# Power Electronics Questions and Answers – Thyristors-3

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This set of Power Electronics Multiple Choice Questions & Answers (MCQs) focuses on “Thyristors-3”.

1. During the transition time or turn-on time
a) The forward anode voltage decreases from 90 % to 10 % & the anode current also decreases from 90 to 10 % of the initial value
b) The forward anode voltage increases from 10 % to 90 % & the anode current also increases from 10 % to 90 % of the initial value
c) The forward anode voltage decreases from 90 % to 10 % & the anode current increases from 10 % to 90 % of the initial value
d) The forward anode voltage increases from 10 % to 90 % & the anode current decreases from 90% to 10% of the initial value

Explanation: During the turn on time, the voltage across the SCR is going down and the current through it is slowly rising as it is going into the conduction mode.

2. For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and
During the delay time the
a) anode current flows only near the gate
b) anode current rises from zero to very high value
c) losses are maximum
d) anode to cathode voltage is zero

Explanation: Initially for a fraction of a microsecond (delay time) after the gate signal is applied the anode current only flows near the gate terminal where the gate current density is maximum, as the gate current takes some time to spread all over the cross section of the device.

3. The minimum value of anode current below which it must fall to completely turn-off the device is called as the
a) holding current value
b) latching current value
c) switching current value
d) peak anode current value

Explanation: The device will remain in the conducting state unless the anode current falls below the holding current value.
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4. For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and
During the rise time the
a) anode current flows only near the gate
b) anode current rises from zero to very high value
c) losses are maximum
d) anode to cathode voltage is zero

Explanation: The losses are maximum during the rise time because both Ia & Va are high.

5. The latching current is _________ than the holding current
a) lower
b) higher
c) same as
d) negative of

Explanation: The latching current is the value of current on which the device will remain in the on state even after removal of the gate signal. Whereas, the holding current is the threshold above which the device will work.

6. For an SCR the total turn-on time consists of
i) Delay time
ii) Rise time and the
The spread time interval depends upon
a) the value of gate current
b) junction temperature
c) area of the cathode
d) area of the anode

Explanation: During the spread time the conduction starts spreading all over the SCR cathode cross-section structure, which depends upon the structure of the gate & cathode. Higher the cathode area more is the time required for the charges to spread all over.

7. For effective turning off of the SCR after the anode current has reached zero value, ______________
a) chargers are injected by applying reverse anode-cathode voltage
b) chargers are removed by applying reverse anode-cathode voltage
c) chargers are injected by applying gate signal
d) chargers are removed by applying gate signal

Explanation: To enable the device to regain its reverse blocking capabilities, the stored charges in the junctions of the SCR must be removed.

8. To avoid commutation failure
a) circuit turn-off time must be greater than the thyristor turn-off time
b) circuit turn-off time must be lesser than the thyristor turn-off time
c) circuit turn-off time must be equal to the thyristor turn-off time
d) none of the above mentioned

Explanation: If the thyristor turn off time is more than the circuit turn off time, the circuit will be turned off and the thyristor will keep conducting, which is not at all desirable.

9. The gate characteristics of thyristor is a plot of
a) Vg on the X-axis & Ig on the Y-axis
b) Ig on the X-axis & Vg on the Y-axis
c) Va on the X-axis & Ig on the Y-axis
d) Ig on the X-axis & Va on the Y-axis

Explanation: It is the gate current versus the gate voltage plot and gives the minimum and maximum values of gate parameters.

10. The area under the curve of the gate characteristics of thyristor gives the
a) total average gate current
b) total average gate voltage
c) total average gate impedance
d) total average gate power dissipation

Explanation: As the gate characteristics is a plot of Ig vs Vg consisting of two curves one for the maximum values & other for the minimum the area between them gives the total average gate power dissipation. (A very important parameter in designing of the triggering circuits).

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