# UNIVERSITY OF MASSACHUSETS DARTMOUTH ```UNIVERSITY OF MASSACHUSETS DARTMOUTH
DEPARTMENT OF ELECTRICAL ENGINEERING
ECE 442 POWER ELECTRONICS
EXAM #1 (TAKE – HOME)
ISSUED MARCH 24, 2010
DUE BACK MARCH 31, 2011
NAME:
Be sure to READ everything before you WRITE anything!
PROBLEM #1 (20%)
A certain power transistor is packaged in a TO-220 case having a thermal
resistance, RJC = 3C/W. The device will be mounted using a mica insulator and silicone
grease (RCS = 1C/W) onto a heat sink having RSA = 6C/W.
The maximum junction temperature of the transistor is TJ = 150C and the device
must operate in an environment whose ambient temperature could reach 75C.
Determine the maximum power that the transistor can dissipate without
exceeding the maximum junction temperature.
PROBLEM #2 (20%)
Two diodes are connected in series to share a total dc reverse voltage of 3kV.
Determine the values of the voltage-sharing resistors R1 and R2 so that the diode
voltages are equal (VD1 = VD2 = Vreverse/2). The reverse leakage currents of the two
diodes are Is1 = 20 mA and Is2 = 30 mA. (Hint: Assume the value of one resistor and
calculate the other.)
PROBLEM #3 (30%)
Two BJTs are connected in parallel as shown below. The total current IT is 200 A. The
collector – to – emitter voltage of transistor Q1 is VCE1 = 1.5 V and that of transistor Q2 is
VCE2 = 1.1 V. Determine the collector current of each transistor and difference in current
sharing if the current sharing series resistances are (a) Re1=10mΩ and Re2=20mΩ, and
(b) Re1=Re2=20mΩ.
PROBLEM #4 (30%)
A single – phase bridge rectifier is supplied from a 120 – V, 60 – Hz source. The load
resistance is R = 200 Ω.
a) Design a C – filter so that the ripple factor of the output voltage is less than 5%.
b) Using the value of capacitor C from part (a), calculate the average load voltage Vdc.
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