a capacitor are series connected across a flyback

transformer winding ( or inductor ). A load resistor is then connected across

the capacitor.

The diode is oriented to block current flow from the flyback transformer ( or

source ) to the capacitor and the load resistor during the charging stage.

Controlling the charging time duration (known as duty cycle) in a cycle can

control the amount of energy stored during each cycle. Stored energy value,

E = ( I x I x L ) / 2, where E is in joules, I = current in amps, L = inductance

in Henries. Current is defined by the differential equation V(t) = L x di/dt.

Applying this equation to applications with constant source voltage and

constant inductance value one obtains the following equation; I = Io + V x t /

L , where I = currents in amps, Io = starting current in amps, V = voltage in

volts across the flyback transformer winding ( or inductor ), L = inductance

in Henries, and t = elapsed time in seconds.

Note that increasing L will decrease the current. Stored energy will

consequently decrease because effects of the “current squared decrease” will

more than offset the effects of the inductance increase. Also be aware that

the flyback transformer ( or inductor ) input voltage is less than the source

voltage due to switching and resistive voltage drops in the circuit.

Discharge Stage

The current ( which creates the magnetic field ) from the source is then

interrupted by opening a switch, thereby causing the magnetic field to

collapse or decrease, hence a reversal in the direction of the magnetic field

flux change ( negative flux change over time ). The negative flux change

induces a voltage in the opposite direction from that induced during the

charging stage. The terms “flyback” or “kickback” originate from the

induced voltage reversal that occurs when the supply current is interrupted.

The reversed induced voltage(s) tries to create ( induce ) a current flow. The

open switch prevents current from flowing through the power supply. With

the voltage reversed, the diode now permits current flow through it, hence

current flows into the capacitor and the load across the capacitor. If current

can flow, then the resulting flow of current is in the direction, which tries to

maintain the existing magnetic field. The induced current cannot maintain

this field but does slow down the decline of the magnetic field.

A slower decline translates to a lower induced flyback voltage. If current

cannot flow, the magnetic field will decline very rapidly and consequently

create a much higher induced voltage. In effect, the flyback action will

create the necessary voltage needed to discharge the energy stored in the

flyback transformer or inductor. This principle, along with controlling the

duration of the charging stage, allows a flyback inductor to increase or

decrease the voltage without the use of a step-up or step-down turns ratio. In

the typical flyback circuit, the output capacitor clamps the flyback voltage to

the capacitor voltage plus the diode and resistive voltage drops.

For a sufficiently large & fully charged capacitor, the clamping capacitor

voltage can be treated as a constant value. The equations V(t) = L x di/dt,

and I = Io + V x t / L can also be applied to the discharge stage. Use the

inductance value of the discharging winding and the time duration of the

discharging stage. The time will either be the cycle time minus the charging

time ( no idle time ), or the time it takes to fully discharge the magnetic field

thereby reaching zero current. The cycle time equals the period which equals

1 / frequency.

Idle Stage: This stage occurs whenever the flyback transformer ( or inductor

) has completely discharged its stored energy. Input and output current ( of

the transformer or inductor ) is at zero value.

Other Principles of Operation

Equal Ampere-Turns Condition: A magnetic field is created by the current

flow through the winding(s). The current creates a magnetizing force, H, and

a magnetic field flux density B. A core dependent correlation will exist

between B and H. B is not usually linear with H. By definition H is

proportional to the product of the winding turns and the current flowing

through the winding, hence ampere-turns. In classical physics, the magnetic

field flux cannot instantaneously change value if the source of the field ( the

current flow ) is removed. When the source current is removed from the

flyback transformer ( or inductor ) the charging stage ends and the discharge

stage begins.

The value of the magnetic field will be the same for both stages at that point

in time ( cannot instantaneously change to another value ). The same

magnetic core is used for both stages, hence if the magnetic field is the

same, then the magnetizing force, H, must be the same. Consequently the

ampere-turns at the end of the charging stage must equal the ampere-turns at

the start of the discharge stage. If there are multiple outputs then the total

amperes turns of all outputs at the start of the discharge stage must equal the

ampere-turns at the end of the charging stage. The same condition applies at

the start of the charging stage. The total ampere-turns of all outputs at the

start of the charging stage must equal the ampere-turns at the end of the

discharge stage. Note that there are zero ampere-turns at both the start and

end of an idle stage when an idle stage exists.

Zero Average Voltage

During steady state operation, the average voltage across the charging

winding must equal the average voltage across the discharge winding, or

equivalently, the volt-seconds of the charging stage must equal the volt-

seconds of the discharge stage. If not, flux density increases over time and

the core saturates. Assuming a 1:1 turns ratio, then from V1 x t1 = V2 x t2

one can obtain t1 / t2 = V2 / V1 for both continuous and discontinuous

modes of operation. For continuous mode operation, t1 + t2 = 1 / operating

frequency.

Conservation of Energy

Power out cannot exceed power in. Sum up output power ( V x I ) of each

output at maximum steady state load plus allowances for parasitic output

power losses ( diode and resistive losses ). Divide power in watts by

operating frequency. The result is the energy in Joules that must be

discharged each cycle into the output storage capacitor during steady state

operation. It is also the amount of energy that must be added to the flyback

transformer ( or inductor ) during the charging stage. The energy being

transferred equals ( Ipeak x Ipeak – Imin. x Imin. ) x L /2.

If operating in the continuous mode, the stored energy will exceed the

energy being transferred because the starting level of stored energy is above

zero ( Imin. > 0 ). The flyback transformer ( or inductor ) must be designed

to handle the peak stored energy, Ipeak x Ipeak x L / 2. The power source

will have to supply the transferred energy plus the parasitic switching and

resistive losses of the charging circuit, plus some power allowance for

transient conditions. Take this value and divide by the power supply voltage.

The result will be the average input current.

Need additional information about Flyback Transformers?

Contact Butler Winding. Ask for engineering assistance.

Electronic Transformer - Inverter Transformer

The term "inverter" is associated with several different electronic

applications.

In logic circuits "inverter" may be a logic inverter, the equivalent of a "Not"

gate. In analogue signal processing an inverter can be a circuit which inverts

the phase of the signal being transmitted. In power conversion applications

an inverter is an electronic transformer which converts power from a Direct

Current (D.C.) source into Alternating Current (A.C.) power. Power

conversion inverters can be divided into two sub-categories, voltage-fed

inverters and current-fed inverters.

Voltage-fed inverters are more common than the current-fed inverters. The

electronic transformers used in inverter circuits are often called inverter

transformers. Inverters produce A.C. power by switching the polarity of the

D.C. power source across the D.C. power source’s load. The early inverters

used mechanical switches to do the switching. Vacuum tubes replaced

mechanical switches in low power applications. Eventually semiconductor

based switches (diodes, transistors, F.E.T.s, S.C.R.s, etc.) replaced both

mechanical and vacuum tube switches.

The schematic in Figure 1A illustrates a very simple inverter circuit. The

circuit does not have an inverter electronic transformer. The switches are

alternated on and off (“cycled”), but are not on at the same time. The load

will see alternating square wave pulses of voltage equal to the source voltage

minus the circuit’s resistive voltage drops. The pulse voltage cannot be

adjusted, but the average load voltage can be made less than the source

voltage by holding both switches “open” (“off”) at the same time.

The portion (ratio < 1) of time during a cycle that a switch is “on” is called

the “duty cycle”. The inverter schematic in Figure 1B utilizes a capacitor

and another switch to provide a lower load voltage. One switch controls the

amount of charge delivered to the capacitor hence it also controls the

capacitor voltage. The set of two switches alternately switches the polarity

for the connection between the capacitor and the load. The load voltage

cannot exceed the input source voltage.

The inverter schematic of Figure 1C adds an electronic transformer inverter

with two secondary windings. The switching action sends alternating current

through the inverter transformer’s primary winding. This is referred to as

“push-pull” action. The core has “bipolar” utilization. Bipolar utilization is

discussed further below. The inverter transformer’s turns ratio can permit

either higher or lower load voltage. The inverter transformer’s output is an

A.C. square wave. Output filter networks can be used to obtain sine wave

output. The inverter transformer can also provide electrical isolation

between the inverter transformer’s input and output sides. Full wave

rectification can be applied to the inverter transformer’s outputs to obtain a

D.C. voltage of different value than that of the input source. This is shown in

the schematic of Figure 1D.

Compare the schematic of Figure 2A to the one in Figure 1D. Note in figure

2A the center-tap connections on the electronic transformer windings, a set

of two switches instead of a set of four switches on the input side, the two

diodes on the secondary instead of four, and the output filter inductor

between the capacitor and load. The inverter transformer center-taps allow

use of fewer switches and diodes. The inductors smooth out the current

surges from the rectification thereby maintaining tighter output voltage

regulation (less ripple voltage). The circuit in Figures 2A depicts a typical

“Push-Pull” “Forward Converter” circuit. Be aware that the name for a

“Forward Converter” circuit (and transformer) varies from industry to

industry and from person to person. It may also be referred to as “Buck”,

“inverter”, “D.C. converter”, “feed forward”, and others. There are also

unipolar versions and there are bipolar versions that utilize saturable

transformers to trigger transistor switching.

Butler Winding makes electronic transformers and inverter transformers in a

wide variety of shapes and sizes. This includes; various standard types of

“core with bobbin” structures (E, EP, EFD, PQ, POT, U and others), toroids,

and some custom designs. Our upper limits are 40 pounds of weight and 2

kilowatts of power. We have experience with foil windings, litz wire

windings, and perfect layering. For toroids, we can (and have done) sector

winding, progressive winding, bank winding, and progressive bank winding.

Most of our production is 100% tested on these machines. For more

information on Butler Winding’s capabilities, click on our “capabilities”

link.

The Difference between Bipolar and Unipolar Applications

Since the connections of the electronic transformer "inverter" are alternated,

the current direction through the electronic transformer will also alternate.

Consequently the magnetic field polarity of the inverter transformer’s core

will alternate between positive and negative flux directions. This is known

as “bipolar” utilization of the inverter transformer’s core. This is graphically

illustrated in Figure 2B. The “B-H” curve shown is also known as a

hysteresis loop. The area inside the loop is related to the core loss. A thinner

loop means less core loss. Also note the residual flux density point. In a

Unipolar application the flux density, B, would never return to zero value.

It would stop at Br when the current (hence also the magnetizing force, H)

returns to zero. The applied voltage reversal (by switching action) ensures

that the flux density returns to zero. Bipolar utilization permits use of a

smaller core than unipolar utilization because it permits a larger change in

the core’s flux density. Fewer turns are needed to handle the same amount of

power. Compare Figure 2B to Figures 3C, 4C, and 5C.

Unipolar utilization occurs if the magnetic flux remains in one direction. The

value may vary up and down but does not cross zero value. A unipolar

application is illustrated in Figures 3A, 3B, and 3C. Some designers may

refer to the transformer in Figure 3A as an inverter transformer, but it is not.

It is serving as a pulse transformer with a resistive load. If we assume it to be

an ideal transformer, then there is no core loss, no leakage inductance, does

not store any energy, and the residual flux density is zero. Figure 3B shows

the expected output if a rectangular voltage pulse is placed across the

transformer (turn switch on, then off). The output will also be a rectangular

pulse without any distortion. There will be a change in amplitude because of

the transformer’s turns ratio. The ideal transformer’s lack of stored energy

eliminates the possibility of an inductive kickback voltage spike. This circuit

does not produce an A.C. output, hence no true inverter action.

A non-ideal electronic transformer has finite inductance hence it stores some

inductive energy in its magnetic field. A lower inductance results in more

stored energy. Consider the non-ideal gapped transformer in the circuit

shown in Figure 4A. The gap lowers the inductance of the transformer;

consequently more current can flow when the switch is closed (compared to

no gap). When the switch is closed the transformer directly couples power to

the load plus it stores energy in its magnetic field. The field is created by the

magnetizing current. The current flow due to the load does not contribute to

the stored energy. When the switch is opened the magnetic field collapses.

The collapse creates an inductive kickback voltage of reversed polarity. The

induced secondary voltage causes current to flow through the load resistor in

the reversed direction. (This is how a flyback transformer functions.) The

load sees alternating current although it usually has an asymmetrical

waveform. One could claim that the circuits and transformer have inverter

action.

The energy stored in the electronic transformer’s magnetic field is dissipated

as heat produced by current flowing through the load resistor. Current of

declining value will continue to flow until either all of the stored energy is

dissipated or the switch is closed again. If completely dissipated, then the

output shown in Figure 4B and the generalized hysteresis loop of Figure 4C

apply. The transformer is said to be operating in discontinuous mode. The

load voltage and load current reach zero value, and the core’s flux density

reaches its residual value. Note that the flux density averaged over time is

greater than zero. This holds for all unipolar applications. If the switch is

closed again before all the energy is dissipated, then the output shown in

Figure 5B and the generalized hysteresis curve of Figure 5C applies. The

transformer is said to be operating in the continuous mode. The load voltage

and load current remain above zero value, and the flux density does not

reach its residual value. The output waveform in Figure 5B is more

rectangular than that of Figure 4B.

The circuits in Figures 4A and 5A are not very practical inverter transformer

circuits. To be useful the transformer must store as much energy as it

directly couples to its load. Consequently, the transformer will tend to be

lightly loaded and designed to have appreciable magnetizing current. Output

filters would be required to produce a more symmetrical output waveform.

These circuits find little use as shown here. There are D.C. biased unipolar

applications, which function as inverters. They are not discussed here.

Saturable Transformers as Inverter Transformers

Figure 6A shows a “Royer Inverter Circuit” schematic that uses saturable

transformers. The saturable transformer also functions as the inverter

transformer. Figure 6B shows a “Jensen Circuit” which uses a saturable

transformer and a power transformer. The power transformer functions as

the inverter transformer. Both of these circuits make use of “push-pull”

switching to achieve the inverter action. The feature of these two circuits is

the transistor switching action that is activated by a voltage spike created

when the saturable transformer enters saturation.

An oscillation develops which maintains the necessary switching action.

The theory of operation is not discussed here. It may be available on this

website at some future date from the issue date of this website page. Check

the available links.

Additional Technical Information

This website covers a variety of transformer and inductor applications.

Check the available website links to see if your topic of interest is available.

If a link is not available or if a link does not provide enough information,

please feel free to contact Butler Winding and ask for engineering or

technical assistance. Select the “Contact” link for contact information.

Buck Boost Transformer - Push Pull Transformer

When it comes to power conversion, the buck boost or "push pull"

transformer application is well known. The buck boost transformer

configuration is widely used in converting direct current (D.C.) voltage into

another value of D.C. voltage, and in inverters. Inverters convert direct

current into alternating current (A.C.). The push pull transformer is usually

the preferred choice in high power switching transformer applications

exceeding one kilowatt. It is usually used in a circuit known as a "forward

converter" circuit. Be aware that the name for the "forward converter" circuit

varies from industry to industry and from person to person. It may also be

referred to as an "inverter", "D.C. converter", "buck", "feed forward", and

others. A basic "forward converter" transformer circuit is illustrated in

Figure 1A. It is not a push pull transformer application. The output inductor

reduces ripple voltage. Pulse width modulation is used to control the value

of the output voltage

A center-tapped buck boost transformer application circuit is illustrated in

Figure 2A. Figure 2A only shows one output. Multiple voltage outputs are

possible by using either a tapped secondary winding or using multiple

secondaries. Some other buck boost transformer versions are discussed

further below. They are illustrated in Figures 3, 4, 5, and 6. (These include

some push pull transformers without the center-taps.)

The core of the transformer in Figure 1A is operated in a unipolar fashion.

Unipolar operation is depicted graphically in Figure 1B. The core's magnetic

"B-H" loop remains in one quadrant of the "B-H" grid. A loop occurs once

every cycle. The flux density "B" and the magnetizing force "H" never cross

zero hence always retain the same (or one) polarity. "H" does not have to

return to zero value. The core in a push pull transformer has bipolar

operation. Both "B" and "H" cross zero value and reverse polarity. Bipolar

operation is depicted graphically in Figure 2B. Note that the "dB" value

(change in B) in Figure 2B for the bipolar push pull transformer can be more

than twice the "dB" value shown in Figure 1B for the unipolar forward

converter (assuming the same core material). Push pull transformer (bipolar)

operation permits one to handle the same amount of power in a smaller

package than for that of a unipolar operation. There are tradeoffs. The buck

boost transformer operation requires more switching elements and its control

circuitry is more complicated. Consequently a push pull transformer

application is more expensive. The voltage pulses must be adequately

controlled to avoid phenomena known as saturation walk. Center tapped

push pull transformers have winding capacitance issues at higher

frequencies. Winding imbalances can contribute to saturation walk.

Power ratings for push pull or buck boost transformer can vary from a

fraction of a Watt to Kilowatts. Megawatts is possible, but definitely beyond

Butler Winding's capabilities. Size correlates with power hence size (and

weight) can vary from a fraction of a cubic centimeter (several grams) to

multiple cubic meters (thousands of kilograms). Buck boost transformers

can be wound on toroids, bobbins, and tubes. Core materials vary depending

on the application. Laminated or tape wound grain oriented silicon steel is

common for low frequency inverter buck boost transformers. Ferrite core

materials are common for high frequency switching push pull transformers.

