Homework #1 ECE 481/523 Fall 2009
Due Friday, August 28, 2009
1. Generally, semiconductors are used in the switching mode for power electronic circuits,
since this mode of operation makes it possible to control a large amount of load power with
low power dissipation in the semiconductor switching device. This problem is intended to
illustrate the rather large power dissipation in a transistor when it is used in the active mode
as a linear amplifying element (linear regulator) instead of in the switching mode.
Consider a circuit of Fig. 1P-1 with the supply voltage E = 100 Vdc and a purely resistive
load R = 10 ohms. Assume that a constant base current is supplied such that the collector
current is I amperes. Derive expressions for the power delivered to the load and the power
dissipated in the transistor as a function of E, R, and I. Calculate and plot (graph) the
transistor dissipation and the power delivered to the load as a function of I, for values of I =
0, 1, 2, ... , 10 A. Assume there is negligible leakage current when the transistor is
completely turned off (I = 0). When I = 10 A, assume that there is sufficient base current to
drive the transistor hard into saturation such that the VCE(SAT) is essentially zero.
2. Repeat Problem 1 for the shunt transistor regulator shown in Fig. 1P-2, with the series
transistor Rs = 10 ohms.
3. Consider the circuit of Fig. 1P-1 where the series transistor is now operated as a switch at a
fixed repetition frequency but with varying duty cycle, d
(ON time/switching period). Assume that the transistor on-state voltage drop is equal to 1 V
and constant during the ON interval, and neglect the leakage current and transistor switching
losses. With E = 100 V and R = 10 ohms for this problem, calculate and plot the power
dissipated in the transistor and the power delivered to the load as a function of the duty cycle
of the switching.
