POWER ELECTRONICS LAB

(2008 SCHEME)

LIST OF EXPERIMENTS

1. Sine triangle PWM generation

2. Study of PWM IC TL 494

3. Power BJT and MOSFET drive circuits

4. Battery charger circuit

5. Buck DC-DC Converters

6. Step up DC-DC converter

7. Push pull DC- DC Converter

8. Application of opto-coupler IC MCT2E

9. AC phase control circuit

10. Linear ramp firing circuits

11. Simple SMPS

12. Half bridge and full bridge converters

13. Study of DC Drive

14. Regulation Characteristics of DC Drive

15. Basic Inverter Circuits

Internal Marks: 50

1. Attendance - 10

2. Class work - 20

3. Practical internal Test - 20

Note: For University examination, the following guidelines should be followed regarding

award of marks

(a) Circuit and design - 20%

(b) Performance (Wiring, usage of equipments and trouble shooting) - 15%

(c) Result - 35%

(d) Viva voce - 25%

(e) Record - 05%

Experiment No: 1

SINE TRIANGLE PWM GENERATION

Aim:

To generate a sine-triangle PWM gating signal with carrier frequency = 5KHz and

amplitude=6Vpp and to observe the pulse width variations.

Components Required:

Op-amp 741, Capacitors, resistors, function generator, connecting wires, breadboard.

Circuit Diagram:

Design:

Select 741 op-amp as carrier generator and comparator. For the triangular wave circuit

given output amplitude

Vop=2xVsatx R2/R1

Frequency of the triangular output = R1/4R2R3C

Vop=6V, Vsat=10V

Assume R2=100K, then R1=2VsatR2/Vop

= 2x10x10K/6

= 33.3K

Frequency fc=R1/4R2R3C

R1=33K, R2=10K, assume R3=10K, then C=R1/4R2R3Xfc

= 0.0166μF

Select 0.01μF standard

Theory:

In this circuit the amplitude of reference sine wave is compared with the

amplitude of a carrier triangular wave using an op-amp comparator and the output is a

sinusoidal PWM.

A non-inverting Schmitt trigger (regenerative comparator) cascaded with an ideal

integrator using an op-amp is used for generating the carrier triangular wave. The

amplitude and frequency of the triangular wave is decided by the RC components used in

the circuit. The frequency of the PWM output is decided by the carrier frequency fc and

the pulse width variations are decided by the amplitude of the reference sinusoidal signal.

The maximum amplitude of the reference sine wave must be properly selected in such a

way that no saturation of the op-amp occurs and it must be in comparison with triangular

carrier amplitude ie; amplitude of the reference sine wave must be less than the amplitude

of the carrier.

For the op-amp comparator, the positive input is supplied with the reference

sinusoid and negative input with the carrier triangular wave. The output of the

comparator is the required sinusoidal PWM signal’

Procedure:

1. Test the components and setup the circuit as shown in figure.

2. Measure the frequency and amplitude of the carrier triangular wave.

3. Adjust the reference sinusoidal wave in such a way that its frequency is 10 times

lesser than the frequency of carrier triangular wave. Amplitude of the sine wave

should be less than that of triangular wave.

4. Then observe the PWM output and the changes in pulse width variations in

reference voltage.

Waveforms:

Result:

Sine triangle PWM signal is obtained with

Amplitude = ………………….. V

Frequency = …………………… KHz

The waveform is plotted in the graph sheet and variations in pulse width of the

output is observed.

Experiment No: 2

STUDY OF PWM IC TL 494

Aim:

To design and setup a PWM circuit for an output in single ended and pushpull mode

using IC 494 and plot the waveforms.

Components Required:

IC TL494, resistors, capacitors, connecting wires, breadboard, power supply.

Circuit Diagram:

Pushpull mode

Single ended mode

Design:

Fosc= 1.2/CTRT=100KHz

Let CT=1Μf, Vcc=15V, Ic=5Ma,then RC=5/5mA= 1K

The voltage at pin 3 is VCCxe^(-t/RC)

LetV=2.3V,VCC=15V,RC=τ

Let t=5sand τ=3s. Let 47μF , Then R=64 K. Use 100K pot.

Theory:

TL494 is a fixed frequency PWM control circuit. Modulation of output pulses is

accomplished by comparing the saw tooth waveform created by the internal oscillator on

the tuning capacitor (CT) to either of two control signals. The control signals are derived

from the dead time control circuit and the error amplifier. The PWM comparator

compares the control signal created by the error amplifiers.

