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Power Electronics : Question Paper Jun 2013 - Electronics & Communication (Semester 7) | Visveswaraya Technological University (VTU)
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## Power Electronics - Jun 2013

### 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 is power electronics? Draw a neat block diagram of generalized power converter system. State the applications of power electronics.(6 marks) 1 (b) With neat circuit diagram and waveforms, explain the types of power electronics circuits.(12 marks) 1 (c) Compare General-purpose, Fast recovery and Schokky diodes.(2 marks) 2 (a) The maximum junction temperature of a transistor is T3=150°C and the ambient temperature is TA=25°C, if the thermal impedances are RJC=0.4°C/W, Res=0.1°C/W, and RSA=0.5°C/W. Calculate (i) the maximum power dissipation and (ii) the case temperature.(4 marks) 2 (b) With the help of parasitic model and switching model, explain the switching wavefoms of n-type (enhancement) MOSFET.(10 marks) 2 (c) Write a note on isolation of grate and base drives.(6 marks) 3 (a) Using a two model of thyristor, show that $$I_A=\dfrac {\alpha I_{C}+I_{CBOI}+I_{CBO2}}{1-(\alpha_{1}+\alpha_2)}$$ (6 marks) 3 (b) With neat sketch, explain turn-on characteristics of SCR.(6 marks) 3 (c) The input voltage of Fig.Q3(b) is Vs=200V with load resistance of R=5?. The load and stray inductance are negligible and the thyristor is operated at frequency of fs=2 kHz. If the required dv/dt is 100 V/?s and the discharge current is limited to 100A. Determine (i) the value of Rs and Cs (ii) the snubber loss, and (iii) the power rating of the snubber resistor.

dv/dt Protection Circuit (8 marks) 4 (a) Discuss the effect of inductance using the 1-? full converter.(8 marks) 4 (b) What is phase control? Explain the principle of phase control using 1-? half wave controlled rectifier.(8 marks) 4 (c) Compare circulating and non-circulating mode of operation of dual converter.(4 marks) 5 (a) A thyristor circuit is shown in fig.Q5(a), if thyristor T1 is switched on at t=0, determine the conduction time of thyristor are L=10 ?H, C=50?F and V3=200 V. The inductor carries an intial current of lm=250A.

Self Commulated Circuit
(10 marks)
5 (b) What is the principle of complementary commutation? Explain the same with the help of suitable circuit and waveforms.(10 marks) 6 (a) What is the principle of on off control? Explain the same with a single phase full-wave controller.(6 marks) 6 (b) Draw a neat sketch of 1-? AC voltage controller with RL, load and explain its working .(6 marks) 6 (c) A single-phase full wave AC voltage controller has a resistive load R=10? and the input votage is V3=120V (rms), 60Hz. The delay angles of thyristor T1 and T2 are equal ?1 - ?2 - ?/2. Determine (i) the rms output voltage V0 (ii) the input power factor PF
(iii) the average current of thyristor IA and (iv) the rms current of thyristor IR.
(8 marks)
7 (a) With a neat circuit diagram, explain the operation of a step down chopper and also explain constant frequency and variable frequency operation. Derive an expression for output voltage incase step down chopper.(10 marks) 7 (b) A step-up chopper with a pulse width of 200?s operating on 200V, dc supply. Calculate the output voltage, if the blocking period of the devices is 50?s.(2 marks) 7 (c) With a neat mcircuit diagram and quadrant operation, explain class F. chopper.(8 marks) 8 (a) With a neat circuit diagram, of 1-0 half bridge inverter, explain the principle of operation of an inverter.(8 marks) 8 (b) Write brief note on current source inverter.(6 marks) 8 (c) With the help of circuit diagram and waveforms, explain a variable DC-link inverter.(6 marks)