If minimal size is a requirement, nickel-iron alloys may be chosen for the 1

to 20 kilohertz range. Minimal energy storage is desired so cores have

minimal air gaps in their structure.

Butler Winding manufactures buck boost transformers in a wide variety of

shapes and sizes. This includes; various standard types of “core with

bobbin” structures (E, EP, EFD, PQ, POT, U and others), toroids, and some

custom designs. Our upper limits are 40 pounds of weight and 2 kilowatts of

power. We have experience with foil windings, litz wire windings, and

perfect layering. For toroids, we can (and have done) sector winding,

progressive winding, bank winding, and progressive bank winding. Butler

winding has a variety of winding machines, bobbin/tube and toroid. That

includes two programmable automated machines and a taping machine for

toroids. Butler Winding has vacuum chamber(s) for vacuum impregnation

and can also encapsulate. To ensure quality, Butler Winding purchased two


tested on these machines. For more information on Butler Winding's

capabilities, click on our "capabilities" link.

Push Pull - Buck Boost Transformer Rectification

The push pull / buck boost transformer in Figure 3 is the same as the push

pull transformer in Figure 2A except for secondary rectification. Figure 2A

achieves full wave rectification using a center-tap. It requires two diodes.

Figure 3 achieves full wave rectification with a full wave bridge. It requires

four diodes. Four diodes result in more power loss, but elimination of the

center-tap simplifies transformer construction and reduces winding

capacitance. The primary and secondary winding halves as shown in Figure

2A conduct current on alternate half cycles. Their maximum duty cycle is a

0.5 ratio (or 50%). Figure 3 requires approximately half of the secondary

turns of Figure 2A, but its secondary winding may see a maximum duty

cycle near 1 (or 100%), hence its wire must handle twice the r.m.s. current

value. Both transformers are about the same size.

Half Bridge Push-Pull Transformers

Compare figure 4 to figure 2A. Figure 4 is a “half bridge” push pull / buck

boost transformer application. This configuration eliminates the primary

center-tap and reduces primary winding capacitance. The two series

connected capacitors shown in Figure 4 effectively cut the input voltage to

the push pull transformer in half. Consequently, for the same power rating,

the push pull / buck boost transformer requires one quarter of the total

primary turns to support the halved voltage, but it must handle twice the

amount of input current.

The primary winding may see a maximum current duty cycle near 1, hence

its wire may see 4 times the r.m.s current value as wire used in the primary

winding halves of Figure 2A. Both transformers are about the same size. To

achieve the same output voltage, the number of secondary turns is about the

same as that of figure 2A, but the secondary over primary turns ratio is

quadrupled because the primary turns of figure 4 are one quarter of that of

figure 2A. The output of figure 4 is a full wave center-tap configuration.

Alternately, it could be a full wave bridge configuration with approximately

half the number of secondary turns.

Full Bridge Push Pull Transformers

Compare figure 5 to figure 4. Figure 5 is a “full bridge” push pull / buck

boost transformer application. Like the half bridge configuration of figure 4,

this configuration eliminates the primary center-tap, reduces primary

winding capac-itance, & is about the same size. The two series connected

capacitors are replaced by two additional transistors as shown in Figure 4.

The voltage supplied to the input of the push pull transformer of figure 5 is

the same as that for figure 2A. For the same power rating and source

voltage, the push pull transformer of figure 5 requires half the primary turns

as that of figure 2A and it must handle the same amount of input current.

The primary winding of figure 5 may see a max current duty cycle near 1,

hence its wire may see 2 times the r.m.s current value as wire used in the

primary winding halves of Figure 2A. For the same output voltage, the

number of secondary turns is about the same as that of figure 2A, but the

secondary over primary turns ratio is doubled because the primary turns (fig.

5) are halved. The output of figure 5 is a full wave center-tap configuration.

Alternately, it could be a full wave bridge configuration with approximately

half the number of secondary turns.

The Boost Push Pull Transformer Application

The prior push pull transformer applications utilize an inductor in the output

circuit to reduce output voltage ripple. If there were more than one output,

an inductor would be used with each output. An alternate would be to place

one inductor in series with the primary center-tap of a push-pull center-tap

transformer. This circuit is illustrated in Figure 6. To charge the inductor the

two transistors are made to conduct at the same time. Charging current flow

through both halves of the primary winding but in opposite directions

resulting in magnetic cancellation of each other hence the transformer

windings act as a short to ground. Opening one of the transistor switches

results in current flow in only one of the primary winding halves. Alternate

opening of the transistor switches results in a push-pull transformer action.

Control circuitry is more complex.

Need More Technical Information?

A push pull transformers is a type of forward converter transformer. More

information about the theory of operation for forward converter transformers

is available under the category of switch mode (switching) transformers.

Click on the available link for switch mode power transformers. Much of the

theory for flyback transformers also applies to boost inductors Click on the

available link for flyback transformers. There are also links for other types

of transformers, inductors, chokes, etc.



Pulse Transformers

The magnetic flux in a typical A.C. transformer core alternates between

positive and negative values. The magnetic flux in the typical pulse

transformer does not. The typical pulse transformer operates in an “unipolar”

mode ( flux density may meet but does not cross zero ).

A fixed D.C. current could be used to create a biasing D.C. magnetic field in

the transformer core, thereby forcing the field to cross over the zero line.

Pulse transformers usually (not always) operate at high frequency

necessitating use of low loss cores (usually ferrites). Figure 1A shows the

electrical schematic for a pulse transformer. Figure 1B shows an equivalent

high frequency circuit representation for a transformer which is applicable to

pulse transformers. The circuit treats parasitic elements, leakage inductances

and winding capacitance, as lumped circuit elements, but they are actually

distributed elements. Pulse transformers can be divided into two major

types, power and signal.

An example of a power pulse transformer application would be precise

control of a heating element from a fixed D.C. voltage source. The voltage

may be stepped up or down as needed by the pulse transformer’s turns ratio.

The power to the pulse transformer is turned on and off using a switch (or

switching device) at an operating frequency and a pulse duration that

delivers the required amount of power. Consequently, the temperature is also

controlled. The transformer provides electrical isolation between the input

and output. The transformers used in forward converter power supplies are

essentially power type pulse transformers. There exists high-power pulse

transformer designs that have exceeded 500 kilowatts of power capacity.

The design of “signal” type of pulse transformer focuses on the delivery of a

signal at the output. The transformer delivers a “pulse-like” signal or a series

of pulses. The turns ratio of the pulse transformer can be used to adjust

signal amplitude and provide impedance matching between the source and

load. Pulse transformers are often used in the transmittal of digital data and

in the gate drive circuitry of transistors, F.E.T.s, S.C.R.s, and etc. In the

latter application, the pulse transformers may be referred to as “gate

transformers” or “gate drive transformers”. Signal type of pulse transformers

handle relatively low levels of power. For digital data transmission,

transformers are designed to minimized signal distortion. The transformers

might be operated with a D.C. bias current. Many signal type pulse

transformers are also categorized as wideband transformers. Signal type

pulse transformers are frequently used in communication systems and digital

networks.

Pulse transformer designs vary widely in terms of power rating, inductance,

voltage level (low to high), operating frequency, size, impedance, bandwidth

(frequency response), packaging, winding capacitance, and other parameters.

Designers try to minimize parasitic elements such as leakage inductance and

winding capacitance by using winding configurations which optimize the

coupling between the windings.

Butler Winding can make (and has made) pulse transformers in a wide

variety of shapes and sizes. This includes; various standard types of “core

with bobbin” structures ( E, EP, EFD, PQ, POT, U and others ), toroids, and

some custom designs. Our upper limits are 40 pounds of weight and 2

kilowatts of power. We have experience with foil windings, litz wire

windings, and perfect layering. For toroids, we can ( and have done ) sector

winding, progressive winding, bank winding, and progressive bank winding.

Butler winding has a variety of winding machines, bobbin/tube and toroid.


for toroids. Butler winding has vacuum chamber(s) for vacuum





PULSE TRANSFORMER OPERATING PRINCIPLES

Pulse transformer designers usually seek to minimize voltage droop, rise

time, and pulse distortion. Droop is the decline of the output pulse voltage

over the duration of one pulse. It is cause by the magnetizing current

increasing during the time duration of the pulse. To understand how voltage

droop and pulse distortion occurs, one needs to understand the magnetizing (

exciting, or no-load ) current effects, load current effects, and the effects of

leakage inductance and winding capacitance. The designer also needs to

avoid core saturation and therefore needs to understand the voltage-time

product.

Magnetizing ( No-Load ) Current, its Effects, and Its Relation to

Saturation

Consider the simple pulse transformer circuit of Figure 2A and its equivalent

circuit of Figure 2B. There is no source impedance, winding capacitances, or

secondary leakage inductance to worry about. With both switches open,

there cannot be any primary or secondary currents flowing. Now close the

primary switch. Since the secondary load is not connected, the pulse

transformer’s primary winding acts like an inductor placed across a voltage

source. Primary current begins to flow. This is the magnetizing current ( no

secondary current ) and is governed by the differential equation V(t) = L x

d(I)/dt + Rp x I(t), with units of volts, henries, amps, and seconds. If the

power supply has constant voltage, Rp = zero, & L = Lkp+Lm is constant,

the differential equation can be solved for I(t), I(t) = Io + V x t / ( Lkp+Lm ),

where Io = the initial current which equals zero.

Notice that the current increases at a linear rate over time and that the rate in

inversely proportional to the inductance. The current flows through Np turns

creating Np x I(t) amount of magnetizing force ( amp-turns ) which in turns

creates a magnetic flux density in the pulse transformer core. Eventually the

increasing primary magnetizing current would exceed the magnetic flux

capacity of the pulse transformer core and will saturate the core. Once

saturation occurs the primary current rapidly increases towards infinity ( in

theory ). In a real circuit the primary winding resistance ( and source

impedance ) would limit the current. See Figure 3A for graphical illustration.

For non-zero Rp, I(t) = Io + ( V/Rp ) x ( 1 – e to the ( -Rp x t / ( Lkp + Lm ))

power ). The effect of Rp is graphically illustrated in Figures 3B and 3C. Rp

extends the time it takes for the unloaded transformer ( or an inductor ) to

saturate. If Rp is sufficiently large, it prevents the transformer ( or inductor )

from saturating altogether. Regardless of saturation, Rp places an upper limit

on the primary current value.

Voltage Droop

For Rp = 0 the source voltage divides proportional across Lkp and Lm hence

the voltage across Lm = V x Lm / ( Lm+Lkp ) = Vm. The induced secondary

voltage becomes equal to Ns x Vm / Np. For Rp > zero a voltage drop

occurs across Rp. The value of this drop increases in value as the primary

current increases with time, hence Vm decrease over time and consequently

the secondary voltage declines over time. Thus Rp and magnetizing current

contribute to secondary voltage droop. Lkp does not contribute to the droop

in the “no-load” case but does contribute to a lower secondary starting

voltage for both the “no load” and “under load” cases. Droop is graphically

illustrated in Figure 4B. Compare it against the ideal pulse shown in Figure

4A.

Voltage-time product

Pulse transformers, being typically unipolar (D.C.) applications, require the

primary switch to be opened ( thereby removing the voltage source ) before

saturation occurs, whereas A.C. applications reversed the applied voltage

before saturation occurs. Unipolar applications require that sufficient time be

allowed to pass to re-set the core before starting the next pulse. This time

permits the magnetic field to collapse ( reset ).

The field does not completely collapse to zero value ( unless forced to zero,

or lower ) because of core material remanence. A slight air gap may be used

to bring remanence closer to zero value. The gap lowers the pulse

transformer inductance. The flux range between remanence and the

maximum flux is referred to as dB, the maximum change in flux density

during the pulse duration, dt.

The dB of the typical pulse transformer is less than half for that of an A.C.

application because flux in A.C. applications can go from positive Bmax to

negative Bmax. Operating frequency and maximum expected temperature

affect the choice of maximum usable flux density value, Bmax. Saturation

can be avoided by applying the following equation; dB x Np x Ac x Sf = V x

dt x 100000000, where dt is the maximum time duration of the pulse, Ac is

the core’s cross-sectional area and Sf is the core stacking factor ratio. Units

are gausses, turns, square centimeters, volts and seconds. Be aware that dt

does not include reset time, tr. Maximum operating frequency equals 1 / ( dt

+ tr ). The voltage-time product, V x dt is quite useful. The size and cost of a

pulse transformer is roughly proportional to this product.

Kickback Voltage

In the foregoing discussion the primary switch was opened thereby

interrupting the current flowing through the transformer primary. The

resulting collapse in the magnetic field will induce a voltage reversal in the

transformer windings. The more rapid the field collapse is, the higher the

induced voltage. The transformer will try to dissipate the energy stored in its

collapsing magnetic field. If the transformer was under load, the induced

voltage would cause current to flow into the load. In the “no-load” case of

this example, the transformer does not have any readily available place to

dissipate the energy. The transformer will generate the voltage necessary to

dissipate the stored energy, hence a high voltage “kickback” ( or flyback or

backswing ) voltage will occur in the windings. In a real circuit the

transformer will induce eddy currents in its core thereby dissipating the

energy as core loss. In a real circuit the high voltages can damage the

switching elements ( transistors, F.E.T.s, S.C.R.s, etc. ). Many designs

include protective circuitry across the primary winding.

Secondary Load Current Effects and Rise Time

Consider again the simple pulse transformer circuit of Figure 2A and its

equivalent circuit of Figure 2B. Initally, with both switches open, there

cannot be any primary or secondary currents flowing. Close the secondary

load switch and then close the primary switch. Current flows through the

primary winding. The L x dI(t)/dt action induces a voltage in the primary

winding which opposes the source voltage. A voltage, Vsi, is also induced in

the secondary winding causing secondary current to flow. The ampere-turns

created by the secondary current work against the induced voltage that

opposes the source voltage.

Consequently, the source voltage supplies more current flow through the

primary. Currents rapidly increase until either the secondary current or

primary current encounters a current limitation. Examples of such limits are

the secondary load and winding resistances limiting the secondary current or

the source impedance and primary winding resistance and primary leakage

inductance limiting the primary current. Once a limit is encountered, an

equilibrium is quickly established except for the magnetizing current. The

primary current has two components; Irs, the load current transformed (

reflected ) to the primary winding and Im, the magnetizing current. As in the

“no-load” case, the magnetizing current starts at zero and increases over

time. The pulse transformer must be “switched off” before saturation occurs.

In this example the load is resistive, there is no secondary leakage

inductance, and there is no secondary winding capacitance; hence a purely

resistive load current is reflected to the primary winding. The primary

current is larger than it was in the “no-load” case, hence more voltage drop

is expected across the primary winding resistance. Consequently less

voltage, Vm, is available across Lm which results in less induced voltage in

the secondary winding. Secondary current flow through the secondary

winding resistance causes another voltage drop hence lower transformer

output voltage. Under load, both the primary and secondary winding

resistance contribute to a lower secondary voltage. The secondary winding

resistance does not contribute to pulse droop.

The reflected load current, Irs, does not flow thorughthe mutual inductance,

Lm, but doe flow through the primary leakage inductance, Lkp. Lkp restricts

the flow of the primary current ( hence reflected load current also ).

Consequently the reflected load current cannot immediately reach its full

value ( nor can the secondary current ). It is effectively delayed. Until the

reflected load current reaches its full value, a larger voltage drop will occur

across Lkp then there was in the “no-load” case. This larger voltage

diminishes in value over time. Consequently Vm exhibits a time delay in

reaching peak voltage value. This delay is also seen in the secondary output

voltage. This delay is known as rise time. Rise time is graphically illustrated

in Figure 4B.

Effects of Winding Capacitance, Secondary Leakage Inductance, and

Core loss

Now consider the equivalent pulse transformer circuit of Figure 5. The

circuit has all the components of the circuit in Figure 2B, but also has

primary winding capacitance, secondary winding capacitance, core loss, and

secondary leakage inductance. Start with both switches open and no

capacitive energy and no inductive energy. All currents are initially zero.

Close the secondary switch then close the primary switch. The primary

leakage inductance, Lkp, restricts the flow of primary current by opposing

the source voltage. The opposing voltage is generated by Lkp x d(I)/dt

action. Current flow ( from the source ) finds the uncharged winding

capacitance, Cp to be a much easier path, hence a relatively large amount of

current flows into the winding capacitance. This large amount of current

could be called a surge current because it will diminish over time as the

capacitance is charged. The surge causes a relatively large voltage drop

across the primary winding resistance, Rp, thereby initially lowering the

voltage available to Lkp and Lm. Over time, as the surge current diminishes,

the voltage drop across Rp diminishes, and the voltage across Lkp and Lm

reaches full ( peak ) value. The surge effectively delays the peak voltage

across Lm. This in turn delays peak secondary voltage. The delay

contributes to rise time, hence Cp contributes to rise time. As discussed

earlier, Lpk restricts flow of the reflected load current and consequently also

contributes to rise TIME

A similar consequence occurs with the secondary winding capacitance, Cs.

