19-10-2012, 01:33 PM
SHORT QUESTIONS AND ANSWERS EE1301-Power Electronics
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1. Why IGBT is very popular nowadays?
a. Lower hate requirements
b. Lower switching losses
c. Smaller snubber circuit requirements
2. What are the different methods to turn on the thyristor?
a. Forward voltage triggering
b. Gate triggering
c. dv/dt triggering
d. Temperature triggering
e. Light triggering
3. What is the difference between power diode and signal diode?
4. IGBT is a voltage controlled device. Why?
Because the controlling parameter is gate-emitter voltage.
5. Power MOSFET is a voltage controlled device. Why?
Because the output (drain) current can be controlled by gate-source voltage.
6. Power BJT is a current controlled device. Why?
Because the output (collector) current can be controlled by base current.
7. What is the relation between and ?
8. What are the different types of power MOSFET?
a. N-channel MOSFET
b. P-channel MOSFET
9. How can a thyristor turned off?
A thyristor can be turned off by making the current flowing through it to zero.
10. Define latching current.
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.
11. Define holding current.
The holding current is defined as the minimum value of anode current below
which it must fall to for turning off the thyristor.
12. What is a snubber circuit?
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.
13. What losses occur in a thyristor during working conditions?
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.
14. Define hard-driving or over-driving.
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.
15. Define circuit turn off time.
It is defined as the time during which a reverse voltage is applied across the
thyristor during its commutation process.
16. Why circuit turn off time should be greater than the thyristor turn-off time?
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.
17. What is the turn-off time for converter grade SCRs and inverter grade SCRs?
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.
18. What are the advantages of GTO over SCR?
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.
19. What is meant by phase controlled rectifier?
It converts fixed ac voltage into variable dc voltage.