Power Electronics Question and Answers – Power Transistors-2 and Answers

Power Electronics Multiple Choice Questions on “Power Transistors-2”.

1. For a power transistor, if the forward current gain α = 0.97, then β = ?
A. 0.03
B. 2.03
C. 49.24
D. 32.33
Answer: D
Clarification: Use the relation α = β/(β+1).

2. The power electronics devices have a very high efficiency because
A. cooling is very efficient
B. the devices traverse active region at high speed & stays at the two states, on and off
C. the devices never operate in active region
D. the devices always operate in the active region
Answer: B
Clarification: They are efficient due to their higher transition speeds.

3. For a power transistor, which of the following relations is true?
A. Ie>Ic>Ib
B. Ib>Ic>Ie
C. Ic>Ie>Ib
D. Ie=Ib
Answer: A
Clarification: Practically speaking Ie = Ib+Ic. Ie is the highest as it is the sum of the collector and base currents. The base current is the smallest.

4. High frequency operation of any device is limited by the
A. forward voltage rating
B. switching losses
C. thermal conductivity
D. heat Sink arrangements
Answer: B
Clarification: Lower the switching losses higher the frequency of operation of the device.

5. The instantaneous power loss during the delay time of a transistor is given by
A. Ic Vce
B. Ib Vbe
C. Ic Vbe
D. Ib Vce
Answer: A
Clarification: During the delay time only the collector current flows & base to emitter voltage is zero.

6. For a power transistor, the average power loss during the delay time can be given by the equation
A. Ic * Vc
B. 1/T * ∫^Td (Ic Vce) dt
C. Ic * dVc/dt * T
D. 1/T * ∫^{(Td *Tr)} (Ic VC. dt
Answer: B
Clarification: During the delay time only, the collector current flows & base to emitter voltage is zero. Hence the average power can be found, simply by integrating it over the total delay time & dividing by the base time period.

7. A 1mv of i/p gives an output of 1V, the voltage gain as such would be
A. 0.001
B. 0.0001
C. 1000
D. 100
Answer: C
Clarification: 1V/1mv = 1000.

8. Which of the following relations is true for a BJT?
A. Ic ≈ Ie
B. Ib ≈ Ic
C. Ie ≈ Ib
D. Ib ≈ Ie ≈ Ic
Answer: A
Clarification: The collector & emitter current differ only by the base current, which is very very small.

9. Choose the correct statement
A. A transistor will remain on as long the the base current is applied
B. A transistor remains on after a high to low pulse is applied at the base
C. A transistor will remain on as long the the collector current is applied
D. A transistor remains on after a high to low pulse is applied at the collector
Answer: A
Clarification: Unlike the thyristor devices, all the transistor family devices remain in the conducting state as long as the firing pulses are applied. This is a very important property of the transistor devices.

10. Let’s say that a transistor is operating at the middle of the load line, then a decrease in the current gain would
A. move the Q point up
B. move the Q point down
C. result in to & fro motion of the Q point
D. not change the Q point
Answer: B
Clarification:The current gain would decreases the collector current, shifting the Q point below.