EE 603 Power Electronics

Homework # 8 Due March 31, 2006

Problem 1

A schematic of the basic step down converter is shown in Fig. 1.

Fig. 1 Basic step down converter.

Let Vin equal 160V and Vout equal 80V. The load is 3A. The MOSFET switching frequency is 25kHz, L = 3mH, C = 0.1mF and Rg = 800W. The diode has an Iss= 9 10^-14A, Cjo= 100pF, and TT= 100ns with all of the other diode Spice parameters at their default values.

a) What are Cin, Cr, and Cout for the IRF510 MOSFET? Use the IRF510 specification.

b) What are Vtr and Kp for the IRF510 MOSFET at 25C? Use the typical drain characteristics in Fig. 1 of the IRF510 specification and the V_GS = 5V curve.

c) What is the IRF510’s drain resistance (R_D in the level 1 Spice model) based on the specified on resistance and your calculated channel resistance?

d) If the channel is 1mm long what is the IRF510’s W?

e) Estimate the initial capacitor voltage and inductor current.

f) Make a Spice model of the above circuit using a level 1 MOSFET model with your W, and threshold values from above. Insert the MOSFET capacitances as separate external capacitances. Specify the inductor and output capacitor initial conditions. Make the input a pulse voltage from 0V to 15V with rise and fall times equal to 10ns.

g) Estimate the step size you should use.

h) Simulate your circuit for two switching cycles. Plot the inductor current and diode voltage for two cycles. If your drain current oscillates, try going to the convergence options sub-menu in the simulation menu and setting the integration method to Gear. If this does not fix your problem, try reducing the step size.

i) Plot the gate voltage, drain to source voltage and drain current showing their timing relationship. Expand your results, first about turn on and then about turn off, to show the turn on and turn off transients. You may define a new waveform equal to V_DS / 10 and plot it with the gate voltage and the drain current so they all fit on the same scale.

j) Compute the turn on and turn off times analytically. How do they compare to the Spice computed results?

k) Estimate the switching energies and the average switching losses.

l) Estimate the conduction losses.

m) Using the IRF510 specification, find the device’s thermal resistance. Estimate the temperature rise junction to case, case to sink and case to ambient. Does the IRF510 require a heat sink in this application?

n) Why is the Miller time for turn on less than the Miller time for turn off?

o) Add the stray inductance in Fig. 2 and its corresponding loss resistance. Let L_stray = 1mH and R_loss = 1KW.

p) Simulate your circuit for two switching cycles. Plot the gate voltage, drain to source voltage and drain current showing their timing relationship. You may define a new waveform equal to V_DS / 10 and plot it with the gate voltage and the drain current so they all fit on the same scale.

q) Use the permeability of free space () to estimate the length of wire from the Vin source to the diode and MOSFET required to get 1mH of inductance.

Fig. 2 Basic step down converter with stray inductance.

Problem 2

The channel capacitance of a MOSFET is given by

where W is the MOSFET’s channel width, L is the channel length, t_ox is the thickness of the gate oxide, and e_ox is the permittivity of the gate oxide. The permittivity of the gate oxide equal the dielectric constant of the gate oxide e_rox = 3.97 times the permittivity of free space e_o = 8.85 10^-12 F/m.

a) Estimate the channel capacitance of the IRF510 MOSFET assuming t_ox = 10nm.

b) Is your calculated capacitance bigger or smaller than the IRF510’s input capacitance?