Any current supplied by induced secondary voltage must charge Cs as the

secondary voltage tries to rise to peak value. This delays the secondary in

reaching peak voltage, hence Cs also contributes to rise time.

Secondary leakage inductance, Lks, restricts secondary current flow just like

Lkp restricted primary current flow. Lks also delays the secondary peak

output voltage, hence it also contributes to rise time.

Core loss resistance, Rc, provides a relatively small current shunt path across

Lm just like the reflected secondary load current does. It has the same effect

but the effect is much smaller.

To summarize, Winding capacitances and leakage inductances act to

increase rise time. ( They also generate trailing edges which is discussed

later. ) They may also contribute to spurious oscillations. In a typical pulse

transformer design, core loss does not have much effect.

The Trailing Edge

For an ideal pulse transformer, once the primary switch is opened the

secondary pulse should immediately end. This does not happen. The pulse

transformer tries to dissipate the energy stored in Lm and in the parasitic

components Cp, Cs, Lkp, and Lks. The inductance will induce voltages as

their magnetic fields collapse. The capacitor charge will drain, but will not

drain instantaneously. The capacitances may temporarily supply current to

the inductances. As a result, there is a sloped decline of the secondary output

voltage after the primary switch is opened. This sloped decline is referred to

as the “trailing edge”. Some combinations of capactiance and inductance

could produce spurious oscillations ( known as ringing ). A trailing edge is

graphically illustrated in Figure 3B.

Pulse Distortion

Ideally the output pulse waveform should be identical in shape to the input

pulse waveform except for a desired amplitude change due to the “step-up”

or “step-down” turns ratio. Any other deviation is considered to be

distortion. Rise time, droop, trailing edges, and spurious oscillations are all

considered to be signal distortions.

Figure 3B illustrates all of these distortions.

Electronic Transformer - Trigger Transformers

There are many types of eletronic transformers. What distinguishes a trigger

transformer from other types of electronic transformers? Basically, it is

application! As the word “trigger” implies, a trigger transformer is used in a

circuit that initiates some sort of action or event. Once initiated, some

applications may no longer require continued presence of a voltage to

complete the action or event. Other applications may need the voltage but

for a limited amount of time. Regardless, the application provides a voltage

pulse to the trigger transformer’s primary.

The trigger transformer’s turns ratio steps up or steps down the secondary

voltage as needed. The trigger transformer’s secondary then supplies voltage

or current to its load. The load is usually the gate of a semiconductor switch

such as a transistor, F.E.T., S.C.R., etc.. The trigger transformer also

provides voltage isolation between the primary side circuit and the

secondary side circuit. Most circuit designers would refer to the trigger

transformer as a type of pulse transformer. This website provides some

explanation on pulse transformer operation. Click on the “Electronic

Transformers” button and then select “Pulse Transformer”.

One example of a trigger transformer application is the electronic flash in

modern cameras. A basic circuit is shown in Figure 1. A charging circuit

takes energy from a battery and charges two electrolytic capacitors ( approx.

300V ). The negative sides are both connected to ground. One capacitor is

much larger than the other is. It is connected to the electrodes of a glass tube

filled with xenon gas. This capacitor provides the energy needed to produce

the flash, but lacks sufficient voltage to initiate the flash. The primary of the

trigger transformer is attached to the positive side of the smaller capacitor

through a switch.

The trigger transformer secondary is connected to a metal plate(s) or grid(s)

that partially surrounds the glass tube. The trigger transformer is designed to

step up the voltage to high voltage levels. When the switch is closed the

trigger transformer places high voltage across the plates. The high voltage

ionizes the gas inside the tube. The gas becomes conductive. The large

capacitor discharges through the gas thereby producing a bright white flash.

The capacitor rapidly discharges its energy and must be recharged to

produce another flash.

The switch between the trigger transformer and the smaller capacitor is

opened. A small drain resistor is placed across the high voltage plates to

discharge the voltage on the plates. In this example the trigger transformer

aided the initiation ( or triggering ) of the flash by delivering a stepped up

voltage pulse. Figure 1 shows the trigger transformer windings grounded

together. With proper circuit design the trigger transformer could also

provide voltage isolation.

In the preceding example, the trigger transformer ( which is a pulse

electronic transformer ) design does not saturate the core and usually

employs unipolar core utilization. There are trigger transformer applications

that use bipolar core utilization and/or intentionally saturates the core.

Bipolar core utilization mean the magnetic flux alternates between positive

and negative directions. Unipolar means the flux direction remains either

positive or negative. Two examples of this are found in the “Royer Inverter

Circuit” and the closely related “Jensen Circuit”.

These are shown in Figure 2A and 2B. Operating theory will not be

discussed in detail here but is briefly summarized; transformer saturation

repeatedly occurs in alternating directions which in turn triggers ( switches )

the transistors on and off in alternating fashion, thereby creating an A.C.

output voltage. The switching of the transistors forces the current direction

to alternate which then forces the alternating direction of core saturation. For

more information about saturable transformers click on the “Electronic

Transformers” button, then select “Saturable Transformers”.

Figure 3 is a unipolar application which shows how a trigger transformer can

use core saturation can to shorten the time duration of a pulse. The trigger

transformer usually has a high impedance load ( lightly loaded ) hence it acts

much like a saturated inductor but with voltage step up or step down

capability and voltage isolation. The primary winding of the trigger

transformer has much higher impedance than the series resistor until

saturation occurs. Before saturation most of the circuit’s voltage drop is

across the trigger transformer’s primary. The trigger transformer’s turns

ratio can adjust the secondary output voltage. There will be voltage droop.

After saturation, most of the voltage drop is across the resistor, the

secondary output voltage is substantially reduced, and the time duration of

the output pulse has been reduced. The pulse’s time duration can be

calculated from the transformer’s volt-second product. This website provides

some explanation of the volt-second product. Click on the “Electronic

Transformers” button and then select “Pulse Transformer”.

Butler Winding can make ( and has made ) pulse and trigger transformers.

There are a wide variety of shapes and sizes available. This includes; various

standard types of “core with bobbin” structures ( E, EP, EFD, PQ, POT, U

and others ), toroids, and some custom designs. Our upper limits are 40

pounds of weight and 2 kilowatts of power. We have experience with foil

windings, litz wire windings, and perfect layering. For toroids, we can ( and

have done ) sector winding, progressive winding, bank winding, and

progressive bank winding. Butler winding has a variety of winding

machines, bobbin/tube and toroid. That includes two programmable

automated machines and a taping machine for toroids. Butler winding has

vacuum chamber(s) for vacuum impregnation and can also encapsulate. To

ensure quality, Butler Winding purchased two programmable automated

testing machines. Most of our production is 100% tested on these machines.

For more information on Butler Winding’s capabilities, click on our

“capabilities” link.

Gate Drive Transformers - Electronic Transformer

There are many types of transformers. What distinguishes a gate drive

transformer from other types of transformers? Basically, it is application!

Modern day electronic circuits utilize many gated semiconductor devices

such as ordinary transistors, field effect transistors, and S.C.R.s and others.

Gate drive transformers are used in some of these circuits. A signal must be

supplied to ( or removed from ) the device’s gate node to activate ( or

deactivate ) the device. When used, gate drive transformers are located

within the circuitry driving the gate. Gate drive transformers are used to

modify the voltage level to the gate, provide impedance matching, and to

provide voltage isolation. Gate drive transformer may be used to deliver

voltage to the grids or plates of a vacuum tube or flash tube.

Some gate drive transformers simply deliver a voltage pulse or a series of

voltage pulses to a semiconductor gate. A gate drive transformer functioning

in this manner could also be called a pulse transformer. Most circuit

designers would consider these gate drive transformers to be a type of pulse

transformer. If the gate drive transformer’s pulse initiates some action or

event, the gate drive transformer could be called a trigger transformer. Some

applications require a close reproduction of the pulse. The gate transformer

designer will seek to minimize winding capacitance and leakage inductance

because these parasitic components distort the signal. This website includes

information about trigger transformers and pulse transformers. The latter

includes information on the theory of operation. Click on the available links

if you want to view them.

Some amplifying circuits use a gate drive transformer to deliver a signal to a

semiconductor gate. Here the objective is to reproduce the signal, but with

increased power and increased voltage or current. The gate transformer

designer will seek to minimize winding capacitance and leakage inductance

because these parasitic components distort the signal. In most amplifying

circuits the signal is injected into a direct current biased transistor circuit,

hence the gate transformer may have to tolerate a D.C. current bias. Even

though these gate drive transformers drive a gate, circuit designers will

usually refer to them as signal transformers.

Gate drive transformers exist in a variety of shapes and sizes. There is also a

wide variety of core materials available for use with different applications. If

you need more information please contact Butler Winding and ask for

Engineering.

Butler Winding can make ( and has made ) gate drive transformers. There

are a wide variety of shapes and sizes available. This includes; various




windings, litz wire windings, and perfect layering. For toroids, we can ( and

have done ) sector winding, progressive winding, bank winding, and









Current Transformers

What is the purpose of a current transformer? It measures alternating current

flowing through a conductor. Since it is used to measure current, a current

transformer is often classified as a type of instrument transformer. One could

measure the voltage drop across a known resistor. This is okay for low

current applications but is often impractical for high current applications.

The resistor consumes a lot of power (lowering efficiency) unless the

resistor is very low in value, in which case there may be very little voltage to

measure. The resistor could be excessively large.

The resistor’s heat may affect the resistor value, thereby reducing the

accuracy of the measurement. A current transformer can accurately measure

the alternating current and put out a reasonable voltage, which is

proportional to the current, but without as much heat and size that an

appropriate resistor would require. The current transformer can perform its

function with very little insertion loss into the conductor current being

measured. The current transformer also provides voltage isolation between

the conductor and the measuring circuitry. Proper function of a current

transformer requires use of a load resistor. The load resistor is often referred

to as a “burden resistor”.

The best core structure for a current transformer in terms of electrical

performance is a toroidal coil. Many toroidal current transformers have only

one winding. This winding is usually a “high turns” winding which

functions as the secondary winding. In application, the toroidal current

transformer is slipped over an end of a high current wire or buss bar, which

conducts the primary current. Said wire or buss bar constitutes a one turn

primary winding. Split core current transformers are designed so that they

can be assembled around a buss bar without disconnecting the buss bar. "C"-

cores and "U" core structures are commonly used for split-core current

transformers because they are relatively easy to take apart and put back

together around the buss bar. Historically, this has not been practical for

toroidal coils, but there are now some flexible toroids, which permit the

“split-core” feature of installing it around a buss bar. They have limited

application. Some printed circuit board applications will utilize bobbin

wound current transformers with two or more windings. One winding is an

integral part of the circuitry, while the other winding acts the secondary.

Butler Winding can make (and has made) current transformers in a wide

variety of shapes and sizes. This includes toroids, “U” and “C” cores for

split-core applications; various standard types of "core with bobbin"

structures (E, EP, EFD, PQ, POT, and others), and some custom designs.

Our upper limits are 40 pounds of weight and 2 kilowatts of power. We have

experience with foil windings, litz wire windings, and perfect layering. For

toroids, we can ( and have done ) sector winding, progressive winding, bank

winding, and progressive bank winding. Butler winding has a variety of

winding machines, bobbin/tube and toroid. That includes two programmable







Current Transformer Theory of Operation.

In the typical current transformer application, the primary winding consists

of one to a few turns of wire. The primary wire size is much larger than the

secondary wire size. The number of secondary winding turns is a selected

multiple of the primary turns. Figure 1 gives a circuit schematic of a current

transformer application. The current transformer shown represents an ideal

transformer. The ideal transformer has infinite no-load input impedance,

100% magnetic coupling between transformer windings ( hence no leakage

inductance), zero winding resistance, zero core losses, and no capacitance. (

Capacitance, leakage inductance, winding resistance, and core losses are

considered to be parasitic components. ) The output voltage is exactly

proportional to the primary voltage times the turns' ratio. There is no

regulation drop. There are no losses. Since there are no parasitic components

the ideal current transformer is 100% accurate. The conservation of energy

requires that the output power equals the input power, hence Vp x Ip must

equal Vs x Is. Since Vs = Vp x Ns / Np, it can be shown that Is = Ip x Np /

Ns. Is = Vs / RL, hence Ip = Ns x Vs / ( RL x Np ). With an ideal current

transformer there is no phase shift ( except 180 degrees depending on the

choice of output connections ).

The ideal transformer’s secondary resistive load consumes power equal to Is

x Is x RL. This same amount of power must be consumed at the primary

terminals. The secondary load RL can be replaced ( commonly referred to as

“reflected” ) with a resistor across the primary terminals, RLr. By applying

the conservation of energy, one can show that RLr equals Np x Np x RL /

(Ns x Ns), OR RLr equals RL times the turns ratio squared (where turns

ratio = Np / Ns). If Np / Ns is small, then the RLr is very small. The primary

voltage drop is Ip x RLr. A very small value for RLr means that the current

transformer presents a low insertion loss to the primary current and a low

primary voltage drop.

The reflected load impedance acts in parallel to the transformers own input

impedance. The ideal current transformer has infinite input impedance. This

infinite impedance would correlate to an infinite inductance inserted in

series into the path of the primary conductor. Without the load (or burden)

the current transformer acts like an inductor and would completely block the

primary current flow. Any constant value of alternating current would, in

theory, produce an infinite primary voltage drop. In reality the current

transformer’s input inductance (hence also impedance) cannot be infinity.

The current transformer has an inductance value which acts in parallel to the

reflected load. The core has losses that can be represented as a resistor in

parallel with the reflected load and the transformer’s self-inductance (no

load inductance). Without the load resistor the inductance and core loss will

place an upper limit on the primary voltage, but this voltage could still be

substantial. Core saturation is also a possibility. A turns ratio step-up would

result in even higher secondary voltage. Any circuitry beyond the secondary

load resistor could be subjected to high voltage, possibly resulting in circuit

damage. Because of this potential high voltage, the load resistor should

never be removed from the secondary when the current transformer is being

powered. Figure 2A shows an equivalent circuit schematic for a current

transformer with load RL. The ideal (induced) secondary voltage is now

denoted as Vsi and Vs now denotes the voltage at the secondary terminals.

Notice that the schematic contains the ideal current transformer and load as

before plus transformer mutual inductance Lm, secondary winding

resistance Rs, core loss resistor Rc, secondary leakage inductance Lks, and

primary leakage inductance Lkp. Just like for the load resistor, the other

secondary circuit components can be reflected to the primary side of the

transformer. This is illustrated in Figure 2C.

The parasitic components, Rs, Lkp, and Lks, all act to lower the output

voltage across RL, hence the output voltage, Vout, will not equal the

induced secondary voltage Vsi. Rs and Lks act in series with RL and are

reflected to the primary side along with Rs. Their presence presents added

impedance to the primary current hence an increase in primary voltage in

proportion to the impedance. Consequently, RL still has the same voltage

drop and current flow as it did without Lks and Rs even though Vs does not

equal Vout. The phase shift associated with Lks will cause some slight

deviation from the ideal current ratio (which equals the turns ratio).

The current transformer’s self (no-load) inductance Lm and the core loss Rc

shunt current away from the reflected load and reflected parasitic

components. Their impedances act in parallel to the reflected impedances,

consequently lowering the impedance seen by the primary current and the

resulting primary voltage. Less primary voltage means less output voltage

and less secondary current. Consequently Lm and Rc also cause deviation

from the ideal current ratio.

As long as Rc, Lm, Lkp, Lks, and Rs are constant in value, The actual

current ratio will be some fixed ratio times the ideal (or desired) current

ratio. One can compensate for the deviation from the desired current ratio by

appropriate choice of secondary turns. The number of turns will be a little

lower than that for the associated ideal turns ratio. For constant values

accuracy could be 100% except for any turn resolution limitations (full turns

versus fractional turns).

Accuracy concerns arise from non-constant values for Rc, Lm, and to a

lesser degree from Lkp and Lks. These values usually vary with core

induction levels; hence they vary over the range of primary current being

measured. (Air core transformers are stable but magnetic coupling is

relatively poor hence relatively large leakage inductances.) Since Rc and Lm

impedances act in parallel to the reflected load, higher Rc and Lm values

have a smaller effect and consequently increase accuracy. Cores materials

with high permeability and low core loss are preferred for high accuracy

applications.

At higher frequencies winding capacitance becomes a concern. Figure 3

gives an equivalent circuit schematic, which includes winding capacitance.

Leakage inductance and winding capacitance are actually distributed

components, but are shown as lumped approximate equivalent components.

Like Lm, winding capacitances shunt current around the reflected load. The

inductances and capacitances can interact and consequently may produce

spurious oscillations. It is also possible to develop “parallel resonance”.

High frequency coil designs seek to minimize winding capacitances.

If you need assistance with your current transformer design, please contact

Butler Winding and ask for Engineering.

