G. H. Raisoni College of Engineering, Nagpur
Department of Electronics & Communication Engineering
Subject:-Power Electronics
UNIT–I
1) Define latching & holding current as applicable to an SCR. Show these currents
on its VI characteristics.
2) Draw symbols and characteristics of following power semiconductor devices.
i)TRIAC ii) n-channel power MOSFET iii) GTO
3) Explain construction and operation of UJT.
4) Explain dv/dt rating .
5) Explain di/dt rating of a thyristor.
6) Compare performance and operation of power MOSFET and power transistor.
Also suggest two applications of power MOSFET.
7) Draw symbols and characteristics of following power semiconductor devices.
i) IGBT ii) SCS iii) RCT
8) Draw the dynamic characteristics of a thyristor during its turn-on and turn-off
processes. Show the variation of voltages and current through it during these two
dynamic processes. Indicate clearly the various intervals into which turn-on and
turn-off times can be subdivided. Discuss briefly the nature of these curves.
9) Explain the operation of TRIAC in various modes.
10) Explain the switching characteristics of GTO. Indicate clearly the various
intervals into which turn-on and turn-off times can be subdivided. Discuss briefly
the nature of these curves.
11) Draw complete protection scheme for a SCR and explain the use of each and
every component.
12) Write a note on application of TRIAC as AC phase controller.
13) Draw the thermal model of an SCR and explain why heat sink is required.
14) What are power MOSFETs? Mention some important features of power
MOSFETs.
15) Draw the transfer characteristics of an n-channel depletion type MOSFET.
16) Explain the principle and operation of SCR in detail.
17) Explain two transistor analogy of SCR.
18) Write short notes on Field controlled thyristor.
19) Write short note on Asymmetrical SCR.
20) Explain the operation of LASCR.
21) Explain forward voltage triggering of SCR.
22) Explain gate characteristics of SCR.
23) Explain how thyristors are categorized according to their constructions.
UNIT-II
1) Explain in detail how UJT works as a relaxation oscillator.
2) For the auxiliary commutation circuit, the supply voltage V=200 volt, L=10µH
and C=50 µF. Calculate the minimum on time of main SCR and the Peak value
of capacitor current.
3) Explain class B type commutation technique of thyristors
4) Draw a neat circuit diagram of class D Auxiliary commutation. Explain the
working in brief. Also draw all relevant waveforms. Comment on the design
aspect of commutating components.
5) Explain external pulse commutation technique
6) Explain the line commutation technique.
7) Explain with the help of neat circuit diagram and required waveforms the working
of “Integral cycle control.”
8) What do you mean by commutation of SCR? What are different methods of
forced commutation?
9) Explain complementary commutation with circuit diagram and waveforms in
detail.
10) Draw and explain the circuit for Extinction Angle Control, with various
waveforms.
11) Explain UJT as relaxation oscillator to generate the triggering pulses for SCR.
12) Take a circuit for resonant pulse commutation. Consider in this circuit C=20 µF,
L=5 µH Initial voltage across capacitor is Vs=230 V. For a constant load current
of 300 A, calculate a) conduction time for auxilliary thyristor b) voltage across
main thyristor when it gets commutated and c) circuit turn off time for main
thyristor.
13) A class B commutation circuit is connected to d.c. supply of 100 V. the load
current to be commutated is 10A and turn off time of SCR used is 40 µsec.
Design suitable values of commutating components.
14) A class B commutation circuit is connected to 100 V DC supply. The load
resistance is 20 ohm and turn offtime of SCR used is 50 µsec. A) Design
suitable values of commutating components. B) Calculate how long wiil the SCR
conduct after applying a gate pulse.
15) In a class C commutation circuit Vs=200V, R1=10 ohm, R2=100 ohm. Determine
a) peak value of current through thyristors T1 and T2. b) value of capacitor C if
each thyristor has turn off time of 40 µsec. Take a factor of safety 2.
16) Explain in brief how the following techniques can be employed to improve power
factor in case of single phase bridge converters. I) symmetrical angle control ii)
extinction angle control
17) With a neat circuit diagram, explain the RC triggering method used for SCR. The
SCR feeds a resistive load from a single phase ac mains. Draw the waveforms of
applied voltage, load current and the capacitor voltage. Take a firing angle of
α=135 ˚ in the above case.
18) Write short note on symmetrical angle control.
19) What is forced commutation? What are the advantages of forced commutation for
ac-dc converters.
20) Discuss various methods of forced commutation in brief.
21) Give broad classification of commutation techniques and explain any one of them.
UNIT-III
1) Explain the operation of single phase half wave uncontrolled rectifier with
resistive load. Derive the expression for average dc voltage. Draw the
waveforms for input voltage, load voltage and load current
2) A single phase full bridge diode rectifier is supplied from 230 V, 50 Hz source.The
load consists of 10 ohm and a large inductance so as to render the load current
constant. Determine i) average values of output voltage and output current & ii) rms
value of output voltage.
3) A Single phase uncontrolled diode bridge rectifier feeds a resistive inductive load.
