2.0 BACKGROUND

2.1 Elements of Circuits

2.1.1 Transformer

Fig.1: The Transformer

A Transformer is an electrical device that transfers energy between two circuits through

electromagnetic induction. A transformer may be used as a safe and efficient voltage converter to

change the AC voltage at its input to a higher or lower voltage at its output without changing the

frequency. Other uses include current conversion, isolation with or without changing voltage

and impedance conversion.

A transformer most commonly consists of two windings of wire that are wound around a

common core to provide tight electromagnetic coupling between the windings. The core material

is often a laminated iron core. The coil that receives the electrical input energy is referred to as

the primary winding; the output coil is the secondary winding.

Applications of Transformer: Transformers perform voltage conversion, isolation protection,

and impedance matching. In terms of voltage conversion, transformers can step up voltage and

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step down current from generators to high voltage transmission lines, and step down voltage/step

up current to local distribution circuits or industrial customers. The step-up transformer is used to

increase the secondary voltage relative to the primary voltage. The step-down transformer is used

to decrease the secondary voltage relative to the primary voltage. Transformers range in size

from thumbnail-sized units used in microphones to those weighing hundreds of tons

interconnecting the power grid. A broad range of transformer designs are used in electronic and

electric power applications, including miniature, audio, isolation, high frequency, power

conversion, etc.

An isolation transformer is a transformer used to transfer electrical power from a source of

alternating current (AC) power to some equipment or device while isolating the powered device

from the power source, usually for safety reasons. Isolation transformers provide galvanic

isolation and are used to protect against electric shock, to suppress electrical noise in sensitive

devices, or to transfer power between two circuits that must not be connected. A transformer sold

for isolation is often built with special insulation between primary and secondary, and is

specified to withstand a high voltage between windings.

Isolation transformers block transmission of the DC component in signals from one circuit to the

other, but allow AC components in signals to pass. Transformers that have a ratio of 1 to 1

between the primary and secondary windings are often used to protect secondary circuits and

individuals from electrical shocks. Suitably designed isolation transformers block interference

caused by ground loops. Isolation transformers with electrostatic shields are used for power

supplies for sensitive equipment such as computers or laboratory instruments.

Isolation transformers are designed with attention to capacitive coupling between the two

windings. The capacitance between primary and secondary windings would also couple AC

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current from the primary to the secondary. A grounded Faraday shield between the primary and

the secondary greatly reduces the coupling of common-mode noise. This may be another

winding or a metal strip surrounding a winding. Differential noise can magnetically couple from

the primary to the secondary of an isolation transformer, and must be filtered out if a problem.

The ideal transformer induces secondary voltage VS as a proportion of the primary voltage VP

and respective winding turns as given by the equation:

V P

V S=

NP

N S=a (1)

where

a: is the winding turns ratio

V P: is t h e primary voltage

V S : is t he secondary voltage

The power losses due to transformer could be found by

W h ≈ η βmax1.6 (2)

Hysteresis loss is obtained by:

Ph=ηW h=η . f βmax1.6 (3)

Where f is the frequency, η is the hysteresis coefficient and βmax is the maximum flux density.

2.1.2 Diode 

 Diode is an electrical device allowing current to move through it in one direction with far

greater ease than in the other. Semiconductor diode is considered as the most common kind of

diode in modern circuit design. The schematic diagram of diode is shown in Figure 2 below.

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Fig.2: Semiconductor diode schematic symbol: Arrows indicate the direction of electron current flow.

Fig.3: Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.

When the polarity of the battery is such that electrons are allowed to flow through the diode, the

diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode

blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch:

“closed” when forward-biased and “open” when reverse-biased as shown in Figure 3.

