250+ TOP MCQs on Thyristor Ratings and Answers

Power Electronics Multiple Choice Questions on “Thyristor Ratings”.

1. The voltage safety factor (V_SF) for an SCR is the ratio of
A. peak working voltage & peak reverse repetitive voltage
B. dv/dt & di/dt
C. peak repetitive reverse voltage & maximum value of input voltage
D. peak repetitive reverse voltage & rms value of input voltage
Answer: C
Clarification: It is peak repetitive reverse voltage/ the maximum value of input voltage(Vm).

2. The forward dv/dt rating of an SCR
A. increases with increase in the junction temperature
B. decreases with increase in the junction temperature
C. increases with decrease in the rms value of forward anode-cathode voltage
D. decreases with decrease in the rms value of forward anode-cathode voltage
Answer: A
Clarification: If the temperature is high, lesser dv/dt is required to turn on the device as the higher temperature has already excited few of the holes & electrons.

3. The finger voltage of an SCR is
A. minimum value of Vak to turn on the device with gate triggering
B. maximum value of Vak to turn on the device with gate triggering
C. minimum value of Vak to turn on the device without gate triggering
D. maximum value of Vak to turn on the device without gate triggering
Answer: A
Clarification: Finger voltage is the minimum value of Vak (anode to cathode voltage) to turn on the device with gate triggering, it is to be avoided for accidental turn-on of the device.

4. Which among the following anode current waveforms will have the minimum junction temperature?
A. 100 % DC
B. 25 % DC
C. 50 % DC
D. AC
Answer: B
Clarification: N % DC is nothing but a wave with n % duty cycle. Lower the Duty cycle lesser is the current flowing & lesser is the temperature dissipation.

5. An SCR has half cycle surge current rating of 3000 A for 50 Hz. Calculate its one-cycle surge current rating
A. 3121.32 A
B. 2121.32 A
C. 3131.32 A
D. 2131.32 A
Answer: B
Clarification: By equating the energies involved in one cycle & subcycle
I².T = Isb².t
Isb = 3000 A
T = 1/50
t = T/2.

6. For a SCR the maximum rms on-state current is 35 A. If the SCR is used in a resistive circuit for a rectangular wave with conduction angle of 90°. Calculate the average & rms currents respectively.
A. I/4, I/2
B. I/2, I/√2
C. I/4, I²/2
D. I/4, I/√2
Answer: A
Clarification: First convert the conduction angle into N for rectangular wave.
N = 360°/Conduction angle = 4
Therefore, Iavg = I/4
Irms = I/√4 = I/2.

7. For an SCR the average & rms values of current are I/4 & I/2 respectively. Calculate the average on-state current rating (I_TAV). Take maximum RMS on-state current = 35 A.
A. 8.78 A
B. 10.10 A
C. 17.5 A
D. 24.74 A
Answer: C
Clarification: Form factor = RMS/Average vaules of current
Therefore, FF = 2
I^TAV = 35/2 = 17.5.

8. The amp²-sec rating of the SCR specifies
A. The power dissipated by the device when fault occurs
B. The energy dissipated by the device when fault occurs
C. The energy that the device can absorb before the fault is cleared
D. The energy that the device can absorb while operating in the forward blocking mode.
Answer: C
Clarification: The amp²-sec is the energy that the device can handle before the fault is cleared. Its value decides how fast the fault has to be cleared to avoid damage to the device.

9. The maximum rms current of an SCR is 50 A. For a 120° sine wave conduction the form factor (FF) = 1.878
Find the average on-state current rating (I_TAV).
A. 93.9 A
B. 174 A
C. 26.62 A
D. 68.52 A
Answer: C
Clarification: I^TAV = 50/FF.

10. The thermal resistance between junction & the SCR (θ_jc) has the unit
A. Ω/°C
B. W/Ω
C. °C/W
D. ΩW/°C
Answer: C
Clarification: Thermal resistance always has the unit degree temperature per watt.