Toroidal Current Transformers

Like other types of current transformers, the toroidal current transformer

measures alternating current flowing through a conductor. Since they are

used to measure current, current transformers are often classified as a type of

instrument transformer. One way of distinguishing types of current

transformers is by the type of cores used in their construction. The term

“toroidal” refers to the shape of the core that the winding of a toroidal

current transformer is wound on. The core is circular. Its cross-section may

be rectangular or round. The round cross-section gives better electrical

performance. The cores are often called “ring” cores. In contrast, the term

“split-core” in split-core current transformers is used because the

transformer core is split into two pieces which allow it to be assembled and

disassembled around a buss bar without disconnecting either end of the buss

bar. It is possible to make a split-core toroidal current transformer.

Historically, it has been impractical to do so, but there are now some flexible

toroids, which permit the “split-core” feature of installing it around a buss

bar. They have limited application.

Toroidal current transformers give better electrical performance than other

types of current transformers. Their shape minimizes the magnetic path

length, minimizes the winding turn length, produces less stray magnetic flux,

and optimizes magnetic coupling, and minimizes leakage inductance.

The toroidal current transformer is the most common way to measuring large

amounts of alternating (or even pulsing) current. It is preferred over the

measurement of the voltage drop across a known resistor and over split-core

transformers. The resistor is usually impractical for high current

applications. The toroidal current transformer can accurately measure the

alternating current and put out a reasonable voltage, which is proportional to

the current. The toroidal current transformer does so with very little insertion

loss, while an appropriate resistor would produce lots of heat and

consequently produce considerable insertion loss.

Like other current transformers, the toroidal current transformer also

provides voltage isolation between the conductor and the measuring

circuitry. Measurement over a resistor does not.

Proper function of the toroidal current transformer requires use of a load

resistor. The load resistor is often referred to as a “burden resistor”. Presence

of the load resistor enables a current transformer to perform its function with

little insertion loss. Without the load resistor the core could saturate and no

longer have the desired current ratio, or the no-load inductance could limit

primary current flow. Core materials with high permeability and low core

losses give better electrical performance. Further explanation and theory

about the operation of current transformers is given further below.

Current transformers, including the toroidal current transformer, may have

multiple windings. The typical toroidal current transformers have only one

winding. This winding is usually a “high turns” winding which functions as

the secondary winding. In application, the toroidal current transformer is

slipped over an end of a high current wire or buss bar, which conducts the

primary current. Said wire or buss bar constitutes a one turn primary

winding.

Butler Winding can make (and has made) toroidal current transformers in a

wide variety of sizes and in a variety of core materials. Our upper limits are


windings, litz wire windings, and some limited perfect layering. Butler

Winding can (and has done) sector winding, progressive winding, bank


toroid winding machines. That includes toroid-taping machines. Butler

winding has vacuum chamber(s) for vacuum impregnation and can also

encapsulate. To ensure quality, Butler Winding purchased two




Current Transformer Design Specifications

The designer must either determine or be supplied with the information

needed to design the current transformer. The needed information is listed

below along with a brief description if needed. Add any additional items

required by your particular application.

Describe Primary Current – State maximum current value and type of

measurement (r.m.s. average, peak, etc.), Give type of waveform (sine wave,

square wave, triangular, etc.). State either continuous current or describe the

applicable duty cycle.

Give Number of Primary Turns – This is the number of times the primary

conductor (buss bar) passes through the core window.

The Desired Current Ratio – This is simply the desired secondary current

value (at a specified value of primary current) divided by the primary current

value that generates said value of secondary current. Alternatively, a turns

ratio could be specified. but don’t expect the current ratio to exactly equal

the turns ratio.

Define the Output Burden ( Load Resistor ) – Specify the value and type

of the intended secondary load. The type of load is usually resistive ( a

resistor ), but could be inductive or capacitive ( which complicates things ).

Alternatively, the desired output voltage per unit of primary current can be

specified. The value of the load resistor can then be calculated.

Required Accuracy – This is usually expressed as either a maximum

percentage or maximum absolute change over the entire primary current

range. It includes both measurement tolerances and variations over the

operating range(s). It may be expressed over a portion of the operating range

or at specific operating points.

Minimum Inside Window Dimensions – This is the primary conductor (

buss bar ) dimensions plus any additional distance needed to clear any

obstacles encountered during installation of the current transformer..

Dimensional Constraints – Overall width, length, thickness.

Termination – Describe how you want the secondary terminated. Some

possible examples are: terminal block, lead wires ( with or without terminal

lugs), or header ( with p.c.b. pins or pads ). If leads, what length, insulation

type, voltage rating, etc..

Mounting - Describe how you expect it to be mounted. Will it be supported

by the primary conductor ( hang on the buss bar ), or will the current

transformer support the primary conductor.

Voltage Isolation Requirements – In many applications, the current

transformer’s secondary winding rests on the primary conductor ( buss bar ),

hence it must be adequately insulated according the expected conductor

voltage potential and/or the required equipment voltage classification for the

intended application.

Corona Requirements, if applicable – Give test criteria: maximum test

voltage, minimum voltage ramping time, minimum voltage inception value,

minimum voltage extinguish value.

Maximum Temperatures – Specify the maximum ambient temperature and

the maximum expected temperature of the primary ( buss bar ) conductor. If

applicable, state the maximum allowed temperature rise.

Application Standards -- Application standards may exclude use of some

materials and require use of some materials . Some examples of such

standards are minimum temperature ratings ( regardless if actual is less ),

flame retardancy, vibration, out-gassing, and required labeling.

Environmental Restrictions – Examples are: poor cooling due to confined

space, corrosive environment, water spray, ultra-violet light, and vibration.











considered to be parasitic components. ) The output voltage is exactly

proportional to the primary voltage times the turns' ratio. There is no

regulation drop. There are no losses. Since there are no parasitic components

the ideal current transformer is 100% accurate. The conservation of energy

requires that the output power equals the input power, hence Vp x Ip must

equal Vs x Is. Since Vs = Vp x Ns / Np, it can be shown that Is = Ip x Np /

Ns. Is = Vs / RL, hence Ip = Ns x Vs / ( RL x Np ). With an ideal current

transformer there is no phase shift ( except 180 degrees depending on the

choice of output connections ).





the conservation of energy, one can show that RLr equals Np x Np x RL / (

Ns x Ns ), OR RLr equals RL times the turns ratio squared ( where turns

ratio = Np / Ns ). If Np / Ns is small, then the RLr is very small. The primary







series into the path of the primary conductor. Without the load (or burden)




transformer’s input inductance (hence also impedance) cannot be infinity.


reflected load. The core has losses, which can be represented as a resistor in

parallel with the reflected load and the transformer’s self-inductance (no

load inductance). Without the load resistor the inductance and core loss will

place an upper limit on the primary voltage, but this voltage could still be

substantial. Core saturation is also a possibility. A turns ratio step-up would

result in even higher secondary voltage. Any circuitry beyond the secondary

load resistor could be subjected to high voltage, possibly resulting in circuit

damage. Because of this potential high voltage, the load resistor should

never be removed from the secondary when the current transformer is being

powered. Figure 2A shows an equivalent circuit schematic for a current

transformer with load RL.

The ideal (induced) secondary voltage is now denoted as Vsi and Vs now

denotes the voltage at the secondary terminals. Notice that the schematic

contains the ideal current transformer and load as before plus transformer

mutual inductance Lm, secondary winding resistance Rs, core loss resistor

Rc, secondary leakage inductance Lks, and primary leakage inductance Lkp.

Just like for the load resistor, the other secondary circuit components can be

reflected to the primary side of the transformer. This is illustrated in Figure

2C.









deviation from the ideal current ratio (which equals the turns ratio).

The current transformer’s self (no-load) inductance Lm and the core loss Rc

shunt current away from the reflected load and reflected parasitic







current ratio will be some fixed ratio times the ideal (or desired) current




accuracy could be 100% except for any turn resolution limitations (full turns

versus fractional turns).



induction levels; hence they vary over the range of primary current being

measured. (Air core transformers are stable but magnetic coupling is

relatively poor hence relatively large leakage inductances.) Since Rc and Lm

impedances act in parallel to the reflected load, higher Rc and Lm values



applications.







spurious oscillations. It is also possible to develop “parallel resonance”.




Split Core Current Transformers

What is a split-core current transformer? More specifically how does a split-

core current transformer differ from the typical current transformer? Just like

the typical current transformer, the split-core current transformer measures

alternating current flowing through a conductor. The distinguishing feature

of the split core current transformers is that their design permits them to be

assembled around a buss bar without disconnecting the buss bar. The typical

current transformer is usually a toroidal coil, which is slipped over the end

of a buss bar, hence requires disconnecting the buss bar. "C" - cores and "U"

core structures are commonly used for split-core current transformers

because they are relatively easy to take apart and put back together around

the buss bar. Some sort of bracketry or band clamps and holds the assembled

pieces of the split-core current transformer together.

Historically, this has not been as practical ( but is possible ) for toroidal

coils. The bracketry is more complicated. Typically, the coil(s) must be

sector wound on the toroid before cutting the core in half, whereas the “U”

and “C” core structure of the typical split-core current transformer permit

use of bobbin wound coils which can be wound independently of the core.

There are now some flexible toroids, which permit the “split-core” feature of

installing it around a buss bar.

The electrical performance of split-core current transformers is not as good

as that of the continuous toroidal coil. The “circle” like ( or “ring” like )

shape of the toroid usually offers a shorter magnetic path length than other

cores. Since the toroids are continuous, they do not add any air gap to the

core structure. Split-core current transformers ( including toroidal split-

cores ) add some air gap to the core structure. Consequently, the split-core

current transformers will draw more magnetizing ( exciting ) current than a

continuous toroidal current transformer made of the same core material (

assuming comparable size and/or weight. ).

The toroidal shape provides better magnetic coupling and less leakage

inductance than the “C” and “U” core structures commonly used in split-

core current transformers.

Split-core current transformers for lower frequency applications ( power

frequencies ) typically use grain oriented silicon steel or nickel alloys for the

core material. There are some more exotic materials available. The material

is cut into strips and then wound on an arbor ( mandrel ) to form a core. The

core is then cut in half. These are known as “tape-wound” cores because

their construction resembles a roll of tape. Strip thickness varies from 0.025”

down to 0.0005”. The thinner strips have less core loss at higher frequencies

hence they are used in higher frequency applications up to about 10

kilohertz. High accuracy current transformers require low core losses hence

they either utilize the thinner strip thickness, the lower core loss materials

such as the nickel alloys, or both. Ferrite materials are usually used for very

high frequency designs, up to several megahertz. Some very specialized

applications may require a core-less ( air-core ) coil. Some theory of current

transformer operation is given further below

Butler Winding can make ( and has made ) split-core current transformers in

a variety of shapes and sizes. The "U" and "C" cores structures are the most

typical, but Butler Winding is capable of producing a variety of other custom

designs. Butler Winding already works with various standard types of "core

with bobbin" structures ( E, EP, EFD, PQ, POT, and others ), and does some

custom bobbin wound designs. Usually, we can readily adapt our bobbin

winding equipment to wind the split-core current transformer coils you need.

Our upper limits are 40 pounds of weight and 2 kilowatts of power.

We have experience with foil windings, litz wire windings, and perfect

layering. For toroids, we can ( and have done ) sector winding, progressive

winding, bank winding, and progressive bank winding. Butler winding has a

variety of winding machines, bobbin/tube and toroid. That includes two

programmable automated machines and a taping machine for toroids. Butler

winding has vacuum chamber(s) for vacuum impregnation and can also



tested on these machines. For more information on Butler Winding's

capabilities, click on our "capabilities" link.

Current Transformer Design Specifications

The designer must either determine or be supplied with the information

needed to design the current transformer. The needed information is listed

below along with a brief description if needed. Add any additional items

required by your particular application.

Describe Primary Current – State maximum current value and type of

measurement ( r.m.s., average, peak, etc. ), Give type of waveform ( sine

wave, square wave, triangular, etc. ). State either continuous current or

describe the applicable duty cycle.

Give Number of Primary Turns – This is the number of times the primary

conductor ( buss bar ) passes through the core window.

The Desired Current Ratio – This is simply the desired secondary current

value ( at a specified value of primary current ) divided by the primary

current value that generates said value of secondary current. Alternatively, a

turns ratio could be specified. but don’t expect the current ratio to exactly

equal the turns ratio.

Define the Output Burden ( Load Resistor ) – Specify the value and type

of the intended secondary load. The type of load is usually resistive ( a

resistor ), but could be inductive or capacitive ( which complicates things ).

Alternatively, the desired output voltage per unit of primary current can be

specified. The value of the load resistor can then be calculated.

Required Accuracy – This is usually expressed as either a maximum

percentage or maximum absolute change over the entire primary current

range. It includes both measurement tolerances and variations over the

operating range(s). It may be expressed over a portion of the operating range

or at specific operating points.

Minimum Inside Window Dimensions – This is the primary conductor (

buss bar ) dimensions plus any additional distance needed to clear any

obstacles encountered during installation of the current transformer..

Dimensional Constraints – Overall width, length, thickness.

Termination – Describe how you want the secondary terminated. Some

possible examples are: terminal block, lead wires ( with or without terminal

lugs), or header

( with p.c.b. pins or pads ). If leads, what length, insulation type, voltage

rating, etc..

Mounting -- Describe how you expect it to be mounted. Will it be supported

by the primary conductor ( hang on the buss bar ), or will the current

transformer support the primary conductor.

Voltage Isolation Requirements – In many applications, the current

transformer’s secondary winding rests on the primary conductor ( buss bar ),

hence it must be adequately insulated according the expected conductor

voltage potential and/or the required equipment voltage classification for the

intended application.

Corona Requirements, if applicable – Give test criteria: maximum test

voltage, minimum voltage ramping time, minimum voltage inception value,

minimum voltage extinguish value.

Maximum Temperatures – Specify the maximum ambient temperature and

the maximum expected temperature of the primary ( buss bar ) conductor. If

applicable, state the maximum allowed temperature rise.

Application Standards -- Application standards may exclude use of some

materials and require use of some materials . Some examples of such

standards are minimum temperature ratings ( regardless if actual is less ),

flame retardancy, vibration, out-gassing, and required labeling.

Environmental Restrictions – Examples are: poor cooling due to confined

space, corrosive environment, water spray, ultra-violet light, and vibration.











considered to be parasitic components. )

The output voltage is exactly proportional to the primary voltage times the

turns' ratio. There is no regulation drop. There are no losses. Since there are

no parasitic components the ideal current transformer is 100% accurate. The

conservation of energy requires that the output power equals the input

power, hence Vp x Ip must equal Vs x Is. Since Vs = Vp x Ns / Np, it can be

shown that Is = Ip x Np / Ns. Is = Vs / RL, hence Ip = Ns x Vs / ( RL x Np ).

With an ideal current transformer there is no phase shift ( except 180 degrees

depending on the choice of output connections ).





the conservation of energy, one can show that RLr equals Np x Np x RL / (

Ns x Ns ), OR RLr equals RL times the turns ratio squared ( where turns

ratio = Np / Ns ). If Np / Ns is small, then the RLr is very small. The primary







series into the path of the primary conductor. Without the load ( or burden )




transformer’s input inductance ( hence also impedance ) cannot be infinity.


reflected load.

The core has losses which can be represented as a resistor in parallel with

the reflected load and the transformer’s self inductance ( no load inductance

). Without the load resistor the inductance and core loss will place an upper

limit on the primary voltage, but this voltage could still be substantial. Core

saturation is also a possibility. A turns ratio step-up would result in even

higher secondary voltage. Any circuitry beyond the secondary load resistor

could be subjected to high voltage, possibly resulting in circuit damage.

Because of this potential high voltage, the load resistor should never be

removed from the secondary when the current transformer is being powered.

Figure 2A shows an equivalent circuit schematic for a current transformer

with load RL.

The ideal ( induced ) secondary voltage is now denoted as Vsi and Vs now

denotes the voltage at the secondary terminals. Notice that the schematic

contains the ideal current transformer and load as before plus transformer

mutual inductance Lm, secondary winding resistance Rs, core loss resistor

Rc, secondary leakage inductance Lks, and primary leakage inductance Lkp.

Just like for the load resistor, the other secondary circuit components can be

reflected to the primary side of the transformer. This is illustrated in Figure

2C.









deviation from the ideal current ratio ( equals the turns ratio ).

The current transformer’s self ( no-load ) inductance Lm and the core loss

Rc shunt current away from the reflected load and reflected parasitic







current ratio will be some fixed ratio times the ideal ( or desired ) current




accuracy could be 100% except for any turn resolution limitations ( full

turns versus fractional turns ).



induction levels, hence they vary over the range of primary current being

measured. ( Air core transformers are stable but magnetic coupling is

relatively poor hence relatively large leakage inductances. ) Since Rc and

Lm impedances act in parallel to the reflected load, higher Rc and Lm values



applications.







spurious oscillations. it is also possible to develop “parallel resonance”.




Surface Mount Electronic Transformer

Transformers (and inductors) can be classified in several ways: by power

rating, by type of application, by type of construction, by industry, and

others. “Surface mount electronic transformers” refer to a type of

construction that permits attachment of surface mount transformers to a

printed circuit board (PCB). Historically, transformers and other circuit

devices have been mounted on PCBs using “pin-thru” technology.

