Page 128 - Power Electronics Handbook

Chapter 5

Power semiconductor protection

5.1 Introduction

In spite of its relatively large power-handling capability, a power
semiconductor can be easily destroyed due to voltage or currents which
exceed its ratings. High currents cause localised or general heating, and
excessive voltages can result in punch-through of the silicon wafer.
Protection devices are often connected into power semiconductor circuits,
but it must be remembered that this reduce? the overall reliability of the
system, since the component count has been increased, and these
protection components can fail themselves, sometimes destroying the
components which they are supposed to be protecting.
This chapter considers the factors causing failure in power circuits, and
the protection techniques and components which can be used.

5.2 Causes of Pallure in power circuits

The mechanisms which result in failure within power circuits can be related
to the power semiconductor device itself or to external factors within the
system. Device-related factors are:
(i) The breakdown of the component, causing it to operate in a
continuous on-mode. This could be as a result of a current overload
or an overvoltage.
(ii) The power semiconductor operating in a blocking mode, so that other
components in the system are overstressed. This mode may be a
result of a failure in the component itself or due to a failure of the
circuit controlling it.
Circuit-related failure-mode mechanisms are:
(i) Load related, where load short circuits cause excessive current surges
through the controlling devices or switching the load generates
voltage spikes.
(ii) Supply related, usually seen as voltage spikes on the a.c. or d.c.
supply lines.
(iii) Other circuit-generated current or voltage overloads, such as voltage
transients generated when the current changes suddenly within
inductive circuits.

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