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(ELEC351)03Spring_final.pdf
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THE HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY
DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING
Elec 351 Philip Mok
INTRODUCTION TO POWER ELECTRONICS
Spring 2003 Room 2440
FINAL EXAM
(27 May 2003 8:30 am - 11:30 am)
Name: ___________________________ Student No.: _________________________
Seat No.: ___________________________ Signature: _________________________
DIRECTIONS:
(1)
This is a closed book examination.
(2)
Programmable calculators are allowed.
(3)
Answer ALL questions in the blue Examination Booklet provided. Additional loose pages will NOT be accepted and will NOT be marked unless provided by the examiner or invigilator.
(4)
Submit this exam paper together with the examination booklet. In this case, you can refer to the circuit diagrams and labels without redrawing them in the examination booklet.
(5)
Show all your work clearly and justify any assumption you make. Full marks will be awarded only with correct answers and clear justification.
(6)
Most questions are designed to be independent of each other. Not being able to do any part of a question should not stop you from doing the rest.
GOOD LUCK!
Question Maximum Mark* Mark
1 20
2 24
3 12
4 24
5 20
TOTAL 100
*Individual marks for each section are shown in [ ] brackets.
(a)
What is the definition of crest factor (CF)? What is the CF of a perfect sinusoidal current waveform? [2]
(b)
What is the disadvantage of electrical equipment with very low power factor? Who will suffer from this disadvantage? [2]
(c)
Give two advantages of using current-mode control switching converter over voltage-mode control of switching converter. [2]
(d)
Give two advantages of using DCM over CCM operation in switching converters. [2]
(e)
Draw the schematic diagram of a pulse width modulation (PWM) control buck converter. Clearly label the polarities of the error amplifier (EA) and the comparator. [2]
(f)
What is the advantage of PWM controller over PFM controller? [2]
(g)
What are the two major losses in magnetic core in power converter applications? [2]
(h)
Give two advantages of linear regulator over switching converters. [2]
(i) What is the meaning of dropout voltage in linear regulator? [2]
(j) What is the advantage of current fold-back current protection over standard short-circuit
protection in linear regulator? [2]
Problem 2: (24 marks)
(a)
A 100-Hz 10-V peak triangular wave is supplied to a rectifier circuit shown below. Assume the diode is ideal and the RC time constant is much greater than the switching period Ts where Ts = 1/fs. When the load current Io is 1 mA, find the capacitor value C such that the output peak-to-peak ripple voltage Vr is 1 volt. What is the corresponding peak source current Is. [6]
(b)
The circuit below can be used as a rectifier or voltage doubler to convert 110 Vrms or 220 Vrms AC input so