# Ac To Dc Converter Interview Questions

## Power Electronics Interview Questions with Answers

1. What is holding current in SCR?

• It is the minimum current required to hold the SCR in forward conduction state.
• When the forward current becomes less than holding current, SCR turns from forward conduction state to forward blocking state.
• 2. What is latching current in SCR? It is the minimum current required to latch(turn on) the SCR from forward blocking state to forward conduction state.

3. What are the different turn on methods of SCR?

• Forward voltage triggering
• Gate Triggering
• dv/dt triggering
• Temperature triggering
• Light triggering
• 4. What is snubber circuit? The snubber circuit is used for the dv/dt protection of the SCR. It is a series combination of a resistor and a capacitor in parallel with the SCR.

5. What is hard switching of the thyristor? When gate current is several times higher than the required gate current, the SCR is said to be hard fired. It reduces the turn ON time and enhances the di/dt capability.

6. What is firing angle? The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.

7. What is meant by SOA? SOA – Safe Operating Area determines the voltage and current boundary within which the Power Device can be operated without destructive failure.

8. What are the main components used for isolating the Power Circuits, Power Semiconductor from the low-power circuit? Opto-Couplers, Transformers

9. Name some of the current controlled (current driven) devices… SCR, GTO, GTR

10. Name some of the voltage driven ( Voltage controlled) devices IGBT, MCT, IGCT, SIT

11. What is duty cycle? It is the ratio of the ON time of the chopper to total time period of the chopper. D = Ton / [Ton + Toff]

12. Can fuses with an AC voltage rating be used in a DC applications?

• Fuses must be rated for the voltage AC or DC in which they will be used.
• Generally, fuses have a DC voltage rating that is half of the maximum AC voltage rating.
• 13. What are the characteristics of ideal Opamp?

• Infinite open loop voltage gain
• Infinite input impedance
• Zero output impedance
• Infinite Bandwidth
• Zero offset voltage
• 14. For High voltage applications will you prefer MOSFET or IGBT?

• For High voltage applications we have to use IGBT.
• Because MOSFETs are low voltage devices. ie, Their voltage rating is lesser than IGBT.
• General rule is MOSFETs are suitable for applications which has breakdown voltage less than 250V.
• The IGBTs are suitable for applications which has breakdown voltage upto 1000V.
• 15. For High frequency applications will you prefer MOSFET or IGBT? Why?

• For High frequency applications, MOSFET is the right choice of the device.
• Because MOSFET has low switching losses compare to that of IGBT.
• General rule of thumb is for low-frequency applications having frequency range upto 20kHz, we have to use IGBT.
• For high frequency applications having frequency range of more than 200kHz, we have to use MOSFET.
• ### Q. What are the different TURN ON methods of SCR?

A. Forward voltage triggering Gate Triggering dv/dt triggering Thermal triggering (Temperature Triggering) Light triggering (Radiation triggering)

A. The angle between the zero crossing of the input voltage and the instant the SCR is fired is called as delay angle or firing angle.

### Q. What are the types of cyclo-converters?

A. Step up cyclo-converter: In this output frequency is less than the input frequency. Step down cyclo-converter: In this output frequency is more than the input frequency.

### Q. What are the different operation regions of the thyristor (SCR)?

A. SCR or thyristor will have three regions of operations based on the mode in which the device is connected in the circuit.

Reverse blocking region: When the cathode of the thyristor is made positive with respect to the anode and no gate signal is applied. In this region SCR exhibits the reverse blocking characteristics similar to diode.

Forward blocking region: In this region the anode of the thyristor is made positive with respect to the cathode and no gate signal is applied to the thyristor. A small leakage current flow in this mode of operation of the thyristor.

Forward conduction region: when the forward voltage applied between the anode and cathode increases at particular break over voltage avalanche breakdown takes place and thyristor starts conducting current in forward direction. By this type of triggering the device damages the scr. Hence a gate signal is applied before the forward break over voltage to trigger the scr.