 MORE IN Power Electronics
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INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

1(a) Draw the control characteristics of the following: i) SCR; ii) BJT; iii) MCT; iv) GTO; v) MOSFET.
10 M
1(b) Explain any four industrial applications for power electronics.
10 M

2(a) What is MOSFET? Draw static and switching characteristics of a MOSFET and explain the operation of MOSFET as a switch.
12 M
2(b) A simple transistor switch is used connect a 24V DC supply across a relay coil, which has a DC resistance of 200Ω. An input pulse of 0 to 5V amplitude is applied through a series base resistance RB at the base so as to turn on transistor switch. Calculate,
i) ICS ; ii) Value of resistor RB, required to obtain over drive factor of 2 ; iii) Total power dissipation in the transistor that occurs during the saturation state.
8 M

3(a) With neat circuit diagram and waveforms explain R-triggering technique of SCR.
8 M
3(b) Derive an expression for the anode current of thyristor with the help of two transistor analogy.
8 M
3(c) An SCR is connected in seies with a 0.5H inductor and 20Ω resistance. A 100V DC voltage is applied to this circuit. If the latching current of the SCR is 4mA, find the minimum width of the gate trigger pulse required to property turn on the SCR.
4 M

4(a) What do you mean by natural and forced commutation in thyristor circuits? Explain with waveform and circuit complimentary communication.
12 M
4(b) Explain external pulse commutation with neat circuit and waveform and derive an expression for circuit turn off time.
8 M

5(a) What is controlled rectifier? With neat circuit and waveform explain single phase semiconverter with R-load.
12 M
5(b) For a 3ϕ fully controlled SCR bridge converter operating from 400V, 3ph ac supply, calculate the average DC output voltage for a firing angle of 45°. Write the necessary diagram.
8 M

6(a) Explain the classification of chopper with neat circuit diagram and wave form.
12 M
6(b) A step-up chopper has input voltage of 220 V and an output voltage of 660 V. If the non-conducting time of thyristor chopper is 100μsec, compute the pulse width of input voltage. In case pulse width is halved for constant frequency operation, find the new output voltage.
8 M

7(a) What is an inverter? Explain half bridge inverter with R-load.
10 M
7(b) Briefly explain performance parameters of an inverter.
6 M
7(c) With relevant waveform explain the sinusoidal pulse width modulation of an inverter.
4 M

8(a) With the help of neat circuit diagram and waveform explain on-off control and derive an expression for i) RMS value of o/p voltage ; ii) Input power factor.
12 M
8(b) A single phase half wave ac voltage controller has a resistive load of R = 5Ω and input voltage Vs = 120V, 60Hz. The delay angle of thyristor is α = π/3. Determine: i) RMS output voltage ; ii) Input power factor, iii) Average input current.
8 M

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