2.1.3 Optocoupler

An optical coupler, also called opto-isolator, optocoupler, opto coupler, photocoupler or optical

isolator, is a passive optical component that can combine or split transmission data (optical

power) from optical fibers. It is an electronic device which is designed to transfer electrical

signals by using light waves in order to provide coupling with electrical isolation between its

input and output. The main purpose of an optocoupler is to prevent rapidly changing voltages or

high voltages on one side of a circuit from distorting transmissions or damaging components on

the other side of the circuit. An optocoupler contains a light source often near an LED which

converts electrical input signal into light, a closed optical channel and a photosensor, which

detects incoming light and either modulates electric current flowing from an external power

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supply or generates electric energy directly. The sensor can either be a photoresistor, a silicon-

controlled rectifier, a photodiode, a phototransistor or a triac. A schematic diagram for

optocoupler is shown in Figure 4 below.

Fig. 4: Schematic diagram for optocoupler.

Types of Optocouplers

There are many different kinds of optocouplers and at Future Electronics we stock many of the

most common types categorized by number of channels, operating temperature, output voltage,

isolation voltage, packaging type and current transfer ratio (CTR). The parametric filters on our

website can help refine your search results depending on the required specifications.

The most common sizes for maximum output voltage are 30 V, 70 V and 80 V. We also carry

optocouplers with a maximum output voltage up to 80 kV. The maximum operating temperature

can range from 70 oC to 150 oC, with the most common opto coupler chips having a maximum

operating temperature of 100 oC.

2.1.5 Bipolar Junction Transistor (BJT)

Transistors a semiconductor device that controls current between two terminals based on the

current or voltage at a third terminal. It is used for amplification or switching of electrical

signals.

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The BJT is a transistor with three regions and two pn junctions. The regions are named the

emitter, the base, and the collector and each is connected to a lead.

For normal operation, the base-emitter junction is forward-biased and the base collector junction

reverse-biased.

Figure 5.12 shows the typical common-base current–voltage characteristics.

When the collector–base junction is reverse biased, then for constant values of emitter

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MOSFET

The n-type Metal-Oxide-Semiconductor Field-Effect-Transistor (nMOSFET) consists of a

source and a drain, two highly conducting n-type semiconductor regions, which are isolated

from the p-type substrate by reversed-biased p-n diodes. A metal or poly-crystalline gate covers

the region between source and drain. The gate is separated from the semiconductor by the gate

oxide [4]. The basic structure of an n-type MOSFET and the corresponding circuit symbol are

shown in Figure 2.

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Modes of operation:

The operation of a MOSFET can be separated into three different modes, depending on the

voltages at the terminals. In the following discussion, a simplified algebraic model is used.

Modern MOSFET characteristics are more complex than the algebraic model presented here.

For an enhancement-mode, n-channel MOSFET, the three operational modes are:

Cutoff, subthreshold, or weak-inversion mode

When VGS < Vth:

where   is gate-to-source bias and   is the threshold voltage of the device.

According to the basic threshold model, the transistor is turned off, and there is no conduction

between drain and source. A more accurate model considers the effect of thermal energy on

the Fermi–Dirac distribution of electron energies which allow some of the more energetic

electrons at the source to enter the channel and flow to the drain. This results in a subthreshold

current that is an exponential function of gate–source voltage. While the current between drain

and source should ideally be zero when the transistor is being used as a turned-off switch, there

is a weak-inversion current, sometimes called subthreshold leakage.

Triode mode or linear region (also known as the ohmic mode

When VGS > Vth and VDS < ( VGS – Vth )

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The transistor is turned on, and a channel has been created which allows current to flow between

the drain and the source. The MOSFET operates like a resistor, controlled by the gate voltage

relative to both the source and drain voltages.The current from drain to source is modeled as:

I D=μn CoxWL ((V GS−V th) V DS−

V DS2

2 )where   is the charge-carrier effective mobility,   is the gate width,   is the gate length

and   is the gate oxide capacitance per unit area.

Saturation or active mode:

The switch is turned on, and a channel has been created, which allows current to flow between

the drain and source. Since the drain voltage is higher than the source voltage, the electrons

spread out, and conduction is not through a narrow channel but through a broader, two- or three-

dimensional current distribution extending away from the interface and deeper in the substrate.