TL494 is used as a PWM generator. If the output control (pin 13) terminal of the

IC is connected external to the reference voltage pin (pin14) a pushpull output is obtained

(outputs will be complementary with a minimum dead time of 5%). Pin 4 can be used for

adjusting the dead time. If a minimum dead time is required, pin 4 is grounded. The two

outputs are in single ended mode if the pin 13 is grounded.

Output pulsewidth can be varied from minimum to a maximum of 50% (pushpull

mode) when pin 3 is connected to a variable dc voltage. Thus pin3 is grounded to get a

50% duty cycle. In pushpull mode if an external voltage is given to pin 3 (0.5-3.2V) the

pulse width and hence the duty cycle can be varied. Rms value of the output voltage can

be changed by changing the duty cycle.

Frequency of output

f = 1.2/RTCT for single ended mode

f = 0.6/ RTCT for pushpull mode

Procedure:

1. Test the components and setup the circuit as shown in figure.

2. For pushpull mode observe the waveform at pin 9 and pin 10, V1 and V2.

3. Observe the waveform by connecting the control pin (pin 13) to ground for single

ended mode.

Waveforms:

Result:

Designed and setup PWM circuits in single ended and pushpull modes using

IC494 and obtained the waveforms.

Experiment No: 3

AC PHASE CONTROL CIRCUIT

Aim:

To realize a phase controlled ac voltage controller and to use it as a dimmer for a lamp.

Components Required:

Triac BT136, Diac IN 5756, Isolation transformer 230/230V, resistors, capacitors,

connecting wires, bread board, power supply.

Circuit Diagram:

Design:

Charging equation for the capacitor is

Vo= Vcc (1-e-t/RC

)

BT136 diac has the breakdown voltage of 20V

Vcc= 230√2 V

20=230√2 (1-e-t/RC

), So t/RC=0.63

Put t=0.5ms, R=7.9 x 10-4

Use 10K std, C=0.1µF

At 10ms for α=1800, RC=0.158

Put C=0.1µF then R= 100K

Theory:

If a thyristor switch is connected between ac supply and load, the power flow can

be controlled by varying the rms voltage applied to load and controller. The most

common application of voltage controller are industrial heating on load transformer tap

changing, light controls, speed control of polyphase induction motors and magnet

controls. Two controls are normally used

1 ON-OFF control

2 Phase angle control

In phase angle control, thyristor switch connects the load to ac source for a

portion of each cycle of input voltage. For application upto 400Hz triacs are commonly

used. The triac can be considered as two thyristors with their gates connected in parallel.

The flow of power to the load is controlled by delaying the firing angle of triac. Triacs act

as single phase bidirectional controllers. Varying the delay angle varies the power flow in

the positive and negative half cycles.

A diac is a two electrode, bidirectional avalanche diode that can be switched from

OFF state to ON state for either polarity of the applied voltage. The device consists of

two pnp sections in antiparallel order. The device is switched ON when the breakdown

voltage is reached (which is of either polarity).

In phase control circuit the triac is switched on and off in synchronism with the

mains so that only a part of each half cycle is transferred to the load. Triggering is done

by the diac, which ensures that the triac receives a clear first trigger pulse. The voltage

across the capacitor increases as input increases. The diac starts conducting when its

voltage reaches the breakdown point. The rapid conduction of the diac results in a drop in

voltage across it and a large current pulse is injected into the base of triac. This also

discharges the capacitor.

The point of firing is called the firing angle which can be varied by varying the

rate of charging of the capacitor. Since time constants of the capacitor is determined by

charging resistor R and capacitor C, the varying resistor can change charging time of

capacitor there by changing the firing angle of triac.

Procedure:

1. Check the diac and determine the breakdown voltage by connecting a variable dc

source in series with a 1KΩ resistor.

2. Set up the circuit as shown in figure.

3. Vary the firing angle by varying the potentiometer.

4. Observe the voltage across the diac, triac and the load.

Waveforms:

AC Input wave

Voltage across triac

Voltage across load

Result:

The voltages across diac, load and triac at various firing angles were observed and

plotted.

Experiment No: 4

BATTERY CHARGER CIRCUIT

Aim:

To design and set up a battery charger circuit.

Components Required:

LM317, resistors, dc battery, ammeter, voltmeter, connecting wires, bread board, power

supply.

