EX-502 Power Electronics - Dec 2025 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 are the different methods to turn on the Thyristor?
   How to answer

   • Define thyristor turn-on / triggering
   • List methods: gate triggering, forward voltage, dv/dt, temperature, light
   • Briefly explain each method with condition
   • Mention preferred practical method - gate triggering
2. 7 Marks
   bDraw the static V-I characteristics of the SCR and explain its mode of operation.
   How to answer

   • Draw static V-I characteristics with forward/reverse regions
   • Mark forward breakover voltage, holding and latching current
   • Explain forward blocking, conduction and reverse blocking modes
   • Explain gate current effect on breakover
3. 7 Marks
   aWhat is commutation? Explain class B commutation with the help of circuit diagram and waveform.
   How to answer

   • Define commutation / turn-off of thyristor
   • Draw class B self-commutation circuit with LC resonant circuit
   • Explain charging and resonant commutation process
   • Show relevant voltage and current waveforms
4. 7 Marks
   bCompare the power BJT, power MOSFET and IGBT.
   How to answer

   • Compare control type: current vs voltage controlled
   • Compare switching speed, on-state drop, power handling
   • Compare input impedance and drive requirements
   • Summarize in table with applications
5. 7 Marks
   aDraw the single phase full wave bridge type controlled rectifier. Draw the waveforms of input voltage, load voltage and voltage across SCR.
   How to answer

   • Draw single-phase full-wave bridge controlled rectifier with 4 SCRs and load
   • Show input supply voltage waveform
   • Show load voltage waveform for firing angle alpha
   • Show voltage across SCR waveform
6. 7 Marks
   bWhat is the function of freewheeling diodes in the controlled rectifier?
   How to answer

   • Define freewheeling diode connection across inductive load
   • Explain conduction during negative / zero source interval
   • Explain effects: prevents negative load voltage, improves power factor
   • Mention protection of SCRs from overvoltage
7. 7 Marks
   aWhat is meant by rectification mode and inversion mode in single phase fully controlled converter?
   How to answer

   • Define rectification mode with alpha < 90 deg, positive average output
   • Define inversion mode with alpha > 90 deg, negative average output
   • Explain power flow direction in each mode
   • Relate to firing angle and load EMF requirement
8. 7 Marks
   bDescribe a Morgen chopper with associated voltage and current waveform.
   How to answer

   • Draw Morgan chopper circuit with commutating elements
   • Explain charging and commutation operation sequence
   • Show associated voltage and current waveforms
   • Mention characteristics / limitations
9. 7 Marks
   aExplain the operation of 1-phase current source inverter, also describe the merits and demerits of this inverter.
   How to answer

   • Draw 1-phase current source inverter circuit
   • Explain operation with thyristor pairs and commutation
   • List merits e.g. short-circuit protection, simple commutation
   • List demerits e.g. slow response, large inductor
10. 7 Marks
    bExplain the operation of the 3-phase inverter in $180^{\circ}$ mode with the resistive star connected load.
    How to answer

    • Draw 3-phase bridge inverter with star resistive load
    • Explain 180-degree conduction sequence of devices
    • Show phase and line voltage waveforms and steps
    • Mention device conduction pattern 3 devices at a time
11. 7 Marks
    aA step-up chopper is used to feed a load at 400 V DC from a 250V DC source. The inductor current is continuous. If the off time of the switch is $20\mu s$, then find the switching frequency of the chopper in kHz.
    How to answer

    • Write step-up chopper output relation $V_o = V_s/(1-D)$
    • Find duty ratio from 400 V and 250 V
    • Find ON time and total period from $T_{off}=20\mu s$
    • Calculate switching frequency $f=1/T$ in kHz
12. 7 Marks
    bExplain the working of the step-up chopper with the help of neat diagram.
    How to answer

    • Draw step-up chopper circuit with inductor, switch, diode and load
    • Explain ON period energy storage in inductor
    • Explain OFF period energy transfer to load
    • Show waveforms and voltage boosting principle
13. 7 Marks
    aWith the help of a circuit diagram and relevant waveforms explain the operation of a mid-point type step-up cycloconverter.
    How to answer

    • Draw mid-point step-up cycloconverter circuit
    • Explain thyristor groups for positive and negative output
    • Show input and stepped-up output waveforms
    • Explain frequency step-up principle
14. 7 Marks
    bExplain the working of the Buck-Boost converter with the help of neat diagram.
    How to answer

    • Draw Buck-Boost converter power circuit
    • Explain switch ON inductor charging mode
    • Explain switch OFF energy transfer to load
    • Mention output voltage polarity and magnitude relation
15. 7 Marks
    aA single phase voltage controller has an input of 230 V and a load of $10 \Omega$ resistor. For 4 cycles on and 3 cycle off, determine the

    1. (i))RMS output voltage
    2. (ii))Input power factor
    3. (iii))Average and RMS thyristor current
    How to answer

    • Write on-off control formulas: Vo_rms = Vs*sqrt(n/(n+m))
    • Calculate RMS output voltage for 4 on 3 off
    • Calculate input power factor and load power
    • Derive average and RMS thyristor currents
16. 7 Marks
    bWhat are dual converters? Explain the operation of three phase dual converter using circulating current mode of operations. How are firing angles of two converters controlled?
    How to answer

    • Define dual converter with two antiparallel converters
    • Explain three-phase dual converter in circulating current mode
    • Show firing angle relation alpha1 + alpha2 = 180 degrees
    • Explain control of firing angles for four-quadrant operation