Power Semiconductor Devices - Power Electronics - Electrical Engineering Previous Year Questions

1

2007 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2007

The circuit in the figure is a current commutated dc – dc chopper where, Th_M is the main SCR and Th_AUX is the auxiliary SCR. The load current is constant at 10 A. Th_M is ON. Th_AUX is triggered at t = 0. Th_M is turned OFF between

A

0 μs < t ≤ 25 μs

B

25 μs < t ≤ 50 μs

C

50 μs < t ≤ 75 μs

D

75 μs < t ≤ 100 μs

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2

2007 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2007

The resistance R should be

A

4.7 kΩ

B

470 Ω

C

47 Ω

D

4.7 Ω

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3

2007 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2007

The minimum approximate volt-second rating of the pulse transformer suitable for triggering the SCR should be: (volt-second rating is the maximum of product of the voltage and the width of the pulse that may be applied)

A

2000 μV-s

B

200 μV-s

C

20 μV-s

D

2.0 μV-s

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4

2009 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2009

An SCR is considered to be a semi-controlled device because

A

it can be turned OFF but not ON with a gate pulse

B

it conducts only during one half-cycle of an alternating current wave

C

it can be turned ON but not OFF with a gate pulse

D

it can be turned ON only during one half-cycle of an alternating voltage wave

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5

2009 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2009

The circuit shows an ideal diode connected to a pure inductor and is connected to a purely sinusoidal 50Hz voltage source. Under ideal conditions the current waveform through the inductor will look like

A

Graph A

B

Graph B

C

Graph C

D

Graph D

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6

2009 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2009

In the chopper circuit shown, the main thyristor (TM) is operated at a duty ratio of 0.8 which is much larger the commutation interval. If the maximum allowable reapplied dv/dt on TM is 50 V/μs, what should be the theoretical minimum value of C1 ? Assume current ripple through L0 to be negligible.

A

0.2 μF

B

0.02 μF

C

2 μF

D

20 μF

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7

2010 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2010

Figure shows a composite switch consisting of a power transistor (BJT) in series with a diode. Assuming that the transistor switch and the diode are ideal, the I-V characteristic of the composite switch is

A

B

C

D

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8

2010 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2010

The L-C circuit of Q54 is used to commutate a thyristor, which is initially carrying a current of 5 A as shown in the figure below. The values and initial conditions of L and C are the same as in Q54. The switch is closed at t = 0. If the forward drop is negligible, the time taken for the device to turn off is

A

52 μs

B

156 μs

C

312 μs

D

26 μs

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9

2011 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2011

Circuit turn-off time of an SCR is defined as the time

A

taken by the SCR to turn off

B

required for the SCR current to become zero

C

for which the SCR is reverse biased by the commutation circuit

D

for which the SCR is reverse biased to reduce its current below the holding current

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10

2011 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2011

A voltage commutated chopper circuit, operated at 500Hz, is shown below. If the maximum value of load current is 10 A, then the maximum current through the main (M) and auxiliary (A) thyristors will be

A

\(i_{Mmax} = 12\) A and \(i_{Amax} = 10\) A

B

\(i_{Mmax} = 12\) A and \(i_{Amax} = 2\) A

C

\(i_{Mmax} = 10\) A and \(i_{Amax} = 12\) A

D

\(i_{Mmax} = 10\) A and \(i_{Amax} = 8\) A

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11

2012 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2012

The typical ratio of latching current to holding current in a 20 A thyristor is
(A) 5.0
(B) 2.0
(C) 1.0
(D) 0.5

A

5.0

B

2.0

C

1.0

D

0.5

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12

2013 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2013 [Session 1]

Thyristor T in the figure below is initially off and is triggered with a single pulse of width 10 \( \mu s \). It is given that \( L = \frac{100}{\pi} \mu H \) and \( C = \frac{100}{\pi} \mu F \). Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for

A

10 \(\mu s \)

B

50 \(\mu s \)

C

100 \(\mu s \)

D

200 \(\mu s \)

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13

2013 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2013 [Session 1]

Appropriate transition i.e., Zero Voltage Switching (ZVS)/Zero Current Switching (ZCS) of the IGBTs during turn-on/turn-off is

A

ZVS during turn-off

B

ZVS during turn-on

C

ZCS during turn-off

D

ZCS during turn-on

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14

2013 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2013 [Session 2]

Thyristor T in the figure below is initially off and is triggered with a single pulse of width 10 µs. It is given that \( L = \left(\frac{100}{\pi}\right) \mu H \) and \( C = \left(\frac{100}{\pi}\right) \mu F \). Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for

A

10 µs

B

50 µs

C

100 µs

D

200 µs

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15

2013 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2013 [Session 3]

Thyristor T in the figure below is initially off and is triggered with a single pulse of width 10 \(\mu\)s. It is given that \(L=(\frac{100}{\pi})\mu\)H and \(C=(\frac{100}{\pi})\mu\)F. Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for

A

10 \(\mu\)s

B

50 \(\mu\)s

C

100 \(\mu\)s

D

200 \(\mu\)s

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16

2013 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2013 [Session 4]

Thyristor T in the figure below is initially off and is triggered with a single pulse of width 10 \(\mu\)s. It is given that \( L = \left(\frac{100}{\pi}\right)\mu H \) and \( C = \left(\frac{100}{\pi}\right)\mu F \). Assuming latching and holding currents of the thyristor are both zero and the initial charge on C is zero, T conducts for

A

10 \(\mu\)s

B

50 \(\mu\)s

C

100 \(\mu\)s

D

200 \(\mu\)s

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17

2014 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2014 [Session 1]

Figure shows four electronic switches (i), (ii), (iii) and (iv). Which of the switches can block voltages of either polarity (applied between terminals 'a' and 'b') when the active device is in the OFF state?

A

(i), (ii) and (iii)

B

(ii), (iii) and (iv)

C

(ii) and (iii)

D

(i) and (iv)

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18

2014 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2014 [Session 2]

The SCR in the circuit shown has a latching current of 40 mA. A gate pulse of 50 μs is applied to the SCR. The maximum value of R in Ω to ensure successful firing of the SCR is ________.

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19

2016 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2016 [Session 1]

A steady dc current of 100 A is flowing through a power module (S, D) as shown in Figure (a). The V-I characteristics of the IGBT (S) and the diode (D) are shown in Figures (b) and (c), respectively. The conduction power loss in the power module (S, D), in watts, is ______

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20

2016 · Electrical Engineering · Power Electronics · Power Semiconductor Devices

Electrical Engineering (EE) 2016 [Session 1]

The voltage \(v_s\) across and the current \(i_s\) through a semiconductor switch during a turn-ON transition are shown in figure. The energy dissipated during the turn-ON transition, in mJ, is ________.

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