(SEM 5) THEORY EXAMINATION 2017-18 POWER ELECTRONICS

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.