EC-604-C Power Electronics - May 2023 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aWhat do you understand by fast recovery diodes? With the help of diagram explain the static characteristics of power diodes.
   How to answer

   • Define fast recovery diode and reverse recovery time
   • Describe static V-I characteristic regions of power diode: forward conduction, reverse blocking, breakdown
   • Explain effect of reduced recovery time on switching
   • Sketch static characteristic diagram with labels
2. 7 Marks
   bDraw and explain the transfer characteristic of power MOSFET.
   How to answer

   • Define power MOSFET structure in brief
   • Sketch transfer characteristic Id vs Vgs showing threshold voltage
   • Explain cut-off, linear and saturation regions
   • Mention application relevance
3. 7 Marks
   aDraw the static V-I characteristic of the SCR and explain its mode of operation.
   How to answer

   • Sketch static V-I characteristic with forward blocking, conduction, reverse blocking
   • Define latching, holding current, breakover voltage
   • Explain forward blocking, forward conduction, reverse blocking modes
   • Explain gate current effect on breakover
4. 7 Marks
   bDescribe with neat diagram the working of IGBT with its transfer characteristics.
   How to answer

   • Show IGBT structure and equivalent circuit diagram
   • Explain gate controlled turn-on and conductivity modulation
   • Sketch transfer characteristic Ic vs Vge
   • Mention input and output characteristics
5. 7 Marks
   aWhat do you mean by Commutation? Explain in brief about External Pulse commutation.
   How to answer

   • Define commutation in thyristor circuits
   • Classify natural and forced commutation
   • Explain external pulse commutation circuit operation
   • Give waveforms and commutation steps
6. 7 Marks
   bExplain the effect of source inductance on the operation of single phase full bridge converter.
   How to answer

   • Define source inductance and overlap phenomenon
   • Explain overlap angle and commutation interval in full bridge
   • Show effect on output voltage reduction and waveforms
   • Give expression for voltage loss due to Ls
7. 7 Marks
   aWhy does unequal voltage sharing take place among series connected thyristors during steady state and dynamic state? How is equal voltage sharing obtained in both the states?
   How to answer

   • Explain leakage current mismatch in steady state and capacitance/charge mismatch in dynamic state
   • Describe static equalization with shunt resistors
   • Describe dynamic equalization with RC snubber
   • Give design considerations for equal sharing
8. 7 Marks
   bFind the number of thyristors each with a rating of 500 V and 75 A required for each branch of series and parallel combination for a circuit for a total voltage and current rating of 7.5 kV and 1 kA. Also compare the series and parallel string efficiency. Assume derating factor of 14 %.
   How to answer

   • Calculate number of series devices using total voltage, device voltage and derating factor
   • Calculate number of parallel branches using total current, device current and derating
   • Compute series and parallel string efficiencies
   • Compare efficiencies and comment on derating
9. 7 Marks
   aDescribe the operation of a 1-$\phi$ full bridge inverter. Draw wave shapes of output current when:

   1. i)Load is pure resistive
   2. ii)Load is pure inductive
   How to answer

   • Explain 1-phase full bridge inverter circuit and switching sequence
   • Draw output current waveform for resistive load in phase with voltage
   • Draw output current waveform for inductive load with lag and feedback diodes
   • Compare conduction of thyristors and diodes
10. 7 Marks
    bExplain the different harmonic control techniques for an inverter.
    How to answer

    • Define harmonics in inverter output
    • Explain single pulse width, multiple pulse and sinusoidal PWM
    • Explain selective harmonic elimination
    • Compare techniques on THD and control
11. 7 Marks
    aExplain the operation of a 1-$\phi$ fully controlled bridge converter feeding a RL load. Also draw relevant output voltage and current waveforms.
    How to answer

    • Explain 1-phase fully controlled bridge with RL load and firing angle
    • Describe continuous and discontinuous conduction modes
    • Sketch output voltage and current waveforms with thyristor conduction
    • Mention effect of inductance and freewheeling
12. 7 Marks
    bWith the help of neat circuit diagram and relevant waveforms, explain the operation of a bridge type step down cycloconverter, with resistive load.
    How to answer

    • Show bridge type step-down cycloconverter circuit with resistive load
    • Explain positive and negative group converter operation
    • Sketch input, output voltage and current waveforms
    • Explain frequency division principle
13. 7 Marks
    aDescribe the operation of 1-$\phi$ full wave ac voltage controller with inductive load. Derive expression for output voltage.
    How to answer

    • Explain 1-phase full wave AC voltage controller with inductive load
    • Describe thyristor triggering and extinction angle
    • Derive RMS output voltage expression
    • Sketch voltage and current waveforms
14. 7 Marks
    bA motor single phase voltage controller has input voltage of 230 Volts, 50 Hz and R= 15 ohms. For 6 cycles on and 4 cycles off, determine r.m.s output voltage and input power factor.
    How to answer

    • Write on-off control RMS voltage formula Vo = Vs*sqrt(n/(n+m))
    • Calculate Vo from 230 V with 6 on and 4 off cycles
    • Calculate load power and input VA for power factor
    • Obtain input power factor
15. 7 Marks
    aDescribe the working of step-up converter and derive the expression for output voltage.
    How to answer

    • Explain step-up chopper circuit with inductor, switch and diode
    • Describe ON and OFF period energy storage and transfer
    • Derive Vo = Vs/(1-D) using volt-second balance
    • Sketch waveforms of current and voltage
16. 7 Marks
    bDescribe fly-back SMPS with relevant equivalent circuit and waveforms.
    How to answer

    • Show flyback SMPS circuit with transformer and switch
    • Explain energy storage in magnetizing inductance and transfer to secondary
    • Draw equivalent circuits for ON and OFF modes
    • Sketch switching waveforms