Page 68 - Power Electronics Handbook

Chapter 2

Thermal design

2.1 Introduction

The parameter which has the greatest impact on the design of power
semiconductor devices is the power which they generate and that which can
be dissipated through it. This power dissipation results in temperature rises
and bond fracture, due to uneven expansion between the silicon dice and
its joints within the package. Special precautions must be taken to dissipate
this heat to the ambient in order to prevent excessive temperature rises
within the device.
This chapter describes the losses which occur within power semiconduc-
tors, their thermal characteristics, and the techniques which are used to
cool the devices.

2.2 Power losses in lrgelncoo1ductors

There are four main sources of power loss within a semiconductor, as
follows:
(9 Power loss during forward conduction. For a diode this is given by the
product of the current through the device and the forward voltage
drop across it. The same applies for a thyristor, but since the
conduction angle can now be varied the power loss curves are usually
given in data sheets as in Figure 2.1, where the average current over
the whole conduction cycle is used. For a transistor the forward
conduction loss is given by the product of the collector current and
voltage, to which is added the base dissipation, equal to the product
of the base current and voltage. Usually the base losses are small
compared to the collector loss.
Leakage loss, when the power semiconductor is blocking voltage in
the forward or reverse direction. This can occur when a diode or
thyristor is reverse biased, or when a transistor or thyristor is forward
biased but not turned on. These losses are usually small in
comparison with the forward conduction losses.
(iii) Switching losses which occur during turn-on or turn-off of the power
semiconductor. Although relatively small, these losses can became
appreciable when the device is being operated at high frequencies.

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