8/18/2019 Assignment3-2015

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CONCORDIA UNIVERSITY

DEPARTMENT OF ELETRICAL AND COMPUTER ENGINEERING

ELEC 433/6461 – Power Electronics/Power Electronics I

Assignment #3: dc-dc converters

Due date: November 9, 2015 in the mailbox of Dr. Lopes (EV5.175).

1) 

Consider a buck dc-dc converter switching at 25 kHz. Its input voltage can vary between 36 V and 48 V

while the output voltage is regulated at 24 V by duty cycle control. The load is expected to vary between

120 and 360 W. a) Design the output inductor so that the converter operates in the continuous conduction

mode (CCM) at all cases; b) Select an output filter capacitor so that the output voltage ripple is less than 5%

of the rated voltage at all times for CCM; c) Compute the peak voltage and peak current across the diode; d)

Compute the maximum peak and average current across the switch.

2)  An electric motor bike is based on a 36V/500 W permanent magnet dc motor (PMDC) fed through a class C

dc-dc converter operating with PWM. Assume that its rated current is 20 A, its armature resistance ( Ra) is

0.9 Ω  and armature inductance ( La) is 2 mH. Assume that the 2 switches of the converter operate

complementarily. A) What should be the duty cycle of the top switch when EA is equal to 24 V and: a) the

motor bike accelerates with rated average current; b) the motor bike brakes with rated average current? C) in

the latter case, what would be the average current at the battery?

3)  Consider a 4-quadrant dc-dc converter feeding a dc motor. The dc bus voltage is 500 V, the armature

inductance ( La) is 5 mH and the armature resistance ( Ra) can be neglected. The back EMF ( E a) is equal to

100 V and the terminal voltage (V t ) is 250 V. Compute the minimum value of the triangular carrier

frequency so that the converter operates in the boundary condition (CCM/DCM) with a 10 A (average)

current when operating with: A) Bipolar PWM and B) Unipolar PWM. Hint: Draw the waveforms of vt  and

ia.