Power Electronics Question and Answers – Thyristor Mounting and Answers

Power Electronics Multiple Choice Questions on “Thyristor Mounting”.

1. The usual way to accomplish higher gate current for improved di/dt rating is by using
A. varistors
B. pilot thyristors
C. twisted cables
D. op-amps
Answer: B
Clarification: Pilot SCR is an SCR which is fired which activates the firing circuit and fires the main SCR.

2. Inter-digitating of gate-cathode regions in SCR devices improves the
A. I²t rating
B. di/dt rating
C. dv/dt rating
D. thermal resistance
Answer: B
Clarification: Inter-digitating is the inter-mixing of the gate-cathode area to improve the di/dt ratings. di/dt rating is improved by providing more cathode conduction area during the delay and rise time.

3. The dv/dt rating of SCR can be improved by using
A. cathode-short structure
B. anode-short structure
C. gate-short structure
D. centre gate thyristor
Answer: A
Clarification: Cathode shorts are realized by overlapping metal on cathode n+ layers with a narrow p-region in between.

4. The total thermal resistance between junction and ambient θ_jA is 10°C/W. θ_jc is 2°C/W. θ_cs is 4°C/W. θ_sA = ?
A. 4°C/W
B. 2°C/W
C. 10°C/W
D. 16°C/W
Answer: A
Clarification: θ_jA = θ_jc+ θ_cs+ θ_sA.

5. Which of the following thermal resistance parameters depends on the size of the device and the clamping pressure?
A. θ_sA
B. θ_cs
C. θ_jc
D. None
Answer: B
Clarification: The case-to-sink thermal resistance depends on the size of the device, pressure, grease between the interface, etc.

6. The sink to ambient thermal resistance of SCR θ_sA
A. depends on the flatness of the device
B. depends on the length of the device
C. depends on the current carrying capabilities
D. is independent on thyristor configuration
Answer: D
Clarification: It does not depend on any of the device configurations.

7. P_av x (θ_jc + θ_cs + θ_sA) =
A. Maximum specified temperature
B. Energy lost
C. Difference in temperature between junction & ambient
D. Sum of junction & ambient temperature
Answer: C
Clarification: P_av = (T_j – T_a)/θ_jA
θ_jA = (θ_jc + θ_cs + θ_sA).

8. Heat dissipation from heat sink mainly takes place by
A. radiation
B. convection
C. absorption
D. none of the mentioned
Answer: B
Clarification: Heat DISSIPATION (rejection of heat to the atmosphere) takes place through convention only.

9. For low power SCRs (about 1 Ampere current) _____________ type of mounting is used
A. lead
B. stud
C. bolt-down
D. press-fit
Answer: A
Clarification: Lead mounting is a very simple time of mounting used for low power devices.

10. In the ___________ type of mounting the SCR is pressed between two heat sinks
A. bolt-down mounting
B. stud-mounting
C. press-pak mounting
D. cross-fit mounting
Answer: C
Clarification: In the press-pak type the device is pressed or clamped between two heat-sinks & external pressure is applied from both the sides.