Transformer wires are terminated to pin type terminals. Holes are drilled in

the PCB’s copper circuitry to accommodate the transformer pins. The

transformer pins are inserted through these holes and then soldered to the

copper circuitry. Engineers have developed solder pastes, adhesives, and

assembly processes that permit attaching transformer terminals to PCBs

without using holes. Flat areas (known as pads) on the transformer terminals

are soldered directly to copper circuitry surfaces hence the term surface

mount transformer. This process eliminates the need to drill holes for the

pins, thereby reducing the cost to manufacture a PCB.

Surface mount electronic transformers (and inductors) are usually wound on

surface mount bobbins, but are also available as toroidal coils. The toroidal

coil is mounted on a “header” equipped with surface mount terminals. The

bobbins (or headers), used with surface mount transformers, come in a

variety of materials: plastics, phenolic, glass, Teflon and others. Most of

these are molded. Some are fabricated. Some bobbins and headers are “self

leading”. The winding wire is also used to form the surface mount terminal

by looping the wire under a pre-formed flat edge thereby forming a

reasonably flat terminal area.

Surface mount electronic transformers (and inductors) are available in a

variety of shapes. Surface mount electronic transformers shapes include pot

cores (round), “RM” (square pot cores), “EP”, “E”, “EI”, “EEM”, “EFD”,

“U”, “UI”, “ER”, and some others including custom shapes. Surface mount

transformers in these shapes are usually only available in the smaller sizes.

Designers are adapting more shapes and larger sizes to surface mount

transformer applications. Designers have mechanical concerns about the

larger sizes. The weight of the larger sizes may exceed the weight that

soldered surface mount pads can safely handle under vibration. Over time,

designers hope to develop surface mount transformers (and inductors) in

larger sizes.

Like other electronic transformers, surface mount electronic transformers

(and inductors) can use a variety of core materials: laminated or taped

wound silicon steel alloys, nickel-iron alloys, cobalt alloys; powdered irons

and nickels; ferrite; air core; and/or core materials processed for square loop

or round loop properties; and others.

Butler Winding can make (and has made) surface mount electronic

transformers (and inductors) in a wide variety of materials and sizes. Butler

Winding can also do a variety of custom transformers. Butler Winding’s

upper limits are 40 pounds of weight and 2 kilowatts of power. We have

experience with foil windings, litz wire windings, and perfect layering. For

toroids, we can (and have done) sector winding, progressive winding, bank


winding machines, bobbin/tube and toroid. That includes two programmable

automated machines and a taping machine for toroids. Butler Winding has







general







(applications) of electronic transformers (and inductors). Click on one of the

available links.



Electronic Transformers | Bobbin Wound

Electronic transformers can be classified in several ways: by power rating,

by type of application, by type of construction, by industry, and others.

Bobbin wound electronic transformers refers to a type (or method) of

construction. Toroidal coils are wound directly onto a toroidal core. The core

may be coated or boxed to insulate it form the coil windings. In contrast,

bobbin wound electronic transformer coils are wound independently of the

core. The coil must hold its shape (or form) until the coil is assembled onto

the transformer core. One common method of doing this is to wind the coil

onto a bobbin (also referred to as a spool), hence the term “bobbin wound

transformer”. The bobbin is a pre-formed reasonably rigid part. The bobbin

material is usually (but not always) an insulating material, hence it can

provide electrical isolation between the coil and the adjoining core material

provided suitable creepage distance is used. Multi-section bobbins are

available to provide increased electrical isolation between coil windings.

Bobbin wound electronic transformers are used in a variety of applications,

hence bobbins are made from a variety of materials: plastics, phenolic, glass,

Teflon and others. Most bobbins are molded. Some are fabricated. Bobbin

designs for bobbin wound transformers often provide terminals, pins, and/or

surface mount pads to ease wire termination and to facilitate printed circuit

board mounting.

Bobbin wound transformers (and inductors) are available in a variety of

shapes. Bobbin wound transformers shapes include pot cores (round), “RM”

(square pot cores), “RS” (round slab pot cores) and “DS” (double slab pot

cores), “EP”, “PQ”, “E”, “EI”, “EEM”, “EFD”, “U”, “UI”, “EC”, “ETD”,

“ER”, “EER”, and some others including custom shapes. Bobbin wound

transformers in these shapes are available in several different sizes.

Bobbin wound electronic transformers (and inductors) can also use a variety

of core materials: laminated or taped wound silicon steel alloys, nickel-iron

alloys, cobalt alloys; powdered irons and nickels; ferrite; air core; core

materials processed for square loop or round loop properties; and others.

Butler Winding can make (and has made) bobbin wound transformers (and

inductors) in a wide variety of materials and sizes with pin-thru, surface

mount, and/or flying leads terminations. Butler Winding also does “tube

wound” transformers (and inductors) and air core coils. Our upper limits are





machines, bobbin/tube and toroid.








general?







(applications) of transformers (and inductors). Click on one of the available

links.



Mag-Amp Magnetic Amplifiers

Magnetic amplifiers, also called mag amps for short, provide an electro-

magnetic method of amplification. Mag amps were quite common prior to

the development of solid state transistors. As advances in semiconductor

technology progressed, magnetic amplifiers because a relatively expensive

component. Consequently the use of mag amps declined. A properly made

mag amp is highly reliable, hence they are still used in some applications

with demand the reliability performance criteria that a mag amp can meet.

Another feature of mag amps is the high isolation voltages that can be

achieved between windings with proper design. Mag amps may still be

preferred over semiconductor devices in safety critical applications.

A typical simple mag amp contains two identical coils, each having identical

high permeability square loop magnetic cores and each wound with an

identical winding not shared with the other coil. An alternating voltage

source is connected to one end of these windings and a load is connected to

the other end. The windings are either connected in series or in parallel such

that the cores’ magnetic flux generated by the alternating voltage are out of

phase (in opposite directions).

Alternating current (A.C.) will flow through these windings. Either a shared

second winding is wound on both coils or each coil is wound with a second

identical winding. In the latter case the windings are series connected such

that a direct current (D.C.) flowing through these windings generate

magnetic flux in the cores, which are in phase (in the same direction). These

windings are connected to a variable D.C. current source (which might

consist of series connected D.C. voltage source and a variable resistor).

The D.C. winding(s) is (are) referred to as the control winding(s).

Schematic representations of two typical mag amps are given in Figures 1

and 2 further below. The mag amps shown may also be referred to in

literature as a type of saturable reactor. A mag amp may also be referred to

in literature as a type of transductor.

Air gaps within a mag amp’s core structure are detrimental to mag amp

performance. Proper mag amp performance requires nearly identical

symmetry in core flux excursions; hence leakage flux should be minimized.

Toroidal cores have essentially zero air gaps and the toroidal geometry

maximizes magnetic coupling and minimizes leakage flux. Consequently,

toroids are the core shape of choice.

Other variations of mag amps exist, including a single core version that has

three core legs. The middle leg has a D.C. control winding. The outer legs

have identical A.C. windings. In theory D.C. flux generated in the center leg

divides equally and flows through both outer legs. The A.C. windings are

connected such that their phases do not permit any A.C. flux flow through

the center leg (in theory). There are practical difficulties (in the form of

magnetic tolerances) with this type of mag amp design.

More advanced mag amp circuits use rectifying elements to isolate the load

from the mag amp during core reset. Core reset refers to the volt-second

transition from saturation flux (top flat portion of the B-H loop) to the flux

value at the opposite side of the B-H loop (bottom flat portion of the loop).

Butler winding can make (and has made) mag amps. Butler winding has

several types of toroid winding machines that can be used to wind a variety

of mag amp core sizes. This includes toroid-taping machines. For toroids,

we can (and have done) sector winding, progressive winding, bank winding,

and progressive bank winding. Butler winding also has other types of

winding machines. That includes two programmable automated machines.

We can wind and assemble various standard types of “core with bobbin”

structures (E, EP, EFD, PQ, POT, U and others), and some custom designs.

Our upper limits are 40 pounds of weight and 2 kilowatts of power.

We have experience with foil windings, litz wire windings, and perfect

layering. Butler winding has vacuum chamber(s) for vacuum impregnation

and can also encapsulate. To ensure quality, Butler Winding purchased two




Mag Amp Theory

The following discussion is not intended to give a detailed understanding of

mag amp operation. It is not intended to describe all the variations of mag

amp designs or applications. It is intended to give a basic insight to how a

typical simple mag amp functions. Rectifier aided mag amp circuits are not

discussed. Butler Winding has some but limited experience with mag amps.

If you require more information than the following discussion supplies,

please contact Butler Winding and ask to speak to an engineer about mag

amps. Butler Winding will provide whatever help we reasonably can.

Refer to the schematic of Figure 1 bearing in mind (in theory) that the two

coils have identical windings and identical cores. Because of transformer

action, the A.C. voltage impressed across the mag amp’s A.C. windings will

induce a voltage across each control winding. Because of the opposite

phasing of the A.C. windings, the induced voltages in the D.C. windings will

buck each other and exactly cancel each other (in theory) resulting in zero

A.C. voltage induced across the D.C. source. Consequently, low impedance

D.C. source will not load down the A.C. windings.

Consider the impedance of the A.C. windings with no D.C. current

supplied. The core and windings are designed such that; 1) the core does not

saturate at the maximum intended A.C. voltage, and 2) each A.C. winding

has a relatively much higher impedance than the intended load. Because of

the high impedance, very little A.C. current flows. Consequently, there is

very little voltage drop across the load.

Now consider the impedance of the A.C. windings with a D.C. current

flowing through the control winding. Both cores have a D.C. biasing flux of

equal value and the same phasing. The A.C. windings of Figure 1 are

connected in parallel but with opposite phasing. The total flux in a core is

the sum of the D.C. flux and the A.C. flux. Because of the opposite A.C.

winding phasing, the A.C. voltage increases the core flux of one core while

decreasing the core flux of the other core until saturation occurs. Eventually

the alternating fashion of the A.C. voltage causes the changing flux to

reverse the direction of flux change of both cores. Now apply enough D.C.

current to cause one core to enter saturation. The core’s flux reaches its

maximum values and does not change (ideal theory) while in saturation;

hence no induced voltage will oppose the applied A.C. voltage. The

impedance of that core’s A.C. winding drops to near zero value. There can

be very little voltage drop across that A.C. winding. The other A.C. winding

is connected in parallel to this A.C. winding. This A.C. winding shunts the

current around the other A.C. winding hence the other A.C. winding also

sees very little voltage impressed across it.

Consequently the flux of the other core changes very little (essentially stays

where it is). While a core is saturated there is very little impedance between

the A.C. voltage source and the load impedance. Consequently significant

load current flows during saturation and produces a relatively large voltage

drop across the load. Because of the eventual A.C. voltage reversal, the

saturated core will eventually come out of saturation, high A.C. winding

impedance will occur again, and the load current will again drop to near zero

value.

Eventually the other core saturates resulting in high load current until the

core leaves saturation. The mag amp has seen a complete A.C. cycle and

will proceed to the next cycle. For mag amps, entering saturation is like

closing a switch. The time spent in saturation is the “turn-on” time of the

mag amp switch.

The amount of time spent in saturation is determined by the amount of D.C.

biasing current. A larger D.C. bias current causes the cores to enter

saturation earlier and exit saturation later, thereby increasing the length of

time current is delivered to the load, thereby increasing the average amount

of current delivered to the load in a given period of time. Once a steady

state condition is reached in an idealized mag amp, it can be shown that the

averaged ampere-turns of the load current are proportional to the ampere-

turns of the control current. With appropriate choices of turns ratio,

windings, and cores, one can achieve significant power amplification gain.

The schematic in Figure 2 shows the A.C. windings connected in series.

When one core saturates both of its winding have relatively very low

impedance and can be ignored. The core’s A.C. winding does not shunt the

other A.C. winding, but the other A.C. winding will not maintain its high

impedance level if the D.C. source has a sufficiently low impedance. With

one core saturated the low impedance D.C. source becomes a transformer-

coupled load to the unsaturated A.C. winding. The impedance on the

unsaturated A.C. winding drops to the transformer coupled reflected value of

the low impedance D.C. source. A load current flows which produces a

significant load voltage.

Electronic Transformer - Power Transformers

The most common purpose of a power electronic transformer is to convert

alternating current (A.C.) power from one A.C. voltage (or current) to

another A.C. voltage (or current). Another common purpose is to provide

electrical isolation between electrical circuits. Power is the product of

voltage times current. Power transformers do not change power levels except

for parasitic losses. Input power minus parasitic power losses equals output

power. Ideal power transformers have no losses, hence output power equals

input power. Increasing the output voltage will decrease the output current.

Electric utilities prefer to transmit electricity at low current values to reduce

resistive losses in the power transmission lines.

Lower currents also permit smaller size transmission cables. A power

transformer is used between the generating equipment and the power line(s)

to step-up (increase) the transmission voltage (to high voltage) and decrease

the transmission current. Distribution transformers, which are power

transformers, are used to step-down (decrease) the voltage to voltage levels

needed for industrial and household use. Limited discussion on the theory of

power transformer operation is given further below.

Power electronic transformers may be classified by their power ratings

(fractional VA to mega-VA), their type of construction, and/or by their

intended application. The same basic power transformer may be suitable for

multiple applications hence the same power transformer may be classified

under several overlapping category types. The common person associates

power transformers with the electric utilities, hence they think of pole

transformer and distribution transformers. The power transformers used

inside their appliances and electronic devices do not readily come to mind.

The two broadest categories of power transformers are the electric utility

power transformers and electronic power transformers (1 & 3 phase).

Utility transformers are almost entirely A.C. sinewave transformers. An

electronic power transformer is essentially any electronic transformer

supplying power to electronic circuits. There are many sub-categories: pulse,

inverting, switching (flyback, forward converter), toroidal, square wave,

isolation, and others. Instrument transformers (example current

transformers) are not considered to be power transformers. They measure

voltage or current instead of supplying power.

Electronic transformers / power transformers range in size from a cubic

centimeter to multiple cubic meters. The weight can range from a fraction of

an ounce to multiple tons. The size and weight of a power transformer is

dependent on several factors. A non-exhaustive list includes; desired power

rating, maximum ambient temperature, allowable temperature rise, cooling

method (air or liquid cooled, natural convection or forced), transformer

shape, voltage dielectric requirements, required voltage regulation, operating

frequency, operating waveform, and core material.

Of these, the two most limiting parameters are allowed temperature rise and

required voltage regulation. Operating frequency is a major parameter in

selecting core material. Low frequency applications usually utilize either

tape wound or laminated silicon steel cores. Moderate frequency

applications utilize tape wound or laminated nickel iron cores. High

frequency applications usually use ferrite cores.

Power transformers are produced in a variety of shapes. Toroidal power

transformers are the high performers. They offer the smallest size (by

volume and weight), less leakage inductance, and lower electromagnetic

interference (EMI). Their windings cool better because of the proportionally

larger surface area. Bobbin or tube wound transformers are usually more

economical to build. Long thin cores are more suitable for low frequency

high “Q” transformers. Some shapes, pot cores for example, are self

shielding (reduces EMI).

Butler Winding can make (and has made) electronic transformers and power

transformers in a wide variety of shapes and sizes. This includes; various







machines, bobbin/tube and toroid.







Power Transformers – Overview of Operating Theory

Power transformer design involves many interdependent parameters. It

becomes very difficult to optimize a power transformer design. Most power

transformer designers use an electrical model that allows them to

approximate a transformer design. The preliminary approximate design will

be evaluated, then adjusted as needed to achieve desired objectives. An

electrical model is given further below.

The Ideal Transformer

To better understand power transformers one should become familiar with

the concept of and ideal transformer. An ideal transformer has no parasitic

losses (no core loss, no winding resistance, and no leakage inductance).

Ideal transformers are 100% efficient. An ideal transformer has infinite input

impedance hence the ideal transformer does not draw any current for itself.

Primary current equals zero. Figure 1A shows the schematic on an ideal

transformer with primary turns Np and secondary turns Ns.

In the ideal (and the typical) electronic transformer, the primary and

secondary windings share the same core and see the same amount of

magnetic flux. Due to the applied alternating voltage, the magnetic flux is

repeatedly changing value and the direction (polarity) of “flux change” is

repeatedly reversing its direction. This change in flux induces a voltage in

each of the transformer winding turns equal to the primary voltage, Vp,

divided by the number of primary turns, Np.

The total induced primary voltage equals and opposes the applied primary

voltage. The induced primary voltage limits the flow of primary current. In

the ideal transformer the current value is zero. In non-ideal transformers this

current is greater than zero. This current is known as the magnetizing or

exciting current. The induced secondary voltage, Vs equals the number of

secondary turns times the induced voltage per turn. or equivalently, Vs = Ns

x Vp / Np.

