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EE 366

POWER ELECTRONICS

Presented by: Dr. Philip Yaw Okyere

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Power Semiconductor Switching Devices

Introduction

Power electronic circuits are electronic circuits which

use high-power semiconductor switching devices toconvert or control electrical power.

The electronic circuits are called power electronic

circuits because large amount of power is involved. Applications of power electronic circuits include heat

controls, light controls, motor controls, power supplies

and high-voltage direct-current (HVDC) systems.

The most important components of the electronic

circuit are the semiconductor switching devices. In this

chapter, we present an overview of these devices.

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The Power Diode

It is similar to the pn junction signal diodes but

has larger power, voltage and current handling

capabilities.

However, its frequency response (or switching

speed) is lower.

The diode has the highest rating of all the

semiconductor switches and is the cheapest.

(a)Circuit symbol:

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Static i-v characteristic:

When the diode is forward-biased, it conducts

with a small voltage across it. This is in the order

of 1 V.

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When it is reverse-biased, negligible leakagecurrent flows until the reverse breakdown

voltage is reached. In normal operation, the reversebias voltageshould not reach the breakdown voltage.

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Diode parameters: The most important are the following:

VF = forward voltage drop when the diode is conducting.

VB = reverse voltage breakdown. When this voltage is exceeded, the reverse current

increases rapidly with a small change in reverse

voltage.

The current in this portion is limited by the external

circuit.

Breakdown is not destructive provided the power

dissipation is within a safe value specified in themanufacturers data sheet.

IR= reverse (or leakage) current.

It is found in the range of 10-15A and 10-6A.

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Diode parameters Contd

trr = reverse recovery time.

When the current of a forward-conducting diode is reduced

to zero by applying a reverse voltage (may also be reducedto zero by natural behaviour of the diode circuit), the diodecontinues to conduct in reverse direction due to excessminority carriers in each diode region before again falling to

zero. These excess minority carriers are produced during forward

conduction and they must be removed or recombinedbefore the diode regains its reverse blocking capability.

The short time that must elapse before a diode regains itsblocking ability is called the reverse recovery time.

Before a diode regains its blocking ability, it may beconsidered as short circuit in its natural blocking direction

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Diode parameters Contd

tfr turn-on or forward recovery time:

Diodes require a certain turn-on time before all the majoritycarriers over the junction can contribute to the current flow.

If the rate of rise of the forward current is high and the

forward current is concentrated to a small area of the

junction, the diode may fail. Thus di/dt(the rate of rise

of the forward current)

must be kept low to meet

the turn-on time limit.

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Stable states

Conducting state: iD= IF(forward current) > 0 and

vD= V

F> 0

Blocking state: iD= -IR(reverse current) and vD 0,

The value of vDin this state is determined by the

external circuit.

A conducting diode becomes blocked when the

current iDbecomes zero and a blocking diode

starts conducting as soon as vDbecomes slightly

positive ,i.e. > a threshold voltage VTD 0.7 V

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Maximum ratings: They include

Voltage ratings: The data sheet specifies

VRRM= maximum repetitive peak reverse voltage. It isthe maximum instantaneous value.

VR = continuous maximum reverse voltage or

maximum dc reverse voltage

VRSM= maximum non-repetitive peak reverse voltage.Non-repetitive peak reverse voltage may occuroccasionally due to overvoltage surge.

When the actual voltage across a diode isdetermined, a design engineer will apply a safetyfactor, typically 1.5, to arrive at the appropriatediode.

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Maximum ratings: Contd

Forward current ratings: The data sheet specifies

IF(AV) = maximum average current

IF(RMS) = maximum RMS current

IFSM = maximum peak non-repetitive surge current

A design engineer must ensure that none of themis exceeded.

Junction temperature Tjmax

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Power diode types:

Standard or general purpose diodes:

They have high trr. Typical value is 25 s.

They are used in low-speed applications where

the effect of trris not critical.

Examples are diode rectifiers and convertersfor a low input frequency up to 1 kHz and line-

commutated converters.

Current rating is from less than 1 A to several1000s of amperes and voltage rating from 50 V

to 5 kV.

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Power diode types Contd:

Fast-recovery diodes:

They have low recovery time, normally < 5 s. They are used in dc-dc and dc-ac converters

where the speed of the recovery is critical.

Current rating is from less than 1 A to 100s ofamperes and voltage rating from less than 50 V

to around 3 kV.

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Power diode types Contd:

Schottky diodes:

They have low voltage drop (typically 0.3 V) buthigh IR.

They are ideal for high-current and low-voltagedc power supplies for increased efficiency.

It switches very rapidly from forward conductionto reverse blocking and vice versa (They are thefastest)

Maximum allowable voltage is generally limitedto100 V(i.e. breakdown voltage is low ) andcurrent rating is from 1 to 300 A.

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The Silicon-Controlled Rectifier (SCR)

It belongs to a family of bipolar semiconductor devices

called thyristors. They consist of four semiconductor layers of alternating

p- and n-type material and operate as a switch having a

stable on- and off-states.

Numerous members of the thyristor family exist.

