Section 1 - Power Electronics Basics

Question 1

An SCR (Silicon Controlled Rectifier) is a:

A. Two-layer device

B. Three-layer device

C. Four-layer, three-terminal device

D. Five-layer device

Question 2

An SCR can be turned ON by:

A. Applying a negative gate current

B. Applying a positive gate current when anode is positive with respect to cathode

C. Only by reversing the anode voltage

D. Removing the gate current

Question 3

Once an SCR is turned ON, the gate loses control and the SCR can be turned OFF only by:

A. Applying a negative gate pulse

B. Reducing the anode current below the holding current

C. Increasing the gate current

D. Increasing the anode voltage

Question 4

The latching current of an SCR is:

A. The minimum anode current required to keep the SCR conducting after the gate signal is removed

B. The maximum current the SCR can conduct

C. The current at which the SCR turns off

D. The gate current at breakdown

Question 5

The holding current of an SCR is:

A. Greater than the latching current

B. Less than the latching current

C. Equal to the latching current

D. Equal to the gate trigger current

Question 6

Which of the following is a fully controlled power semiconductor device?

A. Diode

B. SCR (thyristor)

C. Power BJT and IGBT

D. DIAC

Question 7

The main advantage of a power MOSFET over a power BJT is:

A. Lower switching frequency

B. Higher input impedance and faster switching (voltage-controlled device)

C. Higher on-state voltage drop

D. Larger physical size

Question 8

An IGBT combines the advantages of:

A. SCR and diode

B. Power BJT (low conduction loss) and power MOSFET (high input impedance / voltage control)

C. TRIAC and DIAC

D. GTO and thyristor

Question 9

A TRIAC is equivalent to:

A. A single SCR

B. Two SCRs connected in anti-parallel (with a common gate)

C. Two diodes in series

D. An SCR in series with a diode

Question 10

A DIAC is a:

A. Three-terminal unidirectional device

B. Two-terminal bidirectional trigger device

C. Four-layer, three-terminal device

D. Fully controlled switch

Question 11

A GTO (Gate Turn-Off thyristor) can be turned OFF by:

A. A positive gate pulse only

B. A negative gate pulse

C. Only by reducing anode current to zero

D. It cannot be turned off

Question 12

For a single-phase half-wave uncontrolled (diode) rectifier with a purely resistive load, the average DC output voltage is:

A. Vm / pi

B. 2 Vm / pi

C. Vm / (2 pi)

D. Vm

Question 13

For a single-phase full-wave bridge rectifier with a purely resistive load, the average DC output voltage is:

A. Vm / pi

B. 2 Vm / pi

C. Vm / (2 pi)

D. Vm

Question 14

The ripple factor of a single-phase full-wave rectifier (without filter, purely resistive load) is approximately:

A. 1.21

B. 0.48

C. 0.81

D. 1.57

Question 15

The output frequency of a single-phase full-wave rectifier (with 50 Hz input) is:

A. 25 Hz

B. 50 Hz

C. 100 Hz

D. 200 Hz

Question 16

In a single-phase full-wave controlled rectifier (fully controlled bridge) with firing angle alpha and R load, the average DC output voltage is:

A. (2 Vm / pi) * (1 + cos alpha)

B. (Vm / pi) * (1 + cos alpha)

C. (2 Vm / pi) * cos alpha

D. (Vm / pi) * sin alpha

Question 17

A phase-controlled rectifier controls the DC output voltage by:

A. Varying the input voltage

B. Varying the firing (delay) angle of the thyristors

C. Changing the load resistance

D. Changing the number of phases

Question 18

A cycloconverter converts:

A. DC to DC at a different voltage level

B. AC at one frequency to AC at another (usually lower) frequency without an intermediate DC link

C. AC to DC only

D. DC to AC only

Question 19

An inverter is a power electronic converter that:

A. Converts AC to DC

B. Converts DC to AC of desired voltage and frequency

C. Converts one DC level to another

D. Only rectifies AC signals

Question 20

The main purpose of Pulse Width Modulation (PWM) in an inverter is to:

A. Increase harmonic content in the output

B. Control the output voltage and reduce harmonics in the output waveform

C. Reduce the switching frequency

D. Eliminate the need for a filter altogether

Question 21

A DC chopper is used to:

A. Convert AC to DC

B. Convert fixed DC voltage to variable DC voltage

C. Convert DC to AC

D. Convert one AC frequency to another

Question 22

In a step-down (buck) chopper, the average output voltage is:

A. Greater than the input voltage

B. Equal to duty ratio times the input voltage (D * Vs)

C. Independent of duty ratio

D. Equal to input voltage divided by duty ratio

Question 23

In a step-up (boost) chopper operating in continuous conduction, the average output voltage is:

A. D * Vs

B. Vs / (1 - D), where D is the duty ratio

C. Vs * (1 - D)

D. Vs / D

Question 24

An SMPS (Switched-Mode Power Supply) is generally preferred over a linear power supply because it has:

A. Higher efficiency and smaller size for a given power rating

B. Lower efficiency but simpler design

C. Zero switching losses

D. No electromagnetic interference

Question 25

A snubber circuit is used across a power semiconductor device mainly to:

A. Increase the switching frequency

B. Limit dv/dt and di/dt and protect the device during switching transitions

C. Increase the on-state losses

D. Reduce the input voltage

Questions

Section 1 - Power Electronics Basics - Set 1