EC-604-C Power Electronics - Jun 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
   aDescribe the basic structure and V-I characteristics of a Power MOSFET. How does it operate in switching applications?
   How to answer

   • Describe vertical DMOS structure: n+ source, p-body, n-drift, n+ drain
   • Explain V-I characteristics: cut-off, ohmic/linear, active/saturation regions with transfer curve
   • Explain gate voltage control and threshold voltage
   • Explain switching operation: on-state, off-state, switching speed and gate drive
2. 7 Marks
   bDiscuss the different types of power diodes. What are the applications of each type?
   How to answer

   • List types: general purpose, fast recovery, Schottky power diodes
   • Explain construction and characteristics of each type
   • Compare reverse recovery, voltage and current ratings
   • Give applications of each type
3. 7 Marks
   aExplain the basic structure and V-I characteristics of an IGBT.
   How to answer

   • Describe four-layer p-n-p-n structure with MOS gate
   • Explain equivalent circuit and gate control
   • Explain V-I characteristics: forward blocking, active, saturation
   • Mention latch-up and safe operating area
4. 7 Marks
   bExplain the working of a three-phase full-wave fully controlled converter with continuous and constant current.
   How to answer

   • Describe 6-thyristor bridge circuit and 3-phase supply
   • Explain firing sequence and conduction for continuous current
   • Explain output voltage waveform and average voltage vs firing angle
   • Explain effect of constant load current assumption
5. 7 Marks
   aDetermine the RMS value of the output voltage for a single-phase full-wave half-controlled rectifier with a peak input voltage $V_m = 300V$ and a firing angle $\alpha = 30^\circ$.
   How to answer

   • State RMS output voltage formula for single-phase half-controlled rectifier
   • Substitute $V_m = 300V$ and $\alpha = 30^\circ$
   • Evaluate integral / expression for RMS value
   • Give final RMS voltage value with units
6. 7 Marks
   bDiscuss the series and parallel operation of thyristor. What are the challenges and solutions associated with each configuration?
   How to answer

   • Explain need for series connection for high voltage and parallel for high current
   • Discuss challenges: unequal voltage/current sharing due to device mismatch
   • Explain solutions: static and dynamic equalizing circuits, derating
   • Explain gate triggering synchronization and protection
7. 7 Marks
   aDescribe the basic structure and operation of a three-phase bridge inverter. What are the applications of three-phase inverters in power systems?
   How to answer

   • Describe 6-switch bridge topology with DC source and 3-phase load
   • Explain 120-degree and 180-degree conduction modes
   • Explain switching sequence and phase/line voltage waveforms
   • List applications: motor drives, UPS, renewable energy, power transmission
8. 7 Marks
   bDescribe the basic circuit and operation of a single-phase Pulse Width Modulated (PWM) inverter. What are the advantages of using PWM inverters over traditional inverters?
   How to answer

   • Describe single-phase full-bridge with PWM gate signals
   • Explain sinusoidal PWM generation by comparing reference and carrier
   • Explain output voltage control and harmonic reduction
   • List advantages: lower harmonics, voltage control, better efficiency
9. 7 Marks
   aA single-phase series resonant inverter operates at a resonant frequency of 10 kHz with a supply voltage of 100V and a load resistance of $20\Omega$. Calculate the output current at resonance.
   How to answer

   • State resonance condition: inductive reactance equals capacitive reactance
   • At resonance impedance equals load resistance $R$
   • Use formula $I_o = V_s / R$ with given values
   • Calculate output current value in A
10. 7 Marks
    bExplain the principle of on-off control and phase control in AC voltage controllers. How do these methods regulate the output voltage?
    How to answer

    • Define on-off (integral cycle) control principle
    • Define phase control principle with firing angle delay
    • Explain how RMS output voltage is varied in each method
    • Compare harmonics, power factor and applications
11. 7 Marks
    aDiscuss the principle of operation of a single-phase transformer taps changer. How does it control the output voltage in power distribution systems?
    How to answer

    • Describe tap changer on transformer winding concept
    • Explain on-load vs off-load tap changing operation
    • Explain voltage regulation by changing turns ratio
    • Explain role in power distribution voltage control
12. 7 Marks
    bExplain the basic principle of operation of a cycloconverter. How does a single-phase to single-phase cycloconverter work and what are its typical applications?
    How to answer

    • Define cycloconverter as direct AC-AC frequency changer
    • Explain single-phase to single-phase bridge operation with thyristors
    • Explain output frequency synthesis and firing control
    • List applications: low-speed drives, traction, induction heating
13. 7 Marks
    aExplain the principle of operation of a Switched-Mode Power Supply (SMPS). How does it differ from linear power supplies?
    How to answer

    • Describe SMPS block diagram: rectifier, high-frequency switch, transformer, filter
    • Explain PWM switching regulation principle
    • Compare efficiency, size, weight with linear supplies
    • Mention applications and EMI considerations
14. 7 Marks
    bDiscuss the functionality of buck-boost regulators. How do they regulate output voltage when the input voltage can be both higher and lower than the desired output voltage?
    How to answer

    • Describe buck-boost topology with inductor, switch, diode
    • Explain buck mode when input higher and boost mode when input lower
    • Explain duty cycle control of output voltage magnitude and polarity
    • Explain continuous vs discontinuous operation
15. 14 Marks
    Write a short note on any two :

    1. a)Different methods of turning on a thyristor
    2. b)Step-up chopper and Step down choppers
    3. c)Line frequency phase-controlled rectifier using SCRs
    4. d)Single-phase bridge inverter
    How to answer

    • Select any two topics from a) to d)
    • Explain principle, circuit and waveforms for each selected topic
    • Give characteristics, control and applications
    • Add neat labelled diagram for each