Circuit Diagram:

Design:

Voltages across pin 3 and 1 of LM317 is 1.25V

Assume current flowing through RL IS 5Ma

2Ω+R1= 1.25/ 5x 10-3

x R2

Vo= 5x10-3

(RL+R1)+5x10-3

R2

Vo=12V, So R2=2.15K, Use 2.2K std

Theory:

IC LM317 is adjustable voltage regulator. It has only three terminals; adjust, input and

output. LM317 maintains normal 1.25V between its output and adjust terminals. This

voltage is called Vref and can vary from chip to chip 1.2 to 1.3V. LM317 will provide

regulated output current of 1.5A provided that it is not subjected to power dissipation of

more than 15W. This means that it should be electrically isolated from and can be

fastened to a LM317 requires a minimum of voltage of 3V across its input and output

terminals or it will drop out of regulation.

Procedure:

1. Connect the components on breadboard as per the circuit diagram.

2. Apply input voltage of 15V and note the corresponding output voltage.

Result:

Designed and set up a battery charger circuit of desired specification and verified

the output.

Experiment No: 5

APPLICATION OF OPTO-COUPLER IC MCT2E

Aim:

To set up an automatic lighting circuit using IC 555 and opto-coupler MCT2E.

Components Required:

Timer IC 555, Opto-isolator MCT2E, transistor SL100, Diode IN4001, Relay 12V,250Ω,

lamp, resistors, capacitor, connecting wires, bread board, power supply.

Circuit Diagram:

Design:

Let ID1=20mA and ID1max=60Ma

Then IC1=10Ma from the data sheet

R2= (Vcc-2)/ID1= (10-2)/20x10-3

=400Ω, Use 390Ω std.

IC2=12/250= 48mA, hfe=50, IB2=1mA

When Q1 is conducting IR3= IC1=10Ma

R3=12/10x10-3

= 1.2 K

When Q1 is not conducting IR3= IB2= 1mA

When R3+R4= 12-0.7/ 10-3

= 11.3K

R4= 10K

Theory:

The LDR has a resistance of about 20Ω when light fall on it and a resistance of

20KΩ when light does not fall on it. For IC555 when the voltage at the reset pin falls

below 0.7V the output is low. When the voltage at this pin lies above the reset voltage,

the flip flop output depends on the comparator output. The trigger pin (pin 2) is grounded

so that the flip flop is set and the output is high. When current flows through the diode of

the opto-isolator, the transistor is turned on and the output of the opto-isolator goes low.

The transistor Q2 is connected as the relay driver. When Q1 of the optoisolator conducts,

this transistor is off and the electric bulb is connected to the mains. When light falls on

LDR, IC 555 is reset and the voltage at pin 3 goes low. There is no current flow through

the diode of the optoisolator and Q1 is off. This causes Q2 to be turned on. When the

transistor Q2 is off, current flows through the relay and the bulb is disconnected parallel

to the coil to work as a free wheeling diode which provides a path for energy flow from

inductor coil. R4 is selected according to the required brightness level at which switching

is to take place.

Optocouplers are used to provide isolation. The need for isolation may be

explained as follows. For operating power transistors as switches an appropriate gate

voltage or base current must be applied to drive the transistor into saturation for low on

stage voltage. The control voltage must be applied between gate and ground (or emitter

and ground). The source (emitter) is not connected to ground isolation and interfacing

circuits are needed between the logic circuit and power transistor.

In MCT2E an LED is the source and the phototransistor is used as the detector.

Detecting region is the collector base junction. By adding additional elements in

combination with the detector, better sensitivity and response time can be achieved. Some

typical applications of optocouplers are in telephone communication, relay control, dc

power control and high voltage ac switching.

Procedure:

1. Design the circuit components as per required current and voltage.

2. Assemble the circuit as shown in the figure.

3. Connect the bulb in such a way as to turn ON when light is allowed to fall on the

LDR and vice versa.

4. Adjust R1 for threshold setting with reference to ambient light.

Result:

The bulb was lightened at different ambient conditions. Bulb glows when light

was on and turns off when light is there.

Experiment No: 6

LINEAR RAMP FIRING CIRCUIT

Aim:

To design and set up a linear ramp firing circuit for firing an SCR and to control the

firing angle.

Components Required:

Op-amp 741, Transistors BC 177 and 2N2222, Diode 1N4001, Transformer 230V/12V,

Resistors, Capacitors, power supply, connecting wires, bread board.