The onset of this region is also known as pinch-off to indicate the lack of channel region near the

drain. Although the channel does not extend the full length of the device, the electric field

between the drain and the channel is very high, and conduction continues.

MOSFET Critical Parameters

When switch mode operation of the MOSFET is considered, the goal is to switch between the

lowest and highest resistance states of the device in the shortest possible time. Since the practical

switching times of the MOSFETs (~10ns to 60ns) is at least two to three orders of magnitude

longer than the theoretical switching time (~50ps to200ps), it seems important to understand

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the discrepancy. Referring back to the MOSFET models in Figure 2, note that all models include

three capacitors connected between the three terminals of the device. Ultimately, the switching

performance of the MOSFET transistor is determined by how quickly the voltages can be

changed across these capacitors.

Therefore, in high speed switching applications, the most important parameters are the parasitic

capacitances of the device. Two of these capacitors, the CGS and CGD capacitors

correspond to the actual geometry of the device while the CDS capacitor is the capacitance of the

base collector diode of the parasitic bipolar transistor (body diode).

When switch mode operation of the MOSFET is considered, the goal is to switch between the

lowest and highest resistance states of the device in the shortest possible time. Since the practical

switching times of the MOSFETs (~10ns to 60ns) is at least two to three orders of magnitude

longet r han the theoretical switching time (~50ps to200ps), it seems important to understand

the discrepancy. Referring back to the MOSFET models in Figure 2, note that all models include

three capacitors connected between the three terminals of the device. Ultimately, the switching

performance of the MOSFET transistor is determined by how quickly the voltages can be

changed across these capacitors.

Therefore, in high speed switching applications, the most important parameters are the parasitic

capacitances of the device. Two of these capacitors, the CGS and CGD capacitors

correspond to the actual geometry of the device while the CDS capacitor is the capacitance of the

base collector diode of the parasitic bipolar transistor (body diode).

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The CGS capacitor is formed by the overlap of the source and channel region by the gate

electrode. Its value is defined by the actual geometry of the regions and stays constant (linear)

under different

operating conditions.

The CGD capacitor is the result of two effects. Part of it is the overlap of the JFET region and

the gate electrode in addition to the capacitance of the depletion region which is non-linear. The

equivalent CGD capacitance is a function of the drain source voltage of the device approximated

by the following formula:

CGD≈CGD ,0

1+K1 √V DS

Unfortunately, non of the above mentioned capacitance values are defined directly in the transistor data

sheets. Their values are given indirectly by the CISS, CRSS, and COSS capacitor values and must be

calculated as:

CGD = C RSS

CGS = C ISS − CRSS

CDS=C0 ss−C RSS

Further complication is caused by the CGD

Capacitor in switching applications because it is Placed in the feedback path between the input and

output of the device. Accordingly, its effective value in switching applications can be much larger

depending on the drain source voltage of the MOSFET. This phenomenon is called the “Miller” effect

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and it can be expressed as:

CGD,equ=(1+gfs . RL) . CGD

Since the CGD and CDS capacitors are voltage dependent, the data sheet numbers are valid only at the

test conditions listed. The relevant average capacitances for a certain application have to be calculated

based on the required charge to establish the actual voltage change across the capacitors. For most

power MOSFETs the following approximations can be useful

CGD,avg=2.C RSS .√ V DS ,spec

V DS ,off

COSS ,avg=2.COSS ,spec √ V DS , spec

V DS , off

The next important parameter to mention is the gate mesh resistance, RG,I. This parasitic resistance

describes the resistance associated by the gate signal distribution within the device.its very important in

high speed switching cause it is in between the driver and the input capacitor of the device.

The transeconductance of the mosfet called “small gane in the linear region of its operation.” Its

important at which mosfet is turnd-on or off, the relationship between transconductance and drain current

and gate to source voltage is:

gfs=dI D

dV GS

The maximum current of the mosfet in the linear mode is :

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I D=(V GS−V t h ) . g fs

SWITCHING APPLICATIONS

Now, that all the players are identified, let’s investigate the actual switching behavior of the

MOSFET transistors. To gain a better understanding of the fundamental procedure, the parasitic

inductances of the circuit will be neglected. Later their respective effects on the basic operation

will be analyzed individually. Furthermore, the following descriptions relate to clamped

inductive switching because most MOSFET transistors and high speed gate drive circuits used in

switch mode power supplies work in that operating mode.