Derive the expression for average output voltage. Also draw all the relevant
waveforms.
4) A single phase full bridge diode rectifier is supplied from 230 V, single phase ac
supply. Resistive load of 60 ohm is connected across output of the rectifier.
Calculate DC power consumed by the load.
5) Draw and explain three phase full wave uncontrolled bridge rectifier with input
and output waveforms. Also derive the expression for average output voltage.
6) Explain the working of 6 pulse uncontrolled rectifier. Draw the waveforms of
output voltage, voltage across the diode and show the conduction period of all
diodes.
7) For three phase full wave rectifier, Vm=480 volts, find the average value of
output voltage.
8) A single phase half wave rectifier is connected to an ac supply of 240 volts, 50
Hz.determine the current waveform, the average load voltage and average load
current for a load of I) pure resistance of 10 ohm ii) an inductance of 0.1 H in
series with a 10 ohm resistance.
9) A single phase diode bridge rectifier is operated from mains 230 V. Find out the
following.
a. Average and RMS output voltage.,
b. Average and RMS output power
c. Ripple factor and form factor of the output voltage.
d. If RL=10 ohm and the load is large inductance in series with RL.
10) A single phase diode wave rectifier is feeding power to a pure resistor such that
Idc=50 amp at output voltage of Vdc=300V.Assuming normal power efficiency,
find I)form factor ii) ripple factor iii) the efficiency of the rectifier.
11) A three phase uncontrolled bridge rectifier operates with resistive load. Draw the
waveforms of i) input phase voltage. ii) line voltage, iii) output voltage. Derive
for average output voltage.
12) Explain the operation of three phase bridge six pulse (B-6) diode rectifier
supplying highly inductive load with suitable waveforms. Draw input and output
voltage waveforms.
13) The single phase bridge rectifier is supplied from 120 V, 60 Hz source. The load
resistance is R=500 ohm. Calculate value of series inductor L that will limit the
rms ripple current Iac to less than 5% of Idc.
14) What are the advantages of a three phase rectifier over single phase rectifier?
Derive expression for output voltage in case of their phase bridge rectifier.
15) Explain single phase half wave rectifier with free wheeling diode.
16) Explain single phase full wave midpoint diode rectifier.
17) The single phase full converter is operated with symmetrical angle control. The
load current with an average value of Id is continuous, where the ripple current is
negligible. i) Express the input current of converter in fourier series.
UNIT-IV
1) Draw and explain the circuit of single phase full wave fully controlled bridge
rectifier with heavily inductive load with neat waveforms. Also derive the
expression for Vdc.
2) Explain the circuit for three phase half controlled bridge rectifier(with inductive
load) with i/p and o/p voltage waveforms.
3) What is the purpose of connecting free wheeling diode in controlled rectifiers
when the loads are Inductive?
4) Explain the effect of source inductance on the operation of single phase full
converter and derive the expressions for the average load voltage in terms of α
and µ.
5) A single phase fully controlled thyristor bridge converter supplies an inductive
load. Assuming that the output current is virtually constant equal to I0, determine
the following if supply voltage is = 220 V and α=60˚. i) average output voltage ii)
Supply rms current iii)supply power factor
6) With the help of equivalent circuit of dual converter, prove that α1 + α2=180˚
7) Explain the need of freewheeling diode in converters.
8) Write a short note on dual converters.
9) A full converter is operated from 230V/50 Hz mains and is delivering power to a
resistance RL= 10 ohm in series with a large smoothening inductor. Find out for α
=135˚,i)Vo(av) ii)Vo(rms)
10) For a 3 phase semiconverter operating from a 3 phase ideal supply and delivering
power to a purely resistive load, derive an equation for average output voltage if
the triggering angle is α .
11) A three phase full converter is supplying power to a purely resistive load of
RL=100 ohm. The supply is 415 V/50 Hz. Find out the average load voltage and
power dissipation for α=45˚.
12) Explain how the power on the load side can be controlled by using phase angle
control in half wave rectifier with SCR. At which firing angle we get the
maximum power.
13) Show that performance of a single phase full converter influenced by source
inductance is given by Cos(α+ µ ) = cos α - (WLS/Em).Io
14) For a 3 phase full converter, sketch the time variations of input voltage and the
voltage across one SCR for one complete cycle for a firing angle delay of 30 ˚.
15) A single phase fully controlled bridge converter supplies resistive load. By
assuming the constant output current Io, find the following performance factor if
the supply voltage is 230 V and if firing angle is П/6 and R=5 ohm.
i) Average output voltage
ii) supply RMS current
iii) input power factor
iv) voltage ripple factor
16) Draw and explain the working of single phase fully controlled bridge converter.
Draw the output waveforms for load voltage, load current, voltage across any one
SCR for
a. highly inductive load
b. purely resistive load. Take a firing angle of α =60˚.
c. Also explain the effect of free wheeling diode on the performance of
single phase converter.
d. For highly inductive load without FWD, explain the inverting mode
operation for a firing angle of α =135˚.
17) Discuss the effect of source inductance on bridge converter.
