Power Electronics-1 - Dec 2014

Electronics Engineering (Semester 6)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) Differentiate between SCR and IGBT.(5 marks) 1 (b) State and explain the conditions to operate fully controlled bridge converter as an inverter.(5 marks) 1 (c) Draw and explain equivalent circuit of heat sink.(5 marks) 1 (d) Explain UJT with the help of equivalent circuit and characteristics.(5 marks) 2 (a) What do you understands by commutation of SCR. Explain class C commutation with the help of waveforms.(10 marks) 2 (b) Design the beginning circuit for SCR using UJT. The UJT parameters are VS=30, η=0.51, I_p=1 μA, Vv=3.5V Iv=10 mA. Frequency of oscillation is 50 Hz. Width of the trigger pulse is 50 7mu;s.(10 marks) 3 (a) What is the need of freewheeling diode in case rectifier with R-L load. Draw and explain full bridge controlled rectifier with R-L load and freewheeling diode. Derive the relation for load voltage.(10 marks) 3 (b) Draw and explain single AC voltage controller circuit using DIAC and TRIAC. Draw waveforms.(10 marks) 4 (a) Explain the role of snubber circuit to protect SCR against high di/dt and dv/dt.(10 marks) 4 (b) What is the difficulty in parallel connection of SCRS. Draw and explain equalising network with design criterion for components.(10 marks) 5 (a) A single phase semiconverter is operated from 120V, 50Hz, ac supply. The load resistance is 10 Ω. The average O/P voltage is 25% of the maximum possible average output voltage. Determine
i) Firing angle
ii) RMS and average output current
iii) RMS and average thyristor current.(10 marks) 5 (b) Explain three phase bridge full converter with R load with continuous load current.(10 marks) 6 (a) Draw and explain over voltage protection circuit for SCR.(5 marks) 6 (b) Differentiate between symmetrical IGBT and asymmetrical IGBT.(5 marks) 6 (c) Draw and explain RAMP and pedestal triggering circuit using UJT. Draw waveforms.(10 marks) 7 (a) Performance parameters of controlled rectifier.(5 marks) 7 (b) Soft start circuit.(5 marks) 7 (c) Half waving effect in controlled rectifier.(5 marks) 7 (d) GTO switching characteristics.(5 marks)