2. SCR turns on by regeneration but there is no regeneration in BJT.
3. Gate has no control over the conduction once SCR turns on. BJT can be turned off by base drive.
4. BJT’s have higher switching frequencies than SCR’s.
Q2. Explain latching current and holding current of the thyristor?
Holding Current: It is the minimum forward current that flows through the thyristor to keep it in forward conduction mode. When forward current reduces below holding current, thyristor turn off.
Q3. What are the advantages of MOSFET’s over BJT’s?
ii) They have high switching frequencies.
iii) MOSFET’s are majority carrier devices.
iv) MOSFET’s have positive temperature coefficients.
Q4. Why pulsed gate drive is used for SCR?
Q5. Define the delay angle of phase-controlled rectifier.
Q6. Why is power factor of semiconverter better than full converter?
Q7. What are the differences between freewheeling diode and feedback diode?
Freewheeling diode is used to circulate the load energy in the load itself. Feedback diode is used to transfer the load energy to the source. Feedback diode is normally used in inverters. Freewheeling diode is normally used in controlled rectifiers.
Q8. What is inverting operation of the converter?
Q9. What are control strategies of chopper?
1. Fixed frequency operation: in this type of control, the frequency of chopper remains fixed , but on and off periods of chopper change as per the duty cycle.
2. Variable frequency operation: in this type of control, the on or off period remains fixed. Hence frequency of the chopper changes when duty cycle is changed.
3. Current limit control (CLC) : in this type of control, the output is sensed, when output current falls below reference level, chopper is turned on. And when output current rises above reference level, chopper is turned off.
Q10. Explain the use of step-up chopper.
Step-up chopper is normally used for energy transfer. The load energy is transferred to source by step-up operation.
Q11. What is four quadrant chopper?
Q12. What is the main difference between voltage source inverter and current source inverter?
1. In the voltage source inverter, input voltage remains fixed and current varies according to load variations. But in current source inverter, input current remains fixed and voltage varies according to the load.
2. In voltage source inverters, short circuit of the devices can damage them. In current source inverter short circuit cannot damage the circuit.
3. Voltage source inverters need freewheeling diodes. But current source inverters doesn’t need freewheeling diodes.
Q13. What are PWM inverters?
Q14. What is cycloconverter?
Q15. What are the advantages and disadvantages of AC voltage controller?
1. They use line communication, hence no extra communication circuits are required.
2. They have high efficiencies since device losses are reduced.
1. Large ripple and harmonics are present in the output.
2. Output waveforms are not sinusoidal.
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