Circuit Diagram:

Design:

Let the constant current be 10mA and maximum voltage of ramp be 4V

VC=V(1-e-t/RC

)

1xt/C=4V, t=2ms,C=0.01µF

VB=15-1Kx10mA-0.7=4.8V

Vcc x (R2/R1+R2)=4.3V

R2=10K and R1=4.7K

For differentiator- C=0.01µF and R=1K

Theory:

An SCR can be switched from the off state to on state in several different ways

like forward voltage triggering, dV/dt triggering, light triggering and gate triggering. The

instant of turning on the SCR can be controlled by using any of the above mentioned

methods. Gate triggering is the best and the most commonly used method for triggering

the SCR. In the figure the output of an op-amp comparator is used to generate the firing

pulses to fire an SCR. A ramp voltage is applied to one of the terminals of the comparator

and a variable dc voltage is applied to the other terminal. By varying the dc voltage of the

input the instant of triggering can be varied.

A full wave rectifier is used to rectify the ac supply. The rectified signal is applied

to the base of transistor T3 which acts as a switch. The square pulse waveform at the

collector of transistor T3 is fed to another transistor T2. When transistor T2 turns off

capacitor CT charges linearly with constant current provided by transistor T3. When T2

turns on, the capacitor CT discharges instantaneously resulting in a linear ramp. The

waveform is level shifted before applying at the positive input of the comparator. By

using the dc level with which the ramp is compared, the firing angle can be varied.

Procedure:

1. Set up the circuit as shown in figure and apply the input.

2. Observe the waveforms at the collectors of different transistors and at the output

of the comparator.

3. Vary the potentiometer and observe the variations in linearity.

Waveforms:

Result:

The linear ramp firing circuit was set up and the waveforms at the collectors of

transistors were observed. A linear ramp was observed at the output and was plotted.

Experiment No: 7

BUCK DC-DC CONVERTER

Aim:

To study the working of DC-DC buck converter.

Components Required:

Power MOSFET IRF840, timer IC 555, zener diode, resistors, capacitors, power supply,

connecting wires, bread board

Circuit Diagram:

Design:

For 555IC T= 0.693(R1+2R2)C

Tc=0.693(R1+R2)C and Td=0.693R2C

Take Tc=18.2ms and Td=15ms

Select C=0.1µF, then R2=4.7Ω and R1=100Ω

Theory:

A dc-dc buck converter converts fixed dc input voltage to a variable dc output

voltage. The circuit consists of MOSFET as on/off switch which provides train of pulses

of dc voltage to load. An astable multivibrator using IC555 generates train of pulses,

which is applied to the gate of MOSFET as a switching pulse.

During the period Ton the circuit is on and supply voltage is available across the

load. During the period toff the circuit is off and the voltage across the load is zero. A

variable dc voltage is thus available across the load. The average load voltage can be

varied in two different ways

a) Frequency modulation or variable frequency system

b) Pulse width modulation or constant frequency system

Procedure:

1. Connect the circuit as shown in figure

2. Switch on the power supply

3. Calculate the output voltage in each case

4. Plot the waveforms

Waveforms:

Result:

Set up and studied the circuit of a dc-dc buck converter and obtained the required

waveforms.

Experiment No: 8

STEP UP DC-DC CONVERTER

Aim:

To study the working of a step up dc-dc converter.

Components required:

Power MOSFET IRF840, Timer IC555, diode BY126, transistor BC107, transformer,

resistors, capacitors, power supply, connecting wires, breadboard.

Circuit Diagram:

Design:

Tc=0.69(RA+RB)C

Td=0.69RBC

Take Tc=1ms and Td=20ms, f=50Hz, C2=0.1µF

Assume RA=4.7K and C1=1µF, then RB=4.7K

We have Rc=(Vcc-VCE)/Ic= (10-2)/2x10-3

=4.9K, use 4.7K std

Select BC107 transistor with hfe=100, IC=2mA

IB=IC/hfe=2mA/100=20µA

RB= VB/IB=10K

Theory:

A dc-dc converter is an electronic circuit which converts a source of direct current

from one volage level to another. It is a power converter. Dc- dc converter is important in

portable electronic devices like cellular phones, laptops, computers which are supplied

with power from battery primarily. Switch dc-dc converter off as a method to increase

voltage from partially lower battery voltage. This saves space instead of using multiple

batteries to accomplish the same thing. Most dc-dc converter also regulates output

voltage. Electronic software mode dc-dc converter converts one dc level to another by

storing the input energy temporarily and supplies that energy to the output as a difference

voltage.

