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 = 3C/W. The device will be mounted using a mica insulator and silicone grease (RCS = 1C/W) onto a heat sink having RSA = 6C/W. The maximum junction temperature of the transistor is TJ = 150C and the device must operate in an environment whose ambient temperature could reach 75C. 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.