Circuit symbol and structure:

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Static i-v characteristic:

IG4

>IG3

> IG2

>IG1

>IG0

= 0

VBF = forward breakdown voltage

VBR= reverse breakdown voltage

IH = holding current

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Static i-v characteristic:

IL = latching current

IFO = forward leakage current

IRO = reverse leakage current

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Stable states:

Forward-biased blocking state: iT= 0, vAK 0

Conducting- (or on-) state: iT> 0,

(vAK

= 0.753 V )

Reverse-biased blocking state: iT= 0, vAK< 0

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Transfer between states:

From forward-biased blocking state to conducting state:

Current IGof appropriate value must flow into the gate

for a few microseconds. The SCR remains conducting after the gate pulse has

ceased provided that iThas risen above the latchingcurrent IL.

From conducting state to reverse-biased blocking state: The current iTmust be reduced to zero and a reverse

voltage must be maintained across the thyristor for aminimum turn-off time tqto allow stored charge in the

device to recombine. It is only after this time that the device is capable of

blocking a forward voltage without going into itsconducting state.

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Transfer between states Contd:

From reverse-biased blocking state to forward-biased

blocking state or vice versa:

This is determined by the external circuit.

The holding current

The holding current is the minimum anode current

necessary to keep the device in the conducting stateafter it has been carrying a high anode current.

If the anode current is reduced to below the holding

current, the device can move from its conducting

state to forward-biased blocking state.

The holding current is in the order of milliamps and is

less than the latching current

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Other characteristics:

As soon as conduction starts, the gate loses all further

control.

The SCR can be damaged if di/dt soon after triggering

exceeds some values specified by the manufacturer. For

this reason converters, with SCR should always be

connected to the mains via transformers or chokes. The SCR can be turned on by exceeding VBFeven if IG= 0.

This mode of switching is however not recommended

because the SCR can get damaged.

Increasing IGreduces the forward biased voltage

required to turn on the device and with IG= IG4, the

device behaves very much like a diode.

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tq= turn-off time.

Turning the SCR off requires that it be reverse biased

by the external circuit for a minimum time period

called turn-off time.

At the end of the turn-off time, the SCR is capable of

withstanding forward voltage without turning on

provided that its dv/dt is kept below a specified value.

In a line-commutated or naturally-commutated

converter circuit, a reverse voltage appears across the

thyristor immediately after the forward current

becomes zero. In forced-commutation techniques, the commutation

circuits are designed to apply a reverse voltage during

the turn-off process.

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SCR parameters:

They include:

VTM (peak on-state voltage), IH

IL

tq IRM(peak reverse current)

IFM(peak forward off-state current),

IGT(dc gate current to trigger) and

VGT(dc gate voltage to trigger)

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Maximum ratings:

They include:

Anode voltage ratings: VRRM= maximum repetitive peak reverse voltage

VRSM= maximum non-repetitive reverse voltage

VR(DC)

= DC reverse blocking voltage

VDRM= maximum repetitive peak forward off-state

voltage

VDSM= maximum non-repetitive peak forward off-

state voltage

VD(DC)= DC forward blocking voltage

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Maximum ratings Contd:

Anode current and current related ratings:

IT(AV), IF(AV)= maximum average forward on-state current

IT(RMS), IF(RMS) = maximum rms forward on-state current

ITSM = maximum non repetitive surge on-state current

di/dt = both non repetitive and repetitive rate of rise of

current I2t = the time integral of the square of the maximum

sinusoidal overload on-state current. Its value is used to

determine fusing for the device.

Gate ratings

PGMand PG= maximum peak and mean gate power.

VGRM= maximum peak negative voltage

IGFM = maximum peak forward gate current

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Types of SCRs

Phase-control SCRs:

They are also known as converter SCRs.

They generally operate at the line frequency (50 Hz and 60 Hz).

Turn-off time tqis of the order 50 to 100 s.

They have large voltage and current handling

capabilities.

The on-state voltage VTvaries from about 1.15 V for

600-V to 2.5 V for 4000-V devices, and for a 5500-A,

1200-V SCR it is typically 1.25 V. Modern SCRs are the amplifying gate type: An auxiliary

SCR TAis gated on by a gate signal and then the

amplified output of TAis applied as gate signal to the

main thyristor TM.

h l d

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Phase-control SCR Contd:

- The amplifying gate SCR has dv/dt typically of 1000 V/s

and di/dt of 500 A/s which are high.

- This simplifies the circuit design by reducing or

minimizing di/dt limiting inductor and dv/dt protection

circuits.

i hi

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Fast-switching SCRs

- They are used in high speed switching applications with

forced commutation e.g. choppers and inverters.

- They are also known as inverter SCRs.

- They have small tq, generally in the range of 5 to 50 s,

depending on the voltage range, high dv/dt of typically1000 V/s and di/dt of 1000 A/s.

- The on-state voltage of a 2200-A, 1800-V SCR is typically

1.7 V.

- Inverter SCRs with a very limited blocking capability,

typically 10 V and very fast tq(between 3 and 5 s) are

commonly known as asymmetrical SCRs (ASCRs).

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Fast-switching SCRs Contd

- They are used in inverter and induction heating

applications where they are not required to block reverse

voltage because feedback diodes are connected anti-

parallel with them.

Light-activated SCRs:

- These are triggered by a pulse of light guided by opticalfibers to a special sensitive region of the SCR.

- They are used in high voltage dc transmission where

several SCRs are connected in series to meet the high

voltage requirement.

- There are devices which can block 4 kV, conduct up to 3

kA with a forward voltage drop of 2 V at a trigger power

of 5 mW