Figure 1B shows the schematic of the ideal transformer with a resistive load

placed across its secondary terminals. Since there are no transformer losses,

power in equals power out. The induced secondary voltage, Vs causes

current, Is, to flow through the resistive load and secondary winding. The

direction of current Is acts to lower the induced primary voltage which

opposes the applied input primary voltage. Consequently more primary

current flows. The value of the primary current increases until it causes the

opposing induced primary voltage to equal the applied input primary

voltage. Conservation of energy requires that power out to equal power in

hence Ip x Vp = Vs x Is, or Ip = Vs x Is / Vp. Since Vs = Ns x Vp / Np, Ip

can be rewritten as

Ip = ( Ns x Vp / Np ) x Is / Vp, or equivalently, Ip = Ns x Is / Np, or Ns x Is

= Np x Ip. In an ideal transformer, Ip is the secondary winding’s load current

reflected (transformed) to the primary winding. The effective primary

impedance, Zp = Vp / Ip. It can be shown that Zp = Np x Np x Zs / ( Ns x Ns

), where Zs = the secondary load impedance. This equation also holds for

inductive and/or capacitive loads.

The Non-Ideal Transformer

Figure 2 shows an equivalent circuit schematic (electrical model) of a non-

ideal power transformer. Leakage inductance and winding capacitance are

actually distributed circuit elements. The schematic represents leakage

inductance and capacitance as “lumped” circuit components. In effect, the

distributed elements are transformer coupled into equivalent collective lump

sum values. Bear in mind that the “lumped” values will only approximate

real life conditions. At sufficiently low frequencies, the impedance of the

capacitors become sufficiently high to permit ignoring their effect. The

capacitors can be removed for low frequency designs.

The voltage drop Vm across the mutual inductance, Lm, represents the

induced primary voltage. Voltage drops occur over parasitic components Rp

and Lkp when current flows through them. Consequently the induced

primary voltage, Vm, is less than the voltage Vp applied to the primary

terminals. The secondary induced voltage, Vsi, becomes less than that of an

ideal transformer. In similar fashion, voltage drops occur over parasitic

components Rs and Lks when current flows through them. The secondary

terminal voltage, Vs, becomes less than the secondary induced voltage, Vsi.

These voltage drops are known as regulation drops. The decline in

secondary output voltage from its no load voltage with increasing load

current is known as transformer regulation. Percent voltage regulation equals

100% x ( no load Vs – full load Vs ) / full load Vs.

Magnetizing Current and Saturation

Transformer designs must avoid core saturation. Saturation occurs when the

applied ampere turns (Np x Im in Figure 2) generates more magnetic flux

than the core can handle. The reflected secondary load current, Irs in figure

2, does not contribute to saturation. Nor does Icp or Irc. The magnetizing

current, Im, must be held below the value where Np x Im causes saturation.

Np x Im is also known as the magnetizing force.

Saturation can be avoided by applying the following formulae V = 4 x F x

Bm x N x Ac x Sf x f where; V = r.m.s voltage in volts, F = form factor for

the voltage waveform (unitless), Bm = maximum allowed flux density in

Telsa, N = the number of turns, Ac = the core’s cross sectional area seen be

the winding in square meters, Sf is the stacking factor of the core (unitless

ratio < or = to 1), and f = the operating frequency in hertz.

The value of Bm depends on the saturation valve of the particular type of

core material that will be used, and on the maximum heat the core can be

permitted to generate. The latter is dependent on operating frequency. The

theory of saturation is not discussed on this particular web page, but there is

some discussion within the “pulse transformer” web page included with this

web site. Click on the available link.

Bipolar Operation

The cores in A.C. power transformers are usually operated in bipolar mode,

but could be operated in unipolar mode by using a D.C. biasing current

through a transformer winding. Bipolar and unipolar operation is not

discussed on this particular web page, but there is some discussion within

the “push pull transformer”, “inverter transformer” and “pulse transformers”

web page included with this web site. Click on the available links.

Need More Technical Information about Transformers in general?

More information about various types of transformers is available on this

web site. Check out the available list of links that can connect you other web

pages within this web site.



Power Inductors, Chokes and Reactors

Power inductors can be classified in several ways: by inductance value, by

power or current rating, by type of application, by type of construction, by

industry, by material and others. Choke and reactor are other names for

power inductors. Inductors inhibit the flow of electrical current in A.C. or

transient applications. Inductors are used in some A.C. circuits to reduce the

voltage reaching the intended load. Inductors may be used to limit the

amount of A.C. current flow. Since an inductor’s impedance increases with

frequency, they are good for blocking (suppression) of high frequency

electrical noise. Inductors are frequently used for electrical/electronic

filtering purposes.

You can find inductors in tuning and most types of bandwidth filters.

Saturable inductors can be used in signaling circuits to create time delays.

Boost inductors, flyback inductors, and buck inductors are inductors used in

some switching power supplies. Inductors are also used in switching power

supplies to smooth out ripple voltage and ripple current.

Inductors store energy. Transformers are not intended to store energy (but

do store some). Coupled inductors are used in some multi-output switching

power supply designs to improve voltage regulation. In this case, the

inductor is also acting as a transformer because there is transformer coupling

occurring between the multiple outputs. In contrast, a flyback transformer is

technically an inductor. A coil winding is used to create a magnetic field

thereby storing energy in the field. The stored energy is then released to the

output. There is no direct (simultaneous) coupling of energy.

Types or inductor construction include bobbin wound, toroidal, air core (no

core), tube wound, foil wound, wound with litz wire, encapsulated (potted),

laminated, powdered core, and others. An Inductor’s core material is

heavily influenced by the application’s frequency range. Line frequency

applications usually use a laminated or tape wound silicon steel core stack.

Low frequency audio applications may use laminated nickel-iron core stack

or possibly powdered core materials. High frequency applications generally

use a ferrite material.

Inductors are available in a variety of shapes. Bobbin wound inductor

shapes include pot cores (round), “RM” (square pot cores), “RS” (round slab

pot cores) and “DS” (double slab pot cores), “EP”, “PQ”, “E”, “EI”, “EEM”,

“EFD”, “U”, “UI”, “EC”, “ETD”, “ER”, “EER”, and some others including

custom shapes. Bobbins often provide a convenient method of mounting;

pin-through, surface mount, or chassis mount. Toroids are well known.

Toroids are usually preferred when high efficiency and optimum

performance are desired. Tube based construction tends to be more

customized hence a variety of inductor shapes are possible.

Butler Winding can makes power inductors and custom transformers in a

wide variety of materials and sizes with pin-thru, surface mount, and/or

flying leads terminations. Butler Winding also does “tube wound” inductors

and air core coils. Our upper limits are 40 pounds of weight and 2 kilowatts

of power. We have experience with foil windings, litz

wire windings, and perfect layering. For toroids, we provide sector winding,

progressive winding, bank winding, and progressive bank winding. Butler

winding has a variety of winding machines, bobbin/tube and toroid. Butler

Winding has vacuum chamber(s) for vacuum impregnation and can also







Common Mode Choke

Common mode chokes are used because many electrical devices may be

connected to the same power lines (or power supply lines), substantial

electrical noise can exist on these lines. Switching mode power supplies can

generate a lot of high frequency noise which can travel over the power lines

and interfere with the operation of computers and other electronic devices

connected to the power lines. Electro-magnetic interference in the

environment can induce or couple electrical noise into the power lines.

Electrical noise which comes in one power line wire and returns to the noise

source through the other power line wire is differential noise. Electrical

noise which comes through one power line and returns to the noise source

through some type of ground path is common mode noise. The two types of

electrical noise are illustrated in Figure 2 further below.

Differential and common mode chokes (or inductors) are often placed

between electrical (or electronic) equipment and the power lines supplying

power to the electrical equipment. This is illustrated in Figure 1. The

differential choke shown could be replaced by two separate single winding

chokes or by one single winding choke in one line. The chokes reduce

electrical noise both entering and leaving a piece of electrical equipment. A

common mode choke (or filter) is used to reduce common mode (electrical)

noise. Figure 3 (further below) illustrates a common mode choke inserted

into the schematic of Figure 2. Common mode chokes can be designed to

include some differential filtering thereby eliminating the need for a separate

differential choke (or inductor) in some applications. Some theory behind

common mode chokes is discussed further below.

Toroids are the preferred core shape to use in common mode chokes. The

continuous unbroken circular path maximizes magnetic coupling between

windings thereby minimizing leakage inductance. "E" cores are the second

most preferred core shapes for common mode chokes. The toroids are less

costly than the "E" cores, but "E" core bobbins are easier and less costly to

wind. Toroidal coils are usually more costly to mount into an assembly. An

air gap can be easily placed between "E" core halves. A gapped core has

more leakage inductance; hence "E" core structures are usually preferred

when some differential filtering is desired from the common mode choke.

Common mode chokes can be made from other core shapes but usually at

higher cost. Their use occurs when a special characteristic is needed. For

example, an "EFD" core may be used when a low profile is desired.

Selecting the optimum core material for common mode chokes is not easy.

The frequency range of the electrical noise is the major factor. If only power

frequency noise is expected (i.e. 60 Hz. harmonics), then laminated silicon

steel may suffice. Laminated nickel iron or powdered iron or "sendust" type

powder will do for lower audio range noise frequencies. Perhaps moly-

permalloy powders for the upper audio range (depending on noise levels).

Ferrite materials are needed for noise frequencies above 20 kHz. Although

the inductive value of ferrites diminishes rapidly above 1 megahertz, some

ferrite materials are still suitable for common mode chokes because the

resistive component helps maintain a sufficiently high impedance value.

Butler Winding makes common mode chokes in a wide variety of shapes

and sizes. This includes; various standard types of "core with bobbin"

structures (E, EP, EFD, PQ, POT, U and others), toroids, and some custom

designs. For more information on Butler Winding's capabilities, click on our

"capabilities" link

Common Mode Choke Theory

A common mode choke may be used to reduce a type of electrical noise

known as common mode noise. Electro-magnetic interference (E.M.I.) in the

circuit's environment is one source of electrical noise. E.M.I. induces or

couples unwanted electrical signals into the circuit. It is desirable to filter out

the unwanted noise signals without significantly affecting the desired signal.

Environmental sources of E.M.I. often create an independent return path

(ground path) for the electrical noise signals. The return path of the desired

signal is a different path. Because there are two different return paths, a

common mode choke can be used to significantly block (hence reduce) the

unwanted noise signal (at the load) without significant reduction in the

desired signal.

A.C. power lines provide a good example. They are known to carry

significant levels of electrical noise. Their long length gives environmental

E.M.I. ample opportunity to generate unwanted electrical noise into the

power lines. Figure 2 illustrates an application without a common mode

choke. The power line voltage, "Vs", causes current, "Iz", to flow through

the load, "Z". At any non-zero instance, Current "Iz" flows into "Z" through

one power line wire and returns through the other power line wire. E.M.I.

voltage, "Vnc1", causes current "Inc1", to flow through the load "Z".

Similarly, E.M.I. voltage, Vnc2 causes current "Inc2" to flow through the

load "Z". Because the E.M.I is generating both "Vnc1" and "Vnc2" the two

voltages tend to be in phase.

There is very little current flow between them. Current "Inc1" does not flow

through both power line wires. It flows through one power line wire and

through the ground path. Similarly, current "Inc2" does not flow through

both power line wires. It flows through one power line wire and through the

ground path. In this example only "Vnc1" produces electrical noise across

load "Z" because the "Vnc2" end of "Z" is grounded. In practice, the

effective ground point could occur somewhere between the two ends of load

"Z".

Figure 3 illustrates the same application with a common mode choke. The

common mode choke has two windings. Each winding of the common mode

choke is inserted between the end of a power line wire and the load. As in

Figure 1, current "Iz" flows through both power line wires and currents

"Inc1" and "Inc2" each flow through one power line wire and return through

the ground path. Observe that current "Iz" flows through both windings but

in opposing winding directions, while currents "Inc1" and Inc2" each flow

through only one winding and in the same winding direction. The ground

path does not flow through a winding.

The inductance of winding A restricts (reduces) the flow of current "Inc1"

(when compared to Figure 1), thereby reducing the noise voltage across "Z".

Similarly the inductance of winding B restricts (hence reduces) the flow of

current "Inc2". Windings A and B have the same number of turns. The

ampere-turns created by Current "Iz" (but excluding any "Inc1" current

component) flowing through winding A is cancelled by the opposing

ampere-turns created by current "Iz" flowing through winding B. Ideally, the

cancellation results in zero inductance and no restriction (no reduction) of

current "Iz". "Iz" produces the same voltage across load "Z" as it does in

Figure 1. In practice this will not be true. The common mode choke will

have some leakage flux between windings A and B hence incomplete

cancellation. Windings A and B will have some winding resistance. Both of

these will have some effect on "Iz" (reduces "Iz").

In contrast, the load current "Iz" flowing through both windings A and B of

the differential choke shown in Figure 1 do not cancel, hence "Iz" will be

restricted (reduced). Differential chokes are useful when the electrical noise

frequencies are much higher than the operating frequencies. The higher

choke impedance at the high frequencies block the electrical noise while

having a tolerable effect at the operating frequencies.

Some common mode chokes are intentionally designed to have significant

leakage inductance. The leakage inductance acts in series with the load

hence the leakage inductance provides differential noise filtering. One

common mode choke functions like the combined chokes shown in Figure 1

but may differ in levels.

Toroidal Inductors

Toroidal inductors / transformers are the high performers among inductors.

They offer the smallest size (by volume and weight) and lower

electromagnetic interference (EMI). Their windings cool better because of

the proportionally larger surface area. A 360 degree wound toroidal

transformer has a high degree of symmetry. Its geometry leads to near

complete magnetic field cancellation outside of its coil, hence the toroidal

inductor has less EMI when compared against other inductors of equal

power rating. Windings that are less than 360 degrees exhibit more EMI.

Toroidal inductors with a round core cross section are better performers than

toroidal inductors with a rectangular cross section.

The cancellation is more complete for the round cross section. The round

cross section also gives a shorter turn length per unit of cross sectional area,

hence lower winding resistances. Good turn-to-turn coupling is dependent

on the winding being wound a full 360 degrees around the core. As winding

turns are positioned further away from the core less complete turn-to-turn

coupling will occur. Turns on the outer layers see a core cross sectional area

that includes some non-magnetic area (air, insulation, copper). This added

area generates some leakage inductance that adds to the inductance expected

from the core.

Toroidal inductors can be used in any inductor application that can

accommodate its shape. Although usable, toroidal inductors are not always

practical for some applications. Gapped toroidal inductors usually require

that the gap be filled with some type of insulating material to facilitate the

winding process. This is an extra expense. Powdered cores have an effective

distributed gap. These are usually preferred over a filled gap because of

lower cost and reduced gap losses. Some printed circuit boards are space

critical. Mounting a toroidal inductor flat on the board may take up too much

precious board area. Some applications also have restricted height so the

toroidal inductor cannot be mounted vertically.

Generally speaking toroidal inductors are more expensive than bobbin or

tube wound inductors. Sufficient winding wire must first be wound (loaded)

onto the winding shuttle, then wound onto the toroidal transformer’s core.

(For bobbin/ tube wound wire is continuously de-reeled from a spool of

wire.) After that, the best situation, from a cost perspective, is no insulation

required over the winding. If the winding must be insulated, then it must

either be insulated (taped) by hand or the toroidal inductor must be removed

and taken to a separate taping machine.

Some inductors have more than one winding. If additional windings are

required, then the toroidal inductor is placed back on a toroid winding

machine after taping. The shuttle must then be loaded with the wire size and

type for the toroidal inductor’s next winding, thereby adding most cost to the

inductor. Toroidal inductors with a single winding wound on a coated core

may be cost competitive with an equivalent bobbin or tube wound inductor

since the toroidal inductor will not require a bobbin or tube. The cost

differential will then depend on the method and cost of mounting the

inductors.

Toroidal inductor cores are available in many materials: silicon steel, nickel

iron, moly-permalloy powder, iron powdered, amorphous, ferrites, and

others. Silicon steel and nickel iron are available as tape wound cores or

laminated pieces. Non-magnetic toroids are also available to make air core

toroidal inductors.

Butler Winding can make (and has made) toroidal inductors (and

transformers) in a wide variety of materials and sizes. Butler Winding also

does “bobbin wound” and “tube wound” inductors. Our upper limits are 40












Need More Technical Information about Inductors Chokes and

Reactors?


For more information on a particular type of inductor simply click from the

dfollowing list: Common Mode Choke, Surface Mount Inductors, Bobbin

Wound Inductors.



Surface Mount Inductors

Surface mount inductors (and surface mount transformers) can be classified

in several ways: by power rating, by type of application, by type of

construction, by industry, and others. "Surface mount inductors" refer to a

type of construction that permits attachment of surface mount inductors to a

printed circuit board (PCB). Historically, inductors and other circuit devices

have been mounted on PCBs using "pin-thru" technology. Inductor wires are

terminated to pin type terminals. Holes are drilled in the PCB's copper

circuitry to accommodate the transformer pins. The inductor pins are

inserted through these holes and then soldered to the copper circuitry.

Engineers have developed solder pastes, adhesives, and assembly processes

that permit attaching inductor terminals to PCBs without using holes. Flat

areas (known as pads) on the inductor terminals are soldered directly to

copper circuitry surfaces hence the term surface mount inductor (or

transformer). This process eliminates the need to drill holes for the pins,

thereby reducing the cost to manufacture a PCB.

