Power Electronics Special MCQ Part Five

Power Electronics Special MCQ Part Five

Power Electronics Special MCQ Part Five

1.The forward voltage drop during SCR on state is 1.5 V. This voltage drop
(A) remains constant and is independent of load current
(B) increases slightly with load current
(C) decreases slightly with load current
(D) Varies linearly with load current

Answer-B

2.Turn on time of an SCR in series with RL circuit can be reduced by
(A) increasing circuit resistance R
(B) decreasing R
(C) increasing circuit inductance L
(D) decreasing L

Answer-D

3.In an SCR, anode current flows over a narrow region near the gate during
(A) delay time td
(B) rise time tr and spread time tp
(C) td and tp
(D) td and tr

Answer-D

4.Consider the following statements
1. The triac is a five layer device
2. The triac may be considered to consist of two parallel sections of p1 n1 p2 n2 and p2 n1 p1 n4
3. An additional lateral region serves as the control gate
4. The triac is double ended SCR
From above, the correct statements are
(A) all (B) 1, 2, 3 (C) 1 only (D) 1, 4

Answer-A

5.Use of a reverse conducting thyristor in place of anti parallel combination of thyristor and feedback diode in an inverter
(A) effectively minimizes the peak commutating current
(B) decreases the operating frequency of operation
(C) minimizes the effects of load inductances on the commutation performance
(D) causes deterioration in the commutation performance

Answer-C

Power Electronics Special MCQ Part Five

6.Which of the following characteristic of a silicon power diode makes it suitable for use as an ideal diode
(A) It has very low saturation current
(B) It has high value of forward cut in voltage
(C) It can withstand large reverse voltage
(D) When compared with germanium diodes, silicon diodes show a lower degree of temperature dependence under reverse bias conditions
Select the correct answer using the codes given below
(A) 1 and 2 (B) 1, 2, 3 and 4 (C) 2, 3 and 4 (D) 1 and 3

Answer-D

7.Current unbalance in the parallel connected SCRs is due to the non uniformity in the
(A) forward characteristics
(B) reverse characteristics
(C) di/dt withstanding capability
(D) dv/dt withstanding capability

Answer-A

8.The turn on time of an SCR is 30 μs. The pulse train at the gate has a frequency of 2.5 kHz with a mark/space ratio of 1.0. This SCR will
(A) turn on
(B) not turn on
(C) turn on if pulse frequency increased
(D) turn on if pulse frequency decreased

Answer-A

9.Which one of the following statements regarding the two transistor model of the p – n – p – n four layer device is correct
(A) It explains only the turn on portion of the device characteristics
(B) It explains only the turn off portion of the device characteristics
(C) It explains only the negative region portion of the device characteristics
(D) It explains all the regions of the device characteristics

Answer-A

Power Electronics Special MCQ Part Five

10.When compared with BJTs, MOSFETs have lesser turn off time, enabling them to operate at high operating frequencies. What is the reason that can be contributed to that property
(A) High input impedance
(B) positive temperature coefficient
(C) Absence of minority stored charge
(D) Smaller leakage current

Answer-C

11.Satisfactory turn off of a thyristor will takes place if the reverse bias period of the device, after its current has become zero, is
(A) greater than its turn off time
(B) less than its turn off time
(C) greater than circuit time constant
(D) less than circuit time constant

Answer-A

12.A BJT operates as a switch
(A) in active region of transfer characteristics
(B) with no signal condition
(C) under small signal condition
(D) under large signal condition

Answer-D

13.n – p – n transistors are preferred in power electronic circuits over p – n – p transistors because they have
(A) high mobility of holes
(B) high mobility of electrons in p – n – p
(C) low mobility of holes
(D) higher mobility of electrons than the mobility of holes in p – n – p transistors

Answer-D

14.Which one of the following device has bi directional current capability?
(A) SCR
(B) MOSFET
(C) TRIAC
(D) IGBT

Answer-C

Power Electronics Special MCQ Part Five

15.For the V – I characteristics of an SCR,
1. It will trigger when the applied voltage is more than the forward break over voltage
2. Holding current is greater than latching current
3. When reverse biased, a small value of leakage current will flow
4. It can be triggered without gate
Which of the above statements are correct
(A) 1, 2 and 3 (B) 1, 3 and 4 (C) 1, 2 and 4 (D) 2, 3 and 4

Answer-B

16.Consider the following statements regarding thyristor
1. It conducts when forward biased and positive current flows through gate
2. It conducts when forward biased and negative current flows through gate
3. It commutates when reverse biased and negative current flows through the gate
4. It commutates when gate current is withdrawn

Which of these statement(s) is/are correct
(A) 1, 2 and 3
(B) 1 and 2 only
(C) 2 and 3 only
(D) 1 only

Answer-D

17.Consider the following statements
1. Speed of operation of MOSFET is more than the speed of operation of SCR
2. SCRs have low power loss than MOSFETs
3. The current in conducting state can be easily controlled through the gate in SCR
4. MOSFET is not a current controlled device
The correct statements are
(A) 1 and 4 only (B) 1 and 2 only (C) 2 and 3 only (D) 1, 2, 3 and 4

Answer-A

Power Electronics Special MCQ Part Five

18.Match List – I with List – II and select the correct answer using the codes given below the lists
List I
P. di/dt rating limits
Q. dv/dt rating limits
R. i2t limit
S. Junction temperature limit
List II
1. Snubber circuit
2. Heat sink
3. Series reactor
4. Fuse

(A) P – 2, Q – 4, R – 1, S – 3
(B) P – 3, Q – 4, R – 1, S – 2
(C) P – 2, Q – 1, R – 4, S – 3
(D) P – 3, Q – 1, R – 4, S – 2

Answer-D

19.Match List – I (Device) with List – II (Switching times) and select the correct answer using the codes given below the lists
List I
P. TRIAC
Q. SCR
R. MOSFET
S. IGBT
List II
1. 5 – 10 μs
2. 100 – 400 μs
3. 50 – 100 μs
4. 200 – 400 μs

(A) P – 4, Q – 3, R – 2, S – 1
(B) P – 1, Q – 2, R – 3, S – 4
(C) P – 4, Q – 2, R – 3, S – 1
(D) P – 1, Q – 3, R – 2, S – 4

Answer-C

20.A thyristor has a PIV of 650 V; the voltage safety factor is 2. Then the voltage upto which the device can be operated is given by
(A) 1300 V
(B) 650 V
(C) 325 V
(D) 230 V

Answer-B

Power Electronics Special MCQ Part Five