# Power Electronics Question and Answers – Power Transistors-2

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses 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

Explanation: 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

Explanation: 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

Explanation: 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.
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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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation: 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

Explanation:The current gain would decreases the collector current, shifting the Q point below.

Sanfoundry Global Education & Learning Series – Power Electronics.

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