B.
Sc Engineering 3rd Backlog Examination 2017
Course No. EEE-3203 Course Title: Power Electronics
Full Mark: 72 Time: 3 Hours
N. B: (i) Answer any six questions taking three from each section.
(ii) Use separate answer script for each section
(iii) Figures in the margin indicate full marks
SECTION-A
Q.1 (a) What are the differences between the gating characteristics of GTOs and thyristors? 03
(b) 04
Show that the peak reverse current of a power diode is given by I RR = 2QRR di , where symbols have
dt
their usual meaning.
(c) Determine the (i) efficiency, (ii) FF, (iii) RF, (iv) TUF and (v) CF of the input current of a full wave rectifier 05
with center-tapped transformer which has purely resistive load.
Q.2 (a) What are the functions of snubber circuit? Explain diode snubber circuit. 03
(b) Explain the two transistor model of thyristor and hence derive the following equation 05
2 I G + I CB01 + I CB02
IA = ; where symbols have their usual meaning.
1 − ( 1 + 2 )
(c) The junction capacitance of a thyristor is C j 2 = 25 pF and can be assumed independent of off-state voltage. 04
The limiting value of charging current to turn on the thyristor is 12 mA. If a capacitor of 0.02 µF is
connected across the thyristor, determine the critical value of dv
dt .
Q.3 (a) Explain in brief the operation of second quadrant chopper with necessary circuit diagram, wave shapes and 04
equations.
(b) For a step down chopper with R-L load, prove that the maximum ripple current is obtained when duty cycle 04
K=0.5.
(c) The buck regulator in the following figure has an input voltage of Vs = 12V . The required average output 04
voltage is Va = 5V at R=500 Ω and the peak-to-peak output ripple voltage is 20 mV. The switching
frequency is 25 kHz. If the peak-to-peak ripple current of inductor is limited to 0.8 A, determine (i) the duty
cycle, (ii) the filter inductance, L, (iii) the filter capacitance C, and (iv) the critical values of L and C.
Q.4 (a) Draw and explain the operation of a single phase full bridge inverter with proper waveforms. 04
(b) Write short notes on (i) trapezoidal modulations and (ii) staircase modulation. 04
(c) Draw the block diagram of UPS and explain the function of each block. 04
SECTION-B
Q.5 (a) For the following 3-φ half wave converter, draw v 0 , i0 and iT 2 . Also find vdc . Let / 6 . 04
(b) For 3-φ full converter with highly inductive load, draw the output voltage waveshapes for = / 2 . 03
(c) If the converter of the following figure has a purely resistive load of R and the delay angle is = / 4 , 05
determine (i) η, (ii) FF, (iii) RF, (iv) TUF, and (v) PIV of T1.
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Q.6 (a) Classify ac controller on the basis of controlling the switch. Design a single phase full wave controller with 04
one thyristor.
(b) What is cyclo-converter? Explain the operation of cycloconverter with inter group reactor. 04
(c) A 1-φ full wave ac voltage controller in the following figure has a resistive load of R = 20 and input 04
voltage is Vs = 220 V (r.m.s) , 50 Hz. The delay angles of the thyristors T1 and T2 are equal:
1 = 2 = / 4 . Determine (i) the rms output voltage V 0 , (ii) the input PF, (iii) the average current of
thyristors IA and (iv) the rms current of thyristors IR.
Q.7 (a) Define voltage controller. Briefly explain the principle of (i) on-off control, and (ii) phase angle control. 04
(b) Explain the operation of single phase full wave controller with one thyristor. 04
(c) For the following controller with resistive load of R=10 Ω and the rms input voltage Vs = 220 V , 50 Hz. 04
The thyristors switch is on for n=25 cycles and is off for m=75 cycles. Determine (i) the rms output
voltage V 0 , and (ii) the input power factor.
Q.8 (a) For a series resonant inverter with unidirectional switches, prove that the maximum possible output 04
frequency is limited to f max = 1
, where the symbols have their usual meaning.
(2(t q + ))
r
(b) Explain the operation of step down dc-dc converter with R-L load, also derive the expression of maximum 04
ripple current.
(c) The following buck regulator has an input voltage of Vs=12 V. The required average output voltage is 04
Va = 5 V at R=500 Ω and the peak to peak output ripple voltage is 20 mV. The switching frequency is 25
kHz. If the peak-to-peak ripple current of inductor is limited to 0.8 A, determine (i) the duty cycle and (ii)
the filter inductance, L.
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