Electronic Devices and Circuits Multiple Choice Questions on “Power BJTs”.
1. A power transistor is a ____________
A. three layer, three junction device
B. three layer, two junction device
C. two layer, one junction device
D. four layer, three junction device
Answer: b
Clarification: A power BJT has three layers, p-n-p or n-p-n forming two junctions.
p-n-p: two positive (p) layers and one negative (n) layers in between them.
n-p-n: two negative(n) layers and one positive(p) layers.
2. For a power transistor, if base current IB is increased keeping Vce constant; then______
A. IC increases
B. IC decreases
C. IC remains constant
D. IC changes sinusoidal
Answer: a
Clarification: IB is directly proportional to Ic. The IC curve is linearly distributed when using IB as a parameter, and exponentially distributed using VBE as a parameter. So when VBE is constant, the transistor current IC is almost linear with respect to IB.
3. Which one is the most suitable power device for high frequency (>100 kHz) switching application?
A. BJT
B. Power MOSFET
C. Schottkey diode
D. Microwave transistor
Answer: b
Clarification: Power MOSFET has a low turn off time. So it can be operated in a frequency range of 1 to 10 MHz.
4. Insulated-gate bipolar transistor (IGBT) has combinational advantages of ______
A. BJTs and SITs
B. BJTs and MOSFETs
C. SITs and MOSFETs
D. FETs and BJTs
Answer: b
Clarification: IGBT combines advantages of BJTs and MOSFETs. IGBT process high input impedance like a MOSFET and has low on state power loss as in BJTs.
5. A Gate Turn Off (GTO) can be turned on by applying _______
A. positive gate signal
B. positive drain signal
C. positive source signal
D. negative source signal
Answer: a
Clarification: A Gate Turn Off (GTO) like and SCR is a device four layer, three junction semiconductor device with three external terminal (anode, cathode, and gate). GTO can be turned ON and OFF by positive pulse or signal respectively, to the gate terminal.
6. SITH is also known as ________
A. Field controlled diode
B. Field controlled rectifier
C. Silicon controlled diode
D. Silicon controlled rectifier
Answer: a
Clarification: The static induction thyristor (SITHs) is a thyristor with a buried gate structure in which the gate electrodes are placed in n-base region. They are normally on-state, gate electrodes must be negatively biased to hold-off state. It is a self controlled GTO like device. Hence it is sometimes called the field controlled diode.
7. The turn on time of an SCR with inductive load is 20 µs. The pulse train frequency is 2.5 KHz with a mark/space ratio of 1/10, and then SCR will ______
A. Turn On
B. Not turn on
C.Turn on if inductance is removeC.
D. Turn on if pulse frequency is increased to two times
Answer: A
Clarification: Pulse repetition rate (PRR) = 1/(2.5 – 103) = 0.4 ms = 400 µs.
Mark/space ratio = 1/10.
Pulse width = 400/11 = 36.4 µs.
The SCR will ‘turn on’ as the pulse width is greater than SCR turn on time.
8. What are the three terminals of a power MOSFET called?
A. Collector, emitter, Gate
B. Drain, source, gate
C. Collector, emitter, base
D. Drain, emitter, base
Answer: B
Clarification: A power MOSFET has three terminals called Drain, source and gate in place corresponding to the three terminals Collector, emitter and base for BJT.
9. A thyristor can be termed as ______
A. AC switch
B. DC switch
C. Wave switch
D. Square wave switch
Answer: b
Clarification: Thyristor is a unidirectional device, that is it will only conduct current in one direction only, but unlike a diode, a thyristor can be operate as either ran open circuit switch or as a rectifying diode depending on how the thyristor gate is triggereC. In other words, the thyristor can operate only in switching mode.
10. If anode current is 400 A, then the amount of current required to turn off the GTO is about ________
A. 20A
B. 200A
C. 400A
D. 100A
Answer: D
Clarification: Generally, anode current required of GTO is four times of turn off current. So the amount of current required to turn off GTO is 400/4 = 100A.