Power Electronics - Jan 2012
Electronics & Communication (Semester 7)
TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks. 1 (a) What are the important applications of power electronics?(6 marks) 1 (b) Explain peripheral effects of power converter system.(6 marks) 1 (c) Explain different types of power electronic converter circuits.(8 marks) 2 (a) Draw the equivalent model of BJT and explain the switching characteristics of power transistor.(8 marks) 2 (b) Compare the BJT and MOSFET.(5 marks) 2 (c) Discuss the different methods of providing isolation of gate drive circuits from power circuit.(7 marks) 3 (a) Explain the two transistor analogy of an SCR. Derive an expression for anode current. Also explain the process of regeneration cycle.(10 marks) 3 (b) Write a short note on dv/dt and di/dt protection.(5 marks) 3 (c) A UJT is connected across a 20 V DC supply. A valley and peak point voltages are 1 V and 15 V. The period of relaxation oscillation is 20 ms. Find the value of charging capacitor, if a charging resistor of 100 kΩ is used.(5 marks) 4 (a) Explain with the help of wave forms,1-Φ semi-converter (half bridge converter) with R-load. Derive an expression for V0(avg) and V0(rms).(10 marks) 4 (b) A single phase dual converter is operated from 120 V, 50 Hz supply and the load resistance R=10Ω the circulating inductance LT=40 mH delay angle are α1=60° and α2=120°. Calculate the peak circulating current and peak current of converter.(6 marks) 4 (c) What is the use of wheeling diode in a converter circuit?(4 marks) 5 (a) Explain the self communication with the help of neat sketch and obtain the expression for the capacitor voltage and current.(10 marks) 5 (b) Distinguish between the natural and forced communication.(5 marks) 5 (c) In the parallel capacitor turn-off circuit shown in fig.Q5(c), the main SCRT, is to be reverse biased for at least 40 µs for proper communication and holding current of auxiliary SCRT2 is 2 mA. Determine the value of R and C.
(5 marks) 6 (a) With the help of circuit diagram, explain the operation of single phase AC regulator using ON-OFF control. Derive the expression for ms value of load voltage.(10 marks) 6 (b) Explain the single phase bidirectional AC voltage controller with resistive load with waveform.(10 marks) 7 (a) With neat circuit diagram and explain the four quadrant chopper.(8 marks) 7 (b) With neat circuit diagram, explain the principle of operation of step up chopper.(8 marks) 7 (c) A chopper circuit is operating on Time Ration Control (TRC) at a frequency of 2 kHz on a 460 V supply of the load voltage of 350 V. Calculate the conduction period of the thyristors in each cycle.(4 marks) 8 (a) Explain the performance of inverters.(6 marks) 8 (b) With a neat circuit diagram, explain the principle of variable DC link.(8 marks) 8 (c) Write a short note on CSI (Current Source Inverter).(6 marks)