Turn-On procedure

The turn-on event of the MOSFET transistor can be divided into four intervals as depicted in Figure 4.

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In the first step the input capacitance of the device is charged from 0V to VTH. During this interval most

of the gate current is charging the CGS capacitor. A small current is flowing through the CGD capacitor

too. As the voltage increases at the gate terminal and the CGD capacitor’s voltage has to be slightly

reduced.

This period is called the turn-on delay, because both the drain current and the drain voltage of the device

remain unchanged.

This period is called the turn-on delay, because both the drain current and the drain voltage of the device

remain unchanged. Once the gate is charged to the threshold level, the MOSFET is ready to carry current.

In the second interval the gate is rising from VTH to the Miller plateau level, VGS,Miller. This is the

linear operation of the device when current is proportional to the gate voltage. On the gate side, current is

flowing into the CGS and CGD capacitors just like in the first time interval and the VGS voltage is

increasing. On the output side of the device, the drain current is increasing, while the drain-to-source

voltage stays at the previous level (VDS,OFF). This can be understood looking at the schematic in Figure

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3. Until all the current is transferred into the MOSFET and the diode is turned-off completely to be able

to block reverse voltage across its pn junction, the drain voltage must stay at the output voltage level.

Entering into the third period of the turn-on procedure the gate is already charged to the sufficient voltage

(VGS,Miller) to carry the entire load current and the rectifier diode is turned off. That now allows the

drain voltage to fall. While the drain voltage falls across the device, the gate to- source voltage stays

steady. This is the Miller plateau region in the gate voltage waveform. All the gate current available from

the driver is diverted to discharge the CGD capacitor to facilitate the rapid voltage change across the

drain-to-source terminals. The drain current of the device stays constant since it is now limited by the

external circuitry, i.e. the DC current source.

The last step of the turn-on is to fully enhance the conducting channel of the MOSFET by applying a

higher gate drive voltage. The final amplitude of VGS determines the ultimate on-resistance of the device

during its on-time. Therefore, in this fourth interval, VGS is increased from VGS,Miller to its final value,

VDRV. This is accomplished by charging the CGS and CGD capacitors, thus gate current is now split

between the two components. While these capacitors are being charged, the drain current is still constant,

and the drain-to source voltage is slightly decreasing as the on resistance of the device is being reduced.

Turn-Off procedure

The description of the turn-off procedure for the MOSFET transistor is basically back tracking

the turn-on steps from the previous section. Start with VGS being equal to VDRV and the

current in the device is the full load current represented by IDC in Figure 3. The drain-to-source

voltage is being defined by IDC and the RDS(on) of the MOSFET. The four turn-off steps are

shown in Figure5. for completeness.

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Figure 5. MOSFET turn-off time intervals

The first time interval is the turn-off delay which is required to discharge the CISS capacitance

from its initial value to the Miller plateau level. During this time the gate current is supplied by

the CISS capacitor itself and it is flowing through the CGS and CGD capacitors of the MOSFET.

The drain voltage of the device is slightly increasing as the overdrive voltage is diminishing. The

current in the drain is unchanged. In the second period, the drain-to-source voltage of the

MOSFET rises from ID⋅RDS(on) to the final VDS(off) level, where it is clamped to the output

voltage by the rectifier diode according to the simplified schematic of Figure 3. During this time

period – which corresponds to the Miller plateau in the gate voltage waveform - the gate current

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is strictly the charging current of the CGD capacitor because the gate-to-source voltage is

constant. This current is provided by the bypass

capacitor of the power stage and it is subtracted from the drain current. The total drain current

still equals the load current, the beginning of the third time interval is signified by the turn-on of

the diode, thus providing an alternative route to the load current. The gate voltage resumes

falling from VGS,Miller to VTH. The majority of the gate current is coming out of the CGS

capacitor, because the CGD capacitor is virtually fully charged from the previous time interval.

