Power Electronics - Dec 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) Draw the circuit diagram, and control characteristics of GTO,MCT and BJT(6 marks) 1 (b) what are power Electronic circuits? Explain any two of them with cirquit, input and output waveforms.(6 marks) 1 (c) What are the peripheral effects of power electronic equipments and mention the remedies?(5 marks) 1 (d) What are the applications of power MOSFET.(3 marks) 2 (a) Mention the merits and dernerits of power MOSFET(4 marks) 2 (b) Draw the transient model of BJT and explain the switching characteristic of bipolar transistor(8 marks) 2 (c) What is the need of base drive control and, in a power transistor? Explain anti saturation control.(8 marks) 3 (a) Explain the two transistor model of thyristor and derive anexpression for anode current interms of current amplification, factor and leakage current(8 marks) 3 (b) For the thyristor circuit shown ih f ig.Q.3(b), the SCR has a latching current of 50mA and is fixed by a pulse of length 5?sec. Show that withbut resistance R, the thyristor will failto remain on, when the fuing pulse ends and then find,the maximum value of R to ensuring fring. (6 marks) 3 (c) Design a UJT relaxation trigger circuit for SCR with V_BB=20V, ?=0.6, I_p=10?a, V^v=2V and I_V=10 mA. The frequeniy of oscillation is 100Hz and triggering pulse width should be 50?sec(6 marks) 4 (a) With the circuit and waveform, explain the operation of a single phase semi converter with inductive load.(8 marks) 4 (b) A single phase full converter is operated from a 120V, 60Hz supply. The load current with an average of I_n is continuous with negligible ripple current. If the delay angle is ?=?/3, caluclate: (i) Harmonic factor; (ii) Displacement factor, (iii) Power factor.(7 marks) 4 (c) What are the advantages of 1-? dual converter operation with circulating current.(5 marks) 5 (a) Compare natural and foced commutation.(5 marks) 5 (b) Determine the proper values of the commutating components for the circuit shown in Fig.Q5(b). The load current to be commutated is 5A, turn off time is 50?sec, supply voltage is 100V and SCR₂ holding current is 2mA. (7 marks) 5 (c) With the necessary circuit diagram and waveform, explain the operation of a complementary commulation.(8 marks) 6 (a) What are the applications of AC voltage controller?(4 marks) 6 (b) For the AC voltage controller shown in Fig.Q6.(b), calculate the average power in the lad if the thyristor firing angle is fixed at 45° with respect to supply coltage. Derive the necessary equation. (8 marks) 6 (c) Explain the operation of a 1-? controller with inductive loads and derive the expression of rms value of the output voltage.(8 marks) 7 (a) what are the applications of DC choppers?(4 marks) 7 (b) A DC chopper has a resistive load of 20? and input voltage 220V. When the chopper is on its voltage drop is 1.5V and chopping frequency is 10 kHz. If the duty cycle is 80% determine the average output voltage, rms value of the output voltage and chopper on time.(7 marks) 7 (b) Explain the operation of a step down chopper with RL load and also derive an expression of peak-peak output ripple current.(9 marks) 8 (a) Compare voltage inverter and current source inverter.(5 marks) 8 (b) Explain the following performance parameters of a inverter :
(i) Harmonic factor of nth harmonic
(ii) Total harmonic distortion
(iii) Distortion factor (6 marks) 8 (c) The single phase full bridge inverter has aresistive load of R=24 ? and the DC input voltage of V_s-48V. Determine :
(i) Rms output voltage at the fundamental frequency
(ii) Output power
(iii) Peak and average currents of each transistor.(9 marks)