Description
Problem 5.1 (50 points)
This problem uses information extracted from portions of the data sheet for a Vishay Siliconix power
MOSFET SiHF620 (the full datasheet is available at http://www.vishay.com/docs/91027/sihf620.pdf). The onstate resistance of this MOSFET (RDS(on)) is 0.8 Ω at 25°C. This on-state resistance varies with junction
temperature, as illustrated in Fig. 4 (note that Fig.4 plots the normalized variation).
2
The junction-to-case thermal resistance (RthJC) is 2.5°C/W (see above table). Also the junction-to-case
transient thermal impedance versus pulse duration, for a single rectangular power dissipation pulse and
repeated pulses of different duty ratios, is given in Fig. 11.
For both parts of this problem assume that the maximum allowable junction temperature (TJ) is 140˚C and
the maximum ambient temperature (TA) is 50˚C.
a) The MOSFET is first attached to a heat sink using an insulating pad resulting in a case-to-sink
thermal resistance (RthCS) of 0.75˚C/W. Assuming that the MOSFET must carry a (forward) rms
current of 2.5 A, and that switching losses can be ignored, what is the maximum allowable thermal
resistance of the heat sink (RthSA)?
b) The MOSFET is now instead operated in a pulsed fashion, carrying rectangular pulses of current of
magnitude Ip, 1 ms in duration with 99 ms of off time between pulses. If the MOSFET is mounted to
an extremely good heat sink that maintains the case temperature at 50˚C, what is the maximum
allowable current pulse magnitude (Ip)? You may assume that the MOSFET on-state resistance is
always at its 140˚C value.
Problem 5.2 (50 points)
Do Textbook (Fundamentals of Power Electronics 2nd Edition) Problem 5.1 (pg. 126).
Problem 5.3 (50 points) [Additional problem only for ECEN 5797 students]
Do Textbook (Fundamentals of Power Electronics 2nd Edition) Problem 5.2 (pg. 126).
(°C/W) Repeated Pulse (Duty Ratio = 0.01)
