Power electronics Viva Voce or Interview questions part-1

 Q-1 Why IGBT is very popular nowadays?

Ans a. Lower heat requirements

b. Lower switching losses

c. Smaller snubber circuit requirements

Q-2 What are the different methods to turn on the thyristor?

Ans a. Forward voltage triggering

b. Gate triggering

c. dv/dt triggering

d. Temperature triggering

e. Light triggering

Q-3 What is the difference between power diode and signal diode?

Ans 

Power diode

1. Constructed with n-layer, called drift region between p+ layer and n+ layer.

2. The voltage, current and power ratings are higher.

3. Power diodes operate at high speeds.

Signal diode

1. Drift region is not present.

2. Lower

3. Operates at higher switching speed.

Q-4 IGBT is a voltage controlled device. Why?

Ans Because the controlling parameter is gate-emitter voltage.

Q-5 Power MOSFET is a voltage controlled device. Why?

Ans Because the output (drain) current can be controlled by gate-source voltage.

Q-6 Power BJT is a current controlled device. Why?

Ans Because the output (collector) current can be controlled by base current.

Q-7 What are the different types of power MOSFET?

Ans a. N-channel MOSFET

b. P-channel MOSFET

Q-8 How can a thyristor turned off?

Ans A thyristor can be turned off by making the current flowing through it to zero.

Q-9 Define latching current.

Ans The latching current is defined as the minimum value of anode current which it must attain during turn on process to maintain conduction when gate signal is removed.

Q-10 Define holding current.

Ans The holding current is defined as the minimum value of anode current below 

which it must fall to for turning off the thyristor.

Q-11 What is a snubber circuit?

Ans It consists of a series combination of a resistor and a capacitor in parallel with 

the thyristors. It is mainly used for dv / dt protection

Q-12 What losses occur in a thyristor during working conditions?

Ans a. Forward conduction losses

b. Loss due to leakage current during forward and reverse blocking.

c. Switching losses at turn-on and turn-off.

d. Gate triggering loss.

Q-13 Define hard-driving or over-driving.

Ans When gate current is several times higher than the minimum gate current required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor reduces its turn-on time and enhances its di/dt capability.

Q-14 Define circuit turn off time.

Ans It is defined as the time during which a reverse voltage is applied across the thyristor during its commutation process.

Q-15 Why circuit turn off time should be greater than the thyristor turn-offtime?

Ans Circuit turn off time should be greater than the thyristor turn-off time for reliable turn-off, otherwise the device may turn-on at an undesired instant, a process called commutation failure.

Q-16 What is the turn-off time for converter grade SCRs and inverter grade SCRs?

Ans Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.

Q-17 What are the advantages of GTO over SCR?

Ans a. Elimination of commutation of commutating components in forced commutation, resulting in reduction in cost, weight and volume.

b. Reduction in acoustic noise and electromagnetic noise due to elimination of commutation chokes.

c. Faster turn-off, permitting high switching frequencies.

d. Improved efficiency of the converters.

Q-18 What is meant by phase controlled rectifier?

Ans It converts fixed ac voltage into variable dc voltage.

Q-19 Mention some of the applications of controlled rectifier.

Ans a. Steel rolling mills, printing press, textile mills and paper mills employing dc motor drives.

b. DC traction

c. Electro chemical and electro-metallurgical process

d. Portable hand tool drives

e. Magnet power supplies

f. HVDC transmission system

Q-20 What is the function of freewheeling diodes in controlled rectifier?

Ans It serves two process.

a. It prevents the output voltage from becoming negative.

b. The load current is transferred from the main thyristors to the freewheeling diode, thereby allowing all of its thyristors to regain their blocking states.

Q-21 What are the advantages of freewheeling diodes in a controlled in a controlled rectifier?

Ans a. Input power factor is improved.

b. Load current waveform is improved and thus the load performance is better.

Q-22 What is meant by delay angle?

Ans The delay angle is defined as the angle between the zero crossing of the input 

voltage and the instant the thyristor is fired.

Q-23 What are the advantages of single phase bridge converter over single phase mid-point converter?

Ans a. SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge rectifier. Hence for same voltage and current ratings of SCrs, power handled by mid-point configuration is about

b. In mid-point converter, each secondary winding should be able to supply the load power. As such, the transformer rating in mid-point converter is double the load rating.

Q-24 What is commutation angle or overlap angle?

Ans The commutation period when outgoing and incoming thyristors are conducting is known as overlap period. The angular period, when both devices share conduction is known as the commutation angle or overlap angle.

Q-25 What are the different methods of firing circuits for line commutated converter?

Ans a. UJT firing circuit.

b. The cosine wave crossing pulse timing control.

c. Digital firing schemes.