Surface mount inductors (and transformers) are usually wound on surface

mount bobbins, but are also available as toroidal coils. The toroidal coil is

mounted on a "header" equipped with surface mount terminals. The bobbins

(or headers), used with surface mount inductors, come in a variety of

materials: plastics, phenolic, glass, Teflon and others. Most of these are

molded. Some are fabricated. Some bobbins and headers are "self leading".

The winding wire is also used to form the surface mount terminal by looping

the wire under a pre-formed flat edge thereby forming a reasonably flat

terminal area.

Surface mount inductors (and transformers) are available in a variety of

shapes. Surface mount inductor shapes include pot cores (round), "RM"

(square pot cores), "EP", "E", "EI", "EEM", "EFD", "U", "UI", "ER", and

some others including custom shapes. Surface mount inductors in these

shapes are usually only available in the smaller sizes. Designers are adapting

more shapes and larger sizes to surface mount inductor applications.

Designers have mechanical concerns about the larger sizes. The weight of

the larger sizes may exceed the weight that soldered surface mount pads can

safely handle under vibration. Over time, designers hope to develop surface

mount inductors (and transformers) in larger sizes.

Like other inductors, surface mount inductors (and transformers) can use a

variety of core materials: laminated or taped wound silicon steel alloys,

nickel-iron alloys, cobalt alloys; powdered irons and nickels; ferrite; air

core; and/or core materials processed for square loop or round loop

properties; and others.

Butler Winding can make (and has made) surface mount inductors (and

transformers) in a wide variety of materials and sizes. Butler Winding can

also do a variety of custom applications. Butler Winding's upper limits are










For more information on Butler Winding's capabilities, click on our

"capabilities" link.

Need More Technical Information about Inductors Chokes and Reactors?


Saturation and the volt-second product are discussed in the "pulse

transformer" web page. An equivalent circuit for a transformer is included in

the "power transformers" web page. The "inverter transformer" and "push

pull" web pages include some discussion about magnetic "bipolar" and

"unipolar" operating modes. There are web pages for various types

(applications) of transformers (and inductors). Click on one of the available

links.



Bobbin Wound Inductors

Bobbin wound inductors refers to a type or method of construction of

winding inductors chokes and reactors. Toroidal coils are wound directly

onto a toroidal core. The core may be coated or boxed to insulate it form the

coil windings. In contrast, bobbin wound inductor coils are wound

independently of the core. The coil must hold its shape or form until the coil

is assembled onto the inductor core. One common method of doing this is to

wind the coil onto a bobbin (also referred to as a spool), hence the term

"bobbin wound winding inductor". The bobbin is a pre-formed reasonably

rigid part. The bobbin material is usually (but not always) an insulating

material, hence it can provide electrical isolation between the coil and the

adjoining core material provided suitable creepage distance is used. Multi-

section bobbins are available to provide increased electrical isolation

between coil windings.

Bobbin wound inductors are used in a variety of applications, hence bobbins

are made from a variety of materials: plastics, phenolic, glass, Teflon and

others. Most bobbins are molded. Some are fabricated. Bobbin designs for

bobbin wound inductors often provide terminals, pins, and/or surface mount

pads to ease wire termination and to facilitate printed circuit board

mounting.

Bobbin winding inductors (and transformers) are available in a variety of

shapes. Bobbin wound inductor shapes include pot cores (round), "RM"

(square pot cores), "RS" (round slab pot cores) and "DS" (double slab pot

cores), "EP", "PQ", "E", "EI", "EEM", "EFD", "U", "UI", "EC", "ETD",

"ER", "EER", and some others including custom shapes. Bobbin wound

inductors in these shapes are available in several different sizes.

Bobbin wound inductors (and transformers) can also use a variety of core

materials: laminated or taped wound silicon steel alloys, nickel-iron alloys,

cobalt alloys; powdered irons and nickels; ferrite; air core; core materials

processed for square loop or round loop properties; and others.

Butler Winding makes bobbin wound winding inductors chokes and reactors

in a wide variety of materials and sizes with pin-thru, surface mount, and/or

flying leads terminations. Butler Winding also does "tube wound" inductors

and air core coils. Our upper limits are 40 pounds of weight and 2 kilowatts

of power. We have experience with foil windings, litz wire windings, and

perfect layering. For toroids, we can (and have done) sector winding,

progressive winding, bank winding, and progressive bank winding. Most of

our production is 100% tested on these machines. For more information on

Butler Winding's capabilities, click on our "capabilities" link.

Need More Technical Information about Inductors Chokes and

Reactors?


Saturation and the volt-second product are discussed in the "pulse

transformer" web page. An equivalent circuit for a transformer is included in

the "power transformers" web page. Additional inductor choke and reactor

links include common mode choke, toroidal inductors, surface mount

inductors and Inductors chokes and reactors.



Custom Wound Coils and Custom Transformers

First question. Why do our customers buy custom wound coils? Most of

our customers would prefer to buy standard catalog parts, preferably parts

available from multiple sources and kept in stock. Second question. What

distinguishes a custom wound coil, a custom transformer, or a custom

inductor from a standard coil, transformer, or inductor?

An answer to the first question is “the diversity of required parameters”.

The answer requires some further explanation. Consider the following

scenario. There are eight product designers, each from a different company.

They all need a five-volt D.C. output power supply. Designers 1 and 2 need

1 amp of current from their power supply output. Designers 3 and 4 need

2.5 amps. Designers 5 & 6 need 5 amps. Designers 7 & 8 need 10 amps.

Let’s figure what magnetic components this group of designers need. The

group needs four different switching transformers and four different output

inductors. Is that correct? Well, no! Designers 1, 3, and 5 will draw power

from a 12-volt battery while designer 7 will draw power from a 24-volt

battery.

Designers 2, 4, 6, and 8 will draw rectified power from the A.C. line, but

designer 8 wants to be able to draw from either 120 or 240 volts. Of course

they all want the smallest transformer that will do the job. They will all

need different transformers. Oh, I almost forgot. Designers 1, 3, 5 and 7 are

switching at 50 kilohertz. Designers 2, 4, 6 and 8 are concerned about high

frequency E.M.I in their applications. They decide to limit the switching

frequency to 25 kilohertz. Now all the output inductors are different. What

other parameters could be different? Dielectric (hipot) requirements?

Voltage regulation specifications? Boy, what a diversity of required parts!

Do all these needed parts exist in catalogs? Doubtful! Do the

manufacturer’s keep them all in stock? Doubtful!

Custom wound coils, custom wound transformers, and custom wound

inductors can simply mean making coils, transformers, and inductors per the

customer’s drawing and/or specifications. There may be several reasons

why the customer has custom designs. Perhaps the customer could not find

a suitable catalog part. Perhaps the customer is guarding against vendor

obsoleteness, perhaps against a vendor going out of business. Alternatively,

the custom coil, custom transformer, or custom inductor design may have

unique features. If so, it is doubtful that it would be listed in catalogs or be

available “off the shelf”. Another consideration is that a custom wound coil,

custom wound transformer, or custom wound inductor may involve

construction processes that require specialized material, equipment or

handling. High voltage coils may be one example. Because of corona

concerns, coils may have to have “void free” vacuum impregnation with

epoxy, silastic, or another suitable material. A vacuum chamber (or perhaps

vacuum oven) will be required.

Butler winding can make (and has made) custom wound coils, transformers,

and inductors in a variety of shapes and sizes. This includes; various

standard types of “core with bobbin” structures (E, EP, EFD, EC, ETD, PQ,

POT, U and others), toroids, and some custom designs. We have

constructed mag amps and some sensing coils. We have experience with

foil windings, litz wire windings, and perfect layering. For toroids, we can

(and have done) sector winding, progressive winding, bank winding, and



automated machines and a taping machine for toroids. To ensure quality,

Butler Winding purchased two programmable automated testing machines.

Most of our production is 100% tested on these machines. Butler Winding

has a corona-testing chamber for testing high voltage coils. Butler Winding

has a temperature chamber. It can be use to temperature cycle parts.

Whatever your custom coil requirements, here is a high likelihood that

Butler Winding can handle it.

Need design assistance for your custom wound coil, transformer and

inductor requirements? Please feel free to contact Butler Winding and ask to

talk to Engineering. For more information on our capabilities, click o

Custom Winding - Transformers - Inductors

Services and Capabilities

Winding Transformers Inductor Services and Capabilities

Butler Winding manufactures coils, transformers, and inductors. Most of

our business is from custom orders. Custom orders being orders built to

customer drawings and/or to customer specifications; or orders requiring

specialized handling, material and/or equipment. The coils, transformers,

and inductors our customers ask us to build are not readily available as “off

the shelf” parts.

What kind of services and capabilities can Butler Winding offer its customers?

• Transformer and inductor manufacturing

• Inductor and transformer design

• Prototyping

• Customer service

• Quality control

Manufacturing ( Manufacturer of Inductors - Transformers - Coils)

After receiving a customer order Butler Winding will purchases the cores,

bobbins, winding tubes, terminals, magnet wire, lead wire, and/or insulation

needed to build the customer’s parts. Then Butler Winding winds the coils;

terminates the coil windings to pins, pads, terminals, or lead wires as

required (including soldering and/or crimping); assembles (or stacks) the

core(s) into the coils; tests the parts; bakes the parts if required; varnishes or

encapsulates the parts; labels the parts; and then ships the parts. Yes, Butler

Winding also sends a bill for the parts to the customer.

Manufacturing can do any of the above processes as a “value added”

process. One of our customers sends transformers to Butler Winding to

encapsulate (pot) in epoxy. Manufacturing makes every reasonable effort to

meet the customer’s delivery dates.

Manufacturing can vacuum impregnate varnishes and encapsulate (potting)

under vacuum. Manufacturing also has the capability to temperature cycle

parts if needed. Butler Winding has one coil winder capable of winding

with single digit wire gauges. For toroids #12 A.W.G. is the largest single

conductor wire size our machines can handle. Manufacturing equipment

available for use includes:

• Tanac AX3 automated (programmable) winding machine.

• Tanac AX10 automated (programmable) winding machine.

• Bobifil ER-900 MP winding machine (can handle single digit wire

gauges).

• Seven hand winders.

• Two directional motors with a turns counter, speed controlled with

a foot pedal.

• Anacoil Meteor M10 coil winder.

• Bachi Model 115 E/C SCR coil winder

• Jovil Model 200 and Jovil Universal SMC-1 toroid winder.

• Jovil model 200 toroid winder equipped with a taping header.

• Gorman Productor B and model 920A toroid winders.

• Universal model 63 and model 6LS toroid winders.

Various size shuttles for each of the toroid winders.

• Two toroid hook winders.

• Carpenter Manufacturing Co. Model 88E mechanical wire stripper.

• Several handheld mechanical wire strippers, rotating knife edge type.

• Banding Machine, compressed air type, for banding “C” cores.

• Brady Model 3481 label maker

• Two Voltech Instruments AT3600 automated (programmable)

transformer testers

• Two Baking Oven.

• Vacuum Chamber and a Vacuum Oven Chamber.

• Varnish tanks

• Small machine shop – drill press, band saw, sander, and grinder.

• Tenney Temperature Chamber

• EFD Model 1500 XL precision dispensing machine.

• Air compressor.

Transformer - Inductor Design Services

Butler Winding has an engineer on staff who is experienced in transformer

and inductor design. If customer personnel need technical advice, they can

telephone Butler Winding and ask to speak to engineering. Schedules

permitting, our engineer may design transformers for our customer. As of

present date, simple design work is usually done (not always) at no charge as

a courtesy to our customers. A more extensive effort may warrant and incur

a charge to the customer for the service. Schedules permitting, our

engineering may be available for an hourly or daily fee contracted through

Butler Winding.

Prototyping

Butler Winding will make prototypes for customers if requested. A

prototype sample is recommended for parts new to Butler Winding. Small

inexpensive prototypes might be supplied to the customer at no charge as a

courtesy to the customer. There will be a charge for larger more expensive

prototypes. Typically, our engineer will produce a parts list, a coil sheet,

and perhaps an assembly drawing. These will be submitted to production

(manufacturing) for quoting (if applicable) and/or production. If the

necessary materials are on hand production will insert the prototype work

into its schedule after receipt of the purchase order (if applicable). If

materials are not on hand, then material will be procured. Butler Winding

tries to produce prototypes within two weeks, but this is not always

possible.

Customer Service

Butler Winding strives to provide good customer service. Butler winding

recognizes that good customer service entails on time deliveries, polite

friendly personnel, quick response times, monitoring of customer order

status, and attention to detail. Butler winding will accommodate customer

schedule changes and cancellations when practical, but Butler Winding

cannot be expected to suffer monetary loss due to customer changes.

Quality Control

Quality is important to our customers therefore it is important to Butler

Winding. Bad parts cost the customer money and cost the vendor money.

The cost occurs in lost time troubleshooting, communicating, and shipping

return and replacement parts. Tracking the quality of thousands of parts is

not an easy task. To help ensure quality, Butler Winding purchased two

automated programmable transformer testers. Both testers are the Voltech

Insturments model AT3600. Butler Winding does nearly 100% testing of

the parts it produces. Where warranted, inspection steps are added to our

manufacturing process to help ensure a quality part. Also, there are

procedures in place that help catch paperwork mistakes thereby reducing

late deliveries.

Butler Winding personnel are willing to work with customer personnel to

resolve any quality problems that may come up.

Electronic Transformers & Inductor Core Types,

Quick Select Core Type

Magnetics cores can be divided into many types of categories. This

discussion will divide magnetic cores into two major categories, structure

(shape) and material. These major core categories will then be sub-divided

into additional categories.

Further below are a list of core structures and a list of magnetically "soft"

core materials. The lists are not intended to be exhaustive lists. The

associated discussions are intended to be general information, not detailed

information. Butler Winding uses magnetic cores in its production of

transformers and inductors, but does not manufacture any cores other than

stacking laminations to form core stacks. Some additional information can

be obtained from other web pages within Butler Winding’s website. Also

feel free to contact Butler Winding and ask to talk to our engineering

personnel. You can also contact core manufacturers for more detailed core

information.

Butler winding can make (and has made) transformers and inductors in a

wide variety of core shapes, sizes, and materials. This includes; various

standard types of “core with bobbin” structures (E, EP, EFD, EC, ETD, PQ,

POT, U and others), toroids, and some custom designs. We have experience

with foil windings, litz wire windings, and perfect layering. For toroids, we

can (and have done) sector winding, progressive winding, bank winding, and



automated machines and a taping machine for toroids. To ensure quality,

Butler Winding purchased two programmable automated testing machines.

Most of our production is 100% tested on these machines. For more

information on our capabilities, click on our “capabilities” link.

Core Structures

Toroids (rings)

Toroids are the core type geometry of choice for optimizing performance. A

toroid of round cross-section offers better performance than one of

rectangular cross-section, but for practical and economic reasons toroids of

rectangular cross-section are much more prevalent. The symmetry of their

circular geometry minimizes the amount of external magnetic flux

produced. Consequently they produce much lower amounts of unwanted

electromagnetic interference. Unlike other core types, turns can be wound

along the entire length of the core thereby allowing more turns per layer.

The mean turn length will be shorter than that of other core types of equal

power capability hence lower winding resistance and lower winding losses.

Compared against other core types, a toroidal coil has a lot of surface area

from which it can dissipate heat hence it cools much better than other core

types. Cooler windings result in higher efficiency and may allow more

utilization of the core’s capability.

Because of its circular nature, the magnetic path of a toroid is an unbroken

continuous path unless intentionally broken. There is no air gap in the

magnetic path (unless intentionally added) hence optimal use can be made of

high permeability materials. Ferrite toroids and stacks of stamped

lamination rings are examples of this. A tape wound core is the next closest

example. The flux in each layer wound on the core can make a full

revolution and then continues onto the next layer, but the magnetic flux must

eventually pass from layer to layer encountering an air gap between layers in

the process. The gap occurs because the tape strip is not perfectly flat. The

layer to layer passage is distributed the surface area of an entire revolution,

hence the magnetic reluctance of the gap becomes very small and usually

can be ignored. A tape wound core can utilize the advantage of grain

oriented materials (such as grain oriented silicon steel) while stamped rings

cannot.

In some applications it is desirable to have an air gap in the core path. For

mechanical reasons, it is cumbersome to add air gap to a toroid. Large air

gaps produce undesirable flux fringing. Powdered cores combined the

magnetic material with a non-magnetic binder material. Magnetically, the

binding material acts like an air gap, but this gap is distributed throughout

the entire core. Because of this distribution there are no flux fringing

effects. The binder(s) also reduce eddy currents.

Toroids are manufactured in practically all “soft” magnetic materials.

Toroid Cores can be coated with insulation to provide electrical isolation

between the core and the winding(s). Some toroid cores are “boxed” to

provide isolation. Some toroid cores are “boxed” because the core material

is sensitive to stresses produced by the winding processes.

Bar, Slab, or Rod

“Soft” magnetic metal alloys are available in Bar, Rod, or Slab shapes.

