Power electronics MCQ #4

1) An SCR is considered as a semi-controlled device. Because it can be_____.

       (a) turned OFF but not turned ON with the gate pulse

       (b) turned ON but not turned OFF with a gate pulse

       (c) conducts during only one-half cycle of an alternating current wave

       (d) turned ON only during only one-half cycle of an alternating voltage wave

2) Thyristors can be turned off by

    1- reducing the current below the holding level

    2- applying a negative voltage to the anode of the device

    3- reducing gate current

    from these, the correct statements are

       (a) 1 and 3

       (b) 1 and 2

       (c) 1, 2 and 3

       (d) 2 and 3

3) What is a TRIAC?

       (a) Two transistors connected in the anti-parallel mode

       (b) Two thyristors connected in series mode

       (c) Two thyristors connected in the anti-parallel mode

       (d) Two thyristors connected in parallel mode

4) The snubber circuit is used in thyristor circuits for

       (a) triggering

       (b) phase shifting

       (c) di/dt protection

       (d) dv/dt protection

5) For an SCR, DV/DT protection is achieved through

       (a) RL in series with SCR

       (b) RC across SCR

       (c) L in series with SCR

       (d) L across SCR

ANSWER: | 1-b | 2-b | 3-c | 4-d | 5-b |

6) During forward blocking state, a thyristor is associated with

       (a) low current, large voltage

       (b) large current, low voltage

       (c) medium current, large voltage

       (d) low current, medium voltage

7) Once SCR starts conducting a forward current, its gate loses control over

       (a) anode circuit voltage only

       (b) anode circuit current only

       (c) anode circuit voltage and current

       (d) anode circuit voltage, current and time

8) Which of the following device is not the current triggered device?

       (a) Thyristor

       (b) GTO

       (c) TRIAC

       (d) MOSFET

9) Carrier frequency gate drive is used for turn-on of a thyristor to reduce

       (a) di/dt

       (b) turn on time

       (c) dv/dt

       (d) size of pulse transformer

10) The correct sequence of the given devices in the decreasing order of their speed of operation is

       (a) power BJT, PMOSFET, IGBT, SCR

       (b) IGBT, PMOSFET, power BJT, SCR

       (c) SCR, PBJT, IGBT, PMOSFET

       (d) PMOSFET, IGBT, PBJT, SCR

ANSWER: | 6-a | 7-c | 8-d | 9-d | 10-d |

11) The di/dt rating of an SCR specified for its

       (a) decaying anode current

       (b) decaying gate current

       (c) rising gate current

       (d) rising anode current

12) TRIACs are most suitable when the supply voltage is

       (a) dc

       (b) low frequency ac

       (c) high frequency ac

       (d) full wave rectified ac

13) The main advantage of IGBT over SCR in power electronics is

       (a) Reduced weight

       (b) Self commutating capability

       (c) Very high reliability

       (d) Self cooling property

14) Turn on and turn off times of a transistor depends on

       (a) Voltage gain

       (b) Junction temperature

       (c) Junction capacitance’s

       (d) Current gain

15) A cycloconverter can be

       (a) step down

       (b) step up

       (c) step down or step up

       (d) none of the above

ANSWER: | 11-d | 12-b | 13-c | 14-c | 15-c |

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