EE 603 Power Electronics

Homework # 2 Due January 25, 2006

In Mohn/Underland/Robbins browse chapters 1 and 2. Read in chapter 2 sections 2.2, 2.4, 2.6, and 2.8. Start reading chapter 7.

Problem 1

A basic chopper circuit has an input voltage equal to 160V, an output voltage equal to 70V, a 100W resistive load, and operates at 100 kHz.

a) Assuming the inductor current is never zero, what is the duty cycle of the switch?

b) Plot Vs(t) over two cycles. Be sure to label your axis.

c) What is the average value and the rms value of Vs(t)?

d) Derive a simple relationship for the peak-to-peak ripple current in the inductor in terms of the duty cycle.

e) What is the dc component of the current in the inductor at the conditions specified above?

f) Determine L (using your result in d)) so that the peak to peak ripple current at the worst case duty cycle is 10% of the dc current in e).

Problem 2

The MOSFET circuit from the first homework is shown in Fig. 3.1 below. Use the level 1 MOSFET model with Kp = 1.2 A / V² and a threshold voltage of Vt = 2.5V. Leave all of the other MOSFET parameters at their default values. Let the input voltage be a pulse from 0V to 15V with 50us rise and fall times and a pulse width of 50us. Let the frequency be 5kHz. Do a transient simulation of the circuit for two cycles using a step size of 1us.

a) For the circuit in Fig. 2.1 plot the drain to source voltage and the drain current for two cycles.

b) What are the consequences of making the step size equal to 5us or 0.5us?

c) For the circuit in Fig. 2.1 plot the drain to source voltage and the drain current showing only the turn on transient.

d) For the circuit in Fig. 2.2 plot the drain to source voltage and the drain current for two cycles.

e) What ware the consequences of making the step size equal to 5us or 0.5us?

f) For the circuit in Fig. 2.2 plot the drain to source voltage and the drain current showing only the turn on transient.

Problem 3

In problem 2 you used a level 1 MOSFET model with a Kp = 1.2 A / V² and a threshold voltage of Vt = 2.5V with all of the other MOSFET parameters at their default values.

a) Conceive an experiment to verify the Kp and Vt give the correct theoretical drain current in the saturated region. Conduct this experiment using Spice.

Instead of making Kp = 1.2 A / V² use the spice default value which is equal to . In this case the total Kp is .

b) Assume L = 1um, what must W be for Kp = 1.2 A / V²?

c) Using these values of W and L in Spice conduct the same Spice experiment as in a) to verify you get the same drain current.