The MOSFET is in linear operation and the declining gate-to-source voltage causes the drain

current to decrease and reach near zero by the end of this interval. Meanwhile the drain voltage is

steady at VDS(off) due to the forward biased rectifier diode. The last step of the turn-off

procedure is to fully discharge the input capacitors of the device. VGS is further reduced until it

reaches 0V. The bigger portion of the gate current, similarly to the third turn-off time interval,

supplied by the CGS capacitor. The drain current and the drain voltage in the device are

unchanged. Summarizing the results, it can be concluded that the MOSFET transistor can be

switched between its highest and lowest impedance states (either turn-on or turn-off) in four time

intervals. The lengths of all four time intervals are a function of the parasitic capacitance values,

the required voltage change across them and the available gate drive current. This emphasizes

the importance of the proper component selection and optimum gate drive design for high speed,

high frequency switching applications.

Characteristic numbers for turn-on, turn-off delays, rise and fall times of the MOSFET switching

waveforms are listed in the transistor data sheets. Unfortunately, these numbers correspond to the

specific test conditions and to resistive load, making the comparison of different manufacturers’

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products difficult. Also, switching performance in practical applications with clamped inductive

load is significantly different from the numbers given in the data sheets.

PIC Microcontroller

The microcontrollers played revolutionary role in embedded industry after the invention of

Intel 8051. The steady and progressive research in this field gave the industry more efficient,

high-performance and low-power consumption microcontrollers. The AVR, PIC and ARM are

the prime examples. The new age microcontrollers are getting smarter and richer by including

latest communication protocols like USB, I2C, SPI, Ethernet, CAN etc.  

 

 

History:

The PIC microcontroller was developed by General Instruments in 1975. PIC was developed when

Microelectronics Division of General Instruments was testing its 16-bit CPU CP1600. Although the

CP1600 was a good CPU but it had low I/O performance. The PIC controller was used to offload the I/O

the tasks from CPU to improve the overall performance of the system.

 

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In 1985, General Instruments converted their Microelectronics Division to Microchip Technology. PIC

stands for Peripheral Interface Controller. The General Instruments used the acronyms Programmable

Interface Controller and Programmable Intelligent Computer for the initial PICs (PIC1640 and PIC1650).

 

In 1993, Microchip Technology launched the 8-bit PIC16C84 with EEPROM which could be

programmed using serial programming method. The improved version of PIC16C84 with flash memory

(PIC18F84 and PIC18F84A) hit the market in 1998.

Development:

Since 1998, Microchip Technology continuously developed new high performance microcontrollers with

new complex architecture and enhanced in-built peripherals. PIC microcontroller is based on Harvard

architecture. At present PIC microcontrollers are widely used for industrial purpose due to its high

performance ability at low power consumption. It is also very famous among hobbyists due to moderate

cost and easy availability of its supporting software and hardware tools like compilers, simulators,

debuggers etc.

 

PIC18F4520

CPU

Up to 10 MIPS Performance at 3V

C compiler optimized RISC architecture

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8x8 Single Cycle Hardware Multiply

System

Internal oscillator support-31 kHz to 8MHz with 4xPLL

Fail-Safe Clock Monitor- allows safe shutdown if clock fails

Watchdog Timer with separate RC oscillator

Wide operating Voltage range; 2.0V to 5.5V

nanoWatt Power Managed Modes

Run, Idle and SLEEP modes

Idle mode currents down to 5.8uA typical

Sleep mode currents down to 0.1uA typical

Analog Features

10-bit ADC, 13 channels, 100K samples per second

Programmable Low Voltage Detection Module

Programmable Brown-out-Reset Module

Two Analog Comparators multiplexing

Peripherals

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Master Synchronous Serial Port supports SPI™ and I2C™ master and slave mode

EUSART module including LIN bus support

Four Timer modules

Up to 5 PWM outputs

Up to 2 Capture / Compare

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