About this previous year paper
Power Electronics (NEE-502)
Section-Wise Important Questions & Study Notes
SECTION–A
(Attempt all questions | 2 × 10 = 20)
This section tests definitions, characteristics, advantages & disadvantages.
Write short, crisp answers (2–3 lines) and draw small diagrams where asked.
Transfer Characteristics of MOSFET
Graph showing relation between ID and VGS at constant VDS. Explain cut-off, ohmic, and saturation regions.
On–Off Control (Advantages & Disadvantages)
Used in choppers.
Advantage: simple control, high efficiency. Disadvantage: ripple content is high.
Phase Angle Control (Advantages & Disadvantages)
Controls output voltage by delaying firing angle.
Advantage: smooth control. Disadvantage: poor power factor, harmonics.
Cycloconverter
A converter that converts AC at one frequency directly to AC at another frequency without DC link.
Advantages & Disadvantages of Cycloconverter
Advantage: low speed, high power applications. Disadvantage: complex control, harmonics.
Step-Up and Step-Down Chopper
Step-up increases output voltage; step-down reduces output voltage.
Advantages of GTO over SCR
GTO can be turned ON and OFF using gate signal; no commutation circuit needed.
Holding Current & Latching Current
Holding current: minimum current to keep SCR ON.
Latching current: minimum current to latch SCR after triggering.
Applications of DC Chopper
Electric traction, battery-operated vehicles, speed control of DC motors.
Single Phase Bridge vs Mid-Point Converter
Bridge converter gives higher output voltage and better transformer utilization.
SECTION–B
(Attempt any THREE | 10 × 3 = 30)
Answers must be descriptive, include derivations & diagrams.
1. Two-Transistor Model of SCR
Draw equivalent model using PNP & NPN transistors.
Derive anode current equation:
IA=IG+ICO1+ICO21−(α1+α2)I_A = \frac{I_G + I_{CO1} + I_{CO2}}{1 - (\alpha_1 + \alpha_2)}IA=1−(α1+α2)IG+ICO1+ICO2
2. Applications of Power Electronics Devices
Explain use in:
Industrial drives
HVDC transmission
Renewable energy systems
UPS & inverters
Electric vehicles
3. Need of Protection in Power Electronics
Explain why devices need protection from:
Over-voltage
Over-current
High dv/dt and di/dt
Snubber Circuit:
Explain RC snubber with neat diagram and working.
4. Construction & Working of SCR
Explain: PNPN structure
Forward blocking, conduction, reverse blocking Gate triggering
Draw V-I characteristics of SCR.
5. Single Phase to Single Phase Cycloconverter
Explain working with: Bridge circuit
Input & output waveforms Applications (low-speed drives)
SECTION–C
(Attempt any ONE | 10 × 1 = 10)
3(a) Step-Up Chopper
Explain principle and derive average output voltage: Vo=Vs1−DV_o = \frac{V_s}{1 - D}Vo=1−DVs
where D = duty cycle. Draw circuit and waveform.
OR
3(b) Output Characteristics of Power MOSFET
Draw ID vs VDS characteristics and explain regions of operation.