POWER ELECTRONICS

It is preferable to use a train of pulse of high frequency for gate triggering of SCR in order to reduce

1. dv/dt problem
2. di/dt problem
3. the size of the pulse transformer
4. the complexity of the firing circuit

Reverse recovery current in a diode depends upon

1. forward field current
2. storage charge
3. temperature
4. PIV

In a 3-phase full-wave a.c. to d.c. converter, the ratio of output-ripple frequency to the supply-voltage frequency is

1. 2
2. 3
3. 6
4. 12

A fully controlled natural commutated 3-phase bridge rectifier is operating with a firing angle α= 30^°. The peak to peak voltage ripple expressed as a ratio of the peak output d.c. voltage to the output of the converter bridge is

1. 5
2. 1 − (√(3)/2)
3. √(3)− 1
4. √(3)/2

Natural commutation of a thyristor takes place when

1. anode current becomes zero
2. gate current becomes zero
3. voltage across the device becomes zero
4. voltage across the device becomes negative

A four-quadrant chopper cannot be operated as

1. one-quadrant chopper
2. cycloconverter
3. inverter
4. bi-directional rectifier

A boost regulator has an input voltage of 5 V and the average output voltage of 15 V. The duty cycle is

1. 3/2
2. 2/3
3. 5/2
4. 15/2

PWM switching is preferred in voltage source inverter for the purpose of

1. controlling output voltage
2. output harmonics
3. reducing filter size
4. controlling output voltage, output harmonics and reducing filter size

Compared to a single-phase half-bridge inverter, the output power of a single-phase full-bridge inverter is higher by a factor of

1. 12
2. 8
3. 4
4. 2

How many switches are used to construct a 3-phase to 3-phase cycloconverter?

1. 3
2. 6
3. 12
4. 18

A single-phase bridge-type cycloconverter feeds a load R. For an output frequency equal to one-third of the input frequency, sketch output voltage waveform for a firing angle of about 30^°. Also derive an expression for the r.m.s. output voltage.

Explain the processes of dv/dt triggering and temperature triggering.

A 3-phase bridge inverter delivers power to a resistive load from a 450 V d.c. source. For a star-connected load 10~Ω per phase, determine the following for 120^° mode of operation:
(i) r.m.s. value of output phase and line voltages
(ii) r.m.s. value of transistor current
(iii) r.m.s. value of load current
(iv) Load power

Describe the structural features of power diodes. How do these differ from signal diodes?

A single-phase full-converter delivers a constant load current I₀. Express its source current in Fourier series.

Derive the expressions for the following performance parameters with the help of waveforms and Fourier series expression in Question No. 4(a): Input displacement factor, Current distortion factor, Power factor, Total harmonic distortion, Voltage ripple factor, Active and Reactive power inputs.

A battery is charged through a resistor R. If v_s= 220√(2)sinωt, E = 100V (d.c.) and R = 20~Ω, calculate the battery charging current and power supplied to the battery.

Write down any four applications of cycloconverters.

Discuss with relevant waveforms, class-B (resonant-pulse commutation) type of commutation employed for thyristor circuits.

What are forward blocking losses and gate power loss in a thyristor?

For step-down chopper circuit, source voltage V_s= 220V, chopping period T = 2000~μs, on period = 600~μs, load circuit parameters R = 1~Ω, L = 5mH and E = 24V.
(i) Find whether load current is continuous or not.
(ii) Calculate the value of average output current.
(iii) Compute the maximum and minimum values of steady state output currents.

Draw the circuit diagram of step-down chopper, also draw the waveforms of source voltage V_s and output voltage V₀ for duty ratio α= 0.4 and time period T = 10~μs.

Describe unipolar-voltage switching in a single-phase full-bridge inverter by suitable waveforms for carrier wave, reference wave and output voltage. Show the turn-on and turn-off of various switches in waveforms.

Justify why it is called unipolar-voltage switching inverter or three-level PWM inverter.

Explain the operation of boost converter with voltage and current waveforms across the boost inductor. Assume continuous conduction.

Derive its output voltage equation in terms of duty cycle and input voltage.