In dc-dc converter there is an oscillatory winding. The 555 timer IC is connected

as astable multivibrator. The output of multivibrator is fed to the base of transistor which

is connected to the primary of the transformer. The primary voltage is applied according

to the output of 555IC and a corresponding secondary voltage is induced which is the

output of dc-dc converter. A filter capacitor can be connected to the load to filter the

ripple in dc output.

Procedure:

1. Set up the circuit as shown in figure.

2. Apply the dc supply voltage

3. Verify the output at pin 3 of 555IC

4. Verify the output dc voltage

Waveforms:

Result:

Setup and studied the working of a dc-dc converter and obtained the waveforms.

Experiment No: 9

POWER BJT AND MOSFET DRIVE CIRCUIT

Aim:

To design and set up a MOSFET gate drive and power BJT drive circuit.

Components required:

Timer IC555, Transistors BC 107, Power MOSFET IRF840, LEDs, diode 1N4001,

resistors, capacitors, power supply, connecting wires, bread board.

Circuit Diagram:

Design:

T=1.38RC, R1=R2=4.7K, C=0.1µF

Design of R1: Assume IC=2mA, hfe=100, then R1=(10-0.2)/10-3

=10K

Design of R2: IB=IC/HFE=1mA/100=10

RB=(10-0.6)/5x10x10-6

=10K

Design of RL: IC=10mA

RL=(Vcc-0.2)/10=1K

Theory:

The 555 timer is connected as an astable multivibrator. The generated square

wave is given to the base of transistor Q1 and thus the transistor drives the MOSFET or

power BJT.

During the positive half cycle of the square wave which is the output of astable

multivibrator, transistor Q1 is on and MOSFET is off. The collector voltage becomes

VCE(SAT) across transistor Q1. This output is given to the base of a MOSFET or power

BJT. This low voltage makes MOSFET off and Vcc is obtained across the collector of

MOSFET.

Similarly during the negative half cycle of the square wave transistor Q1 is off

and MOSFET is in on state. The voltage Vcc is given to the collector of MOSFET. The

voltage VCE is obtained at the collector of transistor Q1.

Procedure:

1. Check the components and setup the circuit as shown in the figure.

2. Verify the otput

Waveforms:

Result:

Designed and setup the MOSFET and BJT drive circuit using 555 timer.

Experiment No: 10

BASIC INVERTER CIRCUIT

Aim:

To study the working of an inverter circuit.

Components Required:

Timer 555IC, power MOSFET IRF840, transistor SL100, diode 1N4001, transformer,

resistors, capacitors, power supply, connecting wires, breadboard.

Circuit Diagram:

Design:

Tc=0.69(RA+RB)C

Td=0.69RBC

Take Tc=1ms and Td=20ms, f=50Hz, C2=0.1µF

Assume RA=4.7K and C1=1µF, then RB=4.7K

We have Rc=(Vcc-VCE)/Ic= (10-2)/2x10-3

=4.9K, use 4.7K std

Select BC107 transistor with hfe=100, IC=2mA

IB=IC/hfe=2mA/100=20µA

RB= VB/IB=10K

Theory:

An inverter is an electronic device that converts dc to ac. The inverted ac can be

at any required voltage and frequency with the use of appropriate transformers, switching

and control circuit. The inverter circuit performs the opposite function of a rectifier. The

inverter circuit is used to power a wide range of devices that are used in our everyday life

including electronic razors, fluorescent lamps, 12V car batteries etc.

The electrical inverter is a high power electronic oscillator. It is so named because

early mechanical ac to dc is converted to dc to ac. In one sample inverter circuit dc power

is connected to a transformer through the centre tap of the primary winding. A switch is

rapidly switched back and forth to allow current to flow back to the dc source following

two alternating paths through one end of the primary winding and then the other. The

alternation of the direction of current in the primary winding of the transformer produces

alternating current in the secondary winding.

Inverter circuit contains an oscillator circuit consisting of 555 timer IC connected

as astable multivibrator. The inclusion of the diode between pin 7 and 2 ensures that the

duty cycle of the square wave output is maintained at abut 50%. The output of 555 timer

IC drives the base of transistor which is connected to the gate terminal of one of the

MOSFET and the output of 555 timer IC directly drives the gate terminal of the other

MOSFET. So the input voltage is applied to the primary according to the output of the

555 timer IC and a corresponding secondary ac voltage is induced.

Procedure:

1. Set up the circuit as shown in the figure.

2. Apply the input dc voltage.

3. Verify the output ac voltage.

Waveforms:

Result:

Studied the working of an inverter circuit and obtained the required waveforms.