UNIT-V
1) Single phase half bridge inverter has a resistive load of R=3 ohms and the dc
input voltage Edc=50 v. Calculate i)The RMS output voltage at the fundamental
frequency E1 ii)the output power Po iii)the average and peak current of each
thyristor and iv)the peak reverse blocking voltage of each thyristor.
2) Single phase full bridge inverter has a resistive load of R=3 ohms and the dc input
voltage Edc=50 v. Compute i) The RMS output voltage at the fundamental
frequency E1 ii)the output power Po iii)the average and peak current of each
thyristor and iv)the peak reverse blocking voltage of each thyristor.
3) A single-phase bridge inverter delivers power connected RLC load with R=3
ohms and WL=12 ohms. The periodic time T=0.2 ms. What value of C should the
load have in order to obtain load commutation for the SCR’s. The thyristor turn
off time is 12 micro sec. Assume the circuit turn off time as 1.5tq. Also that the
load current contains only fundamental components.
4) For the McMurray single phase inverter circuit, obtain the value of commutating
components L and C for the following parameter values: i)max. load current=80
A ii) tq of SCRs=40 micro sec. iii)Edc(min)=200 v.
5) The single phase Mcmurray full bridge inverter is fed by a dc source of 300v. The
dc source voltage may fluctuate by 15%. The current during commutation may
vary from 20 to 100 A. Obtain the value of commutating components if the
thyristor turn off time is 20 micro sec. Also compute the value of R.
6) Draw and explain the simple SCR series inverter ckt. Employing Class A type
commutation. Draw and discuss the important waveforms. State the limitations of
the same.
7) Draw a Modified series inverter circuit. Explain quantitatively how you can have
output freq. higher than series resonance freq.
8) Give detailed design aspects of series inverter.
9) Draw and explain the operation of three-phase series inverter circuit.
10) Explain the operation of self commutated inverter circuit with the help of ckt.
Diagram.
11) Explain the operation of a single-phase parallel-commutated inverter without
freewheeling diode, with the help of voltage and current waveforms.
12) Give the detailed circuit analysis of single-phase parallel-commutated inverter.
13) Explain the operation of single-phase bridge inverter with the help of load,
voltage and load current waveforms.
14) Draw the circuit of McMurray inverter and explain its operation by drawing
voltage and current waveforms.
15) Draw and explain the operation of three-phase bridge inverter with input circuit
commutation.
16) What is the need for controlling the output voltage at the output terminal of an
inverter?
17) Discuss briefly and compare the various methods employed for the control of
output voltage of an inverter.
18) What is PWM? List the various PWM techniques. How do these differ from each
other?
19) Give the performance comparison of PWM, AVI and CSI.
20) Give the circuit analysis of CSI with resistive load.
21) Explain in brief the voltage control of three phase inverters.
22) State the need for reduction of harmonics in inverter.
23) Outline the various methods for reduction of harmonics or the improvement in
wave shape in short.
UNIT-VI
1) Draw the schematic of step down chopper and derive an expression for output
voltage in terms of duty cycle for the same.
2) With the circuit diagram and output voltage waveforms, explain the working of
Jones chopper.
3) Explain the time ratio control and current limit control, and control strategies used
for chopper.
4) With the help of circuit diagram, explain the working of step up/down chopper.
5) With the help of circuit diagram explain two-quadrant chopper.
6) Explain the continuous and non-continuous conduction mode of type A chopper.
7) With the help of circuit diagram and associated waveforms, explain the principle
of working of type C chopper.
8) With the help of circuit diagram and associated waveforms, explain the principle
of working of type D chopper.
9) Give the classification of chopper commutation.
10) Draw the schematic of type E chopper and explain the working of the same.
11) Describe the voltage-commutated chopper with associated waveforms.
12) Describe the current commutated chopper with associated waveforms.
13) Mention the advantages of Jones chopper over others and give the applications of
this chopper.
14) Explain in brief how average voltage across the load is made more than d.c.
supply voltage using chopper. Derive expression for the average voltage.
15) Discuss the working of load-commutated chopper with associated voltage and
current waveforms. Also derive an expression for which the value of commutating
capacitor of the chopper can be calculated.
16) Enumerate the merits and demerits of load-commutated chopper.
17) Describe the Morgan’s chopper with waveforms.
18) What is Multiphase chopper? List the merits and demerits of Multiphase chopper.
19) A dc chopper circuit connected to a 100v dc source supplies an inductive load
having 40 mH with a resistance of 5 ohms. A freewheeling diode is placed across
the load. The load current varies between the limits of 10 A and 12 A. Determine
the time ratio of the chopper.
20) A chopper circuit is operating on TRC principle at a freq. of 2 KHz on a 220v dc
supply. If the load voltage is 170 V, compute the conduction and blocking period
of thyristor in each cycle.
21) A step up chopper is used to deliver load voltage of 500 V from a 220 V dc
source. If the blocking period of the thyristor is 80 micro sec., compute the
required pulse width.
22) Write short notes on SMPS.
23) Write short notes on UPS.
24) Explain in brief working of push pull SMPS.