These core shapes find use in D.C. applications such as D.C. powered

solenoids and D.C. relays. They can be used in very low frequency (below

50 Hz) A.C. applications. They do have some limited use at A.C. line

frequencies. For a solid core, A.C. core losses per unit weight (or unit

volume) become more pronounced as the cross sectional area increases.

This is why silicon steel, nickel-iron, and cobalt alloy cores use a stack of

laminations. The laminations divide the cross-section into a stack of much

smaller cross-sections. D.C. applications are subject to far less core losses.

They only experience A.C. core losses (and the heat produced) during

transitional events.

Powder Cores extend the useful A.C. frequency range of the materials listed

in the previous paragraph. A non-magnetic binding material is used to bind

the small magnetic powder particles together. The binding material also

serves to insulate the particles from one another thereby reducing eddy

current flow in the core. This extends the useful frequency range, but there

is a trade-off. The binding material adds a distributed air gap to the core.

The distributed air gap reduces the permeability of the core. The core

requires more magnetizing VA. Bars, slabs, and rods can be purchased in

powder iron materials. The selection of sizes is somewhat limited. Larger

sizes can be assembled from smaller sizes.

Ferrites are a magnetic form of ceramics. Ferrite has very high electrical

resistivity. Even at high frequencies the eddy currents remain low. With

suitable gauss de-rating, some types of ferrite cores can use above 1

megahertz. Bar, slabs, and rods can be purchased in ferrite materials, but the

selection of sizes is limited. Larger sizes can be assembled from smaller

sizes.

“C” Cores

“C” Cores are similar to tape wound toroids in that they are made by

winding a long strip of electrical steels of desired width and thickness onto a

mandrel. They differ from tape wound toroids in two characteristics: it is

rectangular with rounded corners, and the wound core is cut in half to form

two “C” shaped mating pieces. (One could argue that two “U” shapes are

formed.) The mating surfaces are polished to minimize air gap between the

two halves. Further reduction in the gap may be achieved by cutting the

core at an angle. One “C” core set (or 2 “C” core halves”) can replace a “U-

U” or “U-I” laminated structure. Two sets of “C” cores (or 4 “C” core

halves) can replace a “E-E” or a “E-I” laminated structure. “C” cores can

take full advantage of grain orientation while their laminated counterparts

only take about 60% to 80%. Because of this, “C” cores performance is

better than that of laminated stacks. The rounded corners also reduce the

weight.

“E” Type Cores: “E-E”, “E-I”, “EFD”, “EEM”, “ER”, and “ETD”

Powdered and Ferrite Cores did not exist In the early development of

transformers and inductors. Cores consisted of stacks of laminations;

patterns cut or stamped out of thin sheets of electrical steels. Most

applications required a lamination pattern (or patterns) that would form a

closed magnetic loop when assembled together. Early patterns included

rings for toroids, “L” shapes, “U” shapes, “E” shapes and “I” shapes (used

with the “E” and the “U”). Patterns were sought that were easy to assemble,

could be interleave to minimize gap effects, and would minimize waste. “E”

shapes used in “E-E” and “E-I” combinations became popular choices.

“Scrapless” “E-I” patterns were developed. The electrical steel stamped out

of two adjacent “E” laminations (placed leg end to leg end) to form the

winding window area became the two “I” laminations to be placed across

the leg ends of the “E” laminations.

In the typical “E” lamination, the center leg (one of three legs) is twice the

width of either outer leg. In theory, magnetic flux flowing out of the center

leg divides equally and flows into the outer two “E” core legs. Since the

outer legs handle half the flux they only need to have half the cross-section

that the center leg has. An “E” core structure occupies two outer sides of the

coil. This constitutes a “shell” type core structure (not explained in detail

here). In contrast, a “U” core or “C” core structure (which has two core

legs) only occupies one side of a coil placed over one of its legs. The “E”

core structure provides better self-shielding than the “U” core structure (but

neither provides good shielding). “E” type cores are easily gapped. For the

typical “E” laminations this requires a “butt stacked” core. There is no

interleaving of laminations.

Since “E” cores have two open coil sides, they provide substantial room to

bring high current lead wires out from the coil. This also permits good heat

dissipation but not as good as a toroid. In contrast, the standard pot core has

a much more restricted space in which to bring out lead wires and restricts

heat flow. It is easier to achieve high voltage electrical isolation with an “E”

core than with a pot core.

Because the core stack is a stack of laminations the typical stack has core

legs of rectangular cross-section. Typically the inductor or transformer coil

is placed over the center core leg. To minimize winding resistance (hence

also minimize winding losses) it is desirable to have a round center leg. A

round center leg also eliminates the sharp bend encountered when winding

wire around a rectangular leg; consequently a round center leg permits use

of larger wire. Achieving round center legs with laminations is possible but

very impractical. With the development of powdered cores and ferrite cores

it became practical to have a round center leg. “EC” and “ETD” are

examples of type “E” cores with round center legs. The combined cross

section of the two outer legs should equal or exceed that of the center leg.

“EC”, “EER”, and “ETD” type ferrite cores were developed for higher

power higher frequency switching transformers.

“EFD”, “EEM”, and “ER” ferrite cores are low profile (low height)

designs.

“EP” Cores

The “EP” core design combines the self-shielding feature of a pot core with

the coil lead accessibility of “E” cores in a small package. The core wraps

around the coil on the top, bottom, and three sides of the coil; but leaves one

side of the coil open to bring out wires. Although the one side is open, the

coil is completely recessed into the core. Because of the one open side, The

amount of self-shielding of the “EP” core (by itself) is less than that of the

pot core. However, the self-shielding improves when a ground plane is

placed over the open side. “EP” cores are usually mounted on a printed

circuit board with the open side against the printed circuit board. If good

shielding is required, a grounded section of copper is provided on the printed

circuit board under the “EP” core and coil. Mounted in this way, the “EP”

shielding comes very close to that of a “Pot” core.

Once mounted the coils becomes completely enclosed. Consequently, heat

dissipation is poor.

The “EP” core has a round center leg to minimize winding losses.

“F” Lamination Cores

“F” shape laminations are similar in function to “E” laminations. One

notable difference is that the “F” lamination can be interleaved at the corners

of the stack and have a “butt stack” in the center leg. An air gap can be

provided in the center leg during stamping of the lamination. The typical

“F” lamination has a hole near each stack corner. A screw is passed through

these holes to secure the stack. If the holes are over-sized a bit, there is

some play available. This play can be used as a way to provide some gap

adjustment in the center leg by sliding the stacked interleaved lamination a

bit. The “F” shape is not typically found in cores other than lamination

stacks.

“Pot” Cores --- Round, round slab (RS & DS), and Square (RM)

Pot’ cores are known for their excellent shielding capability. This occurs

because the core completely surrounds the coil except for two narrow slots

which leads are brought through. Pot cores have round center leg and two

nearly semicircle outer legs. The center leg is usually hollow but may be

solid. Solid ones run cooler because it permits a lower flux density. The

center legs may be ground to provide a gapped core. An insert may be

placed in a hollow gapped core to provide a means to adjust the inductance

of the core (and its windings). There are popular in tuned circuits. The

adjustment allows one to compensate for core tolerances and tuning

capacitor tolerance.

The round slab “Pot” cores are similar to the standard round “Pot” core but

differs because a portion of the core has been removed from the standard

round core design. Consequently, the round slab pot cores have better heat

dissipation and have more room for wire leads. Double slab (DS) “Pot”

cores have two portions of the core removed. In essence the slab “Pot” cores

are a compromise design between a standard “Pot” core design and a “E-E”

core design.

Square “Pot” core designs differ from standard round “Pot” core at the outer

legs. The outer legs have a more “corner-like” appearance to them. This

shape permits tighter packing of the cores on a printed circuit board,

achieving about a 40% saving in mounting area. The coil is more open

hence heat dissipation and lead wire space is better that of the standard

round “Pot” cores, but shielding capability is less.

Pot cores are made almost exclusively in ferrite materials.

Planar Cores

Planar cores are low profile cores. The core material is almost exclusively

ferrite material. The core design is intended for use with windings etched on

a printed circuit board, thereby eliminating the winding of a separate coil.

Etching of the windings puts a limit on the number of available turns hence

the operating frequency must be high to avoid core saturation. If the turns

requirement is sufficiently high, some designers might cement a thin coil to

the board under the core. Since the typical planar core user does not need a

coil, Butler Winding has little experience with planar cores.

“PQ” Cores

“PQ” cores were specifically designed for use in switching mode power

supply circuits. The geometry is optimized to provide power with minimal

size (including mounting area) and weight. Otherwise, its features are the

same as an “E-E” core design. See section above discussing “E” type cores.

“U” and “U-I” Cores

These shapes are available in lamination materials (for stacking), powdered

material (typically powdered iron), and ferrite materials. In laminated form,

their features are similar to that of the “C” core discussed in a prior section.

Heat dissipation is excellent. There is lots of room available for lead wires.

Self-shielding is poor. “U” cores have two core legs. Coils can be placed

over either or both legs. Using coils on separate legs is great for high

voltage isolation between coils. The mean turn length of two coils on

separate legs (sharing the whole winding window) is smaller than 1 coil on

one leg (occupying the whole winding window), hence the two coils

connected in series has less winding resistance. “U” (and “C”) cores may be

used for “split-core” current transformers.

Magnetically “Soft” Core Materials

Silicon Steel – laminations or tape wound

Iron has a very high saturation level. It saturates above 20 kilogauss, but

requires a lot of magnetizing force above 17 kilogauss. Cobalt has a higher

saturation level, but is very expensive. Silicon is added to iron to improve

the iron’s electrical resistivity. Processes have been developed with which

promote grain orientation in the metal. The grain orientation lowers the

losses and extends the boundaries of useful operation. The high saturation

level permits the building of smaller transformers. Silicon steel must be

used in thin strips to minimize its eddy currents; hence it is used for

laminated core stacks or for tape wound cores.

Eddy current become excessive as the operating frequency climbs. Eleven

to fourteen mil thick strips are used for 50 & 60 hertz and at 100 hertz with

some gauss de-rating. Six to seven mils is used for 400 hertz applications.

Two to four mils is used near 1000 hertz. Use above 1000 hertz is possible

but requires strip thickness below 1 mil and requires operating at lower

gauss levels. Silicon steel is very economical within its useful frequency

range. Silicon steels can be process to optimize square loop type properties.

Nickel Iron -- laminations or tape wound

Nickel is a higher permeability lower loss magnetic material when compared

to silicon steel. It is usually used in combination with iron. Saturation for a

fifty-fifty percent combination is around 15 kilogauss. Saturation for an

80% nickel combination is around 8 kilogauss. For the same power rating, a

transformer made with Nickel iron will be larger than a silicon steel

transformer provided they are operated in the silicon steel’s useful frequency

range. At higher frequencies Nickel iron is preferred over silicon steel.

Nickel iron is more expensive than silicon steel. Nickel iron, because of its

higher permeability and lower losses it preferred over silicon steel for high

fidelity applications even at the lower frequencies suitable for silicon steel.

Nickel iron can be operated beyond 10 kilohertz with proper choice of strip

thickness and kilogauss level. Ferrites can match the lower losses of Nickel

iron but cannot match the saturation level or the high permeability.

Nickel iron can be processed to optimize either round loop or square loop

properties

Cobalt Alloys – laminations or tape wound

Because of its expense, cobalt is used only in size and/or weight critical

applications. It finds frequent use in the aviation industry.

Powdered Iron Cores

Iron alloys are ground and thoroughly mixed with a binding material, then

pressed in a press to form a core. The binding material is an insulator; hence

it reduces the eddy currents. This extends the useful frequency range of the

iron. It can be used up to about five kilohertz depending on the A.C.

kilogauss level, above 10 kilohertz at low A.C. gauss levels. The binding

material also provides a distributed air gap in the core structure. The

distributed gap is useful in D.C. applications. Powder iron is frequently used

as ripple filter inductors in D.C. power supplies. The D.C. flux can be high

as long as the A.C. flux is sufficiently small.

There are many types of powdered iron materials. Saturation can range from

to 14 kilogauss depending on type.

Powdered iron cores are available in “E”, “E-I”, “U” and “U-I” shapes.

Ferrous Alloy(s) Powdered Cores

Ferrous Alloy materials are similar to the “Sendust” material originally

develop by Arnold Engineering, but with improvements. Saturation level is

10.5 kilogauss. Like powdered iron, the ferrous alloy is thoroughly mixed

with a binding material, then pressed in a press to form a core. It has lower

core losses than the powdered iron. It is also used for ripple filter inductors

in D.C. power supplies. It becomes the preferred choice over powdered iron

at higher A.C. flux levels.

Molybdenum Permalloy Powdered Cores

These cores are composed of a powdered alloy of about 79% nickel, 4%

molybdenum, and 17% iron. Saturation is about 7.5 kilogauss. Their high

nickel content makes them very expensive. These powdered cores have the

lowest losses of all the powdered cores. It has the best A.C. characteristics

under heavy D.C. biasing. Because of its expense, its use is limited to the

more critical applications that demand its superior properties. D.C. biased

High “Q” coils operating at high frequency in tuned circuits is one example.

Nickel-Iron Powdered Cores

These cores are composed of a powdered alloy of about 50% nickel and 50%

iron alloy. It has the highest saturation level of the powdered cores

mentioned above. Saturation level is 15 kilogauss. Core loss is significantly

lower than the core loss of powdered iron cores. Its’ high saturation level

permit the smallest D.C biased inductors (assuming sufficiently small A.C.

flux).

Ferrites (ceramic structures)

Modern electronic designs demand magnetic devices to operate at ever

increasing high frequencies. Higher frequencies permit smaller magnetic

devices up to a point; that point being excessive heat loss and its associated

temperature rise. Of course sufficiently high temperatures will cause

imminent failures. Even mildly excessive temperatures will shorten

insulation life and eventually cause the magnetic device to fail prematurely.

This can cause a real problem for product manufacturer’s and especially for

their customers if the manufacturer’s products fail within a year or two after

delivery. Winding losses are one source of heat.

The other source is core loss. Core loss is caused by magnetic hysterisis.

The hysterisis produces eddy currents. Eddy currents flow through the

resistance of the core material and produce heat. Core materials with high

electrical resistivity can be operated at higher frequencies and/or higher flux

density levels. Consequently designers sought to discover or develop core

materials with high resistivity. Ferrite core materials were a resulting viable

solution. Ferrites exhibit high permeability and high resistivity. Ferrites are

also reasonably stable (repeatable properties) over time and temperature.

Three basic categories of ferrites are discussed below. The manganese zinc

and manganese nicker categories can be divided into various grades of

ferrites.

Manganese Zinc (MnZn) Ferrites

This general type of ferrite can be manufactured in several different vastly

different grades by altering its composition and processing. Initial relative

permeability (at 25 degrees Centigrade) can range from several hundred to

twenty thousand. Saturation (at 25C) ranges from 3.5 to 5 kilogauss. The

curie temperature can range from 100 to 300 degrees Centigrade. Material

grades have been developed for particular groups of applications such as

power, broadband, E.M.I./R.F.I. filtering, ripple filtering, tuning, and others.

The useful frequency range for most of these materials is 1 megahertz and

less (with suitable flux density de-rating), but some types approach 9

megahertz. Manganese Zinc ferrites have very low porosity.

Nickel Zinc Ferrites (ceramic structure)

This general type of ferrite can also be manufactured in several vastly

different grades by altering its composition and processing. Initial relative

permeability (at 25C) can range from about 15 to about 1200. Saturation

ranges from 2 to 3.5 kilogauss. The curie temperature ranges from 125C to

500C. Material grades have been developed for particular groups of

applications. High frequency E.M.I. suppression is one example. Generally

speaking, nickel zinc ferrites grades have significantly lower permeability

than the manganese zinc grades. Nickel Zinc ferrites are typically used at

frequencies above one megahertz. Manganese zinc ferrites are more

economical below one megahertz. The upper frequency limit for nickel zinc

ferrites ranges from 30 to 1000 megahertz depending on the grade. Nickel

zinc ferrites vary in porosity.

Manganese (Mn) Ferrites

This ferrite material has a unique combination of properties. It is stable with

temperature (repeatable properties), it is dense, and it exhibits some square

loop properties. It is a good choice for high frequency magnetic amplifiers

and other high frequency square loop applications. Its upper frequency limit

is 150 kilohertz.

Non-magnetic Cores:

There exist some applications where it is more economical to produce a coil

without a magnetic core. A low inductance but high current inductor could

be one example. Coil turns are wound on a supporting mandrel and bonded

together into a rigid coil or wound on an insulated form which gives the coil

support such as a bobbin (or spool), a tube, or a non-magnetic toroidal form.

Such coils may be referred to as “air core” coils. The relative permeability

of air and most insulators is one. The permeability of air is constant. It does

not change with temperature, unless conditions induce formation of corona

and/or plasma. Coils wound on insulating forms may have slight inductance

changes due to polarization effects on the molecules of the coil form.

Welcome to the Butler Winding Quote Request Page

We at Butler Winding invite your quotation requests and ask that you select

the type of transformer you would like a quote request for from the list

below.

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