MCQs on Power Electronics

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01․ Compared to transistor, ______________ have lower on state conduction losses and higher power handling capability
Semi conductor diodes.

Thyristor is a solid state device like a transistor and has lower on state conduction losses than transistor and higher power handling capability than transistor.

02․ A thyristor can termed as
AC switch.
DC switch.
Both a and B.
Square wave switch.

Thyristor is unidirectional device, that is it will only conduct current in one direction only, but unlike a diode, the thyristor can be to operate as either an open circuit switch or as a rectifying diode depending upon how the thyristor gate is triggered. In other words thyristors can operate only in the switching mode.

03․ Thyristor is nothing but a
Controlled transistor.
Controlled switch.
Amplifier with higher gain.
Amplifier with large current gain.

Thyristor is a solid state device which is operated as bistable controlled switch, operating from non conducting state to conducting state.

04․ The capacitance of reversed bised junction J2 in a thyristor is CJ2 = 20 pF and can be assumed to be independant of the off state voltage. The limiting value of the charging current to turn on the thyristor is 16 mA. What is the critical value of dv/dt?
600 V/µs.
800 V/µs.
1200 V/µs.
1000 V/µs.

CJ2 = 20 pF and IJ2 = 16 mA. d(CJ2)/dt = 0. dv/dt = ij2/Cj2 = (16 X 10-3)/(20 X 10-12) = 800 V/ µs.

05․ BCT is used for
High power phase control.
High power current control.
Low power current control.
Low power phase control.

The bidirectional control thyristor (BCT) is for high power thyristors integrated on a single silicon water with seperate gate contacts. It is used for high power phase control.

06․ Which of following devices has highest di/dt and dv/dt capability?

The static induction is thyristor with a buried gate structure in which the gate electrode are placed in n-base region. They have high di/dt and dv/dt rating.

07․ Which of the following is disadvantage of fast recovery diodes?
Recovery is only 5 µs.
Recovery is only 50 µs.
Doping is carried out.
None of these.

The diodes with the low reverse recovery time of about 5 µs or less are called fast recovery diode. For voltage above 400 V diffusion technique is used for the fabrication of diodes. In order to shorten the reverse recovery time, platinum or gold doping is carried out. This doping may increase the forward voltage drop diode.

08․ A power semiconductor may undergo damage due to
High di/dt.
Low di/dt.
High dv/dt.
Low dv/dt.

di/dt indicates the maximum rate of rise current from anode to cathode without any harm to the device. When a power semiconductor device is turned-on, conduction starts at a place near to gate. This small area of conduction spreads to the whole area of junction. If the rate of rise of anode current is large as compared to the spreading velocity of carries across the cathode junction, local hot spot will be formed near the gate connection on account of high current density. This cause the junction temperature to rise above the safe limit and as a consequence, semiconductor device may be damaged permanently.

09․ If the anode current is 800 A, then the amount of current required to turn off the GTO is about
20 A.
600 A.
400 A.
200 A.

Generally anode current requred of GTO is 4 times of turn off current. So the amount of current requred to turn off GTO is = 800/4 = 200 A.

10․ Which semiconductor device acts like a diode and two transistor?

The triac is similar in operation to two transistor connceted in reverse parallel but using a common gate. The triac can conduct in both direction.