Page 133 - Power Electronics Handbook

126 Power semiconductor protection

Figure 5.3 Transient overvoltage protection using surge suppression components;
(a) unidirectional operating device; (b) bidirectional operating device; (c) protection of a
diode; (d) protection of a thyristor; (e) bridgecircuit protection

Figure 5.3(c) shows a unidirectional cell connected to protect a diode, and
Figure 5.3(d) a bi-directional cell for protecting a thyristor.
Avalanche diodes are also used for surge protection. They have a low
slope resistance and a wide operating temperature range, but they are
relatively costly and so are usually used to protect expensive components.
Varistors, or voltage-dependent resistors, are made from oxides of metals
or of silicon carbide. They have a bi-directional characteristic and a
temperature range from about -2S"C to +85"C. They are robust and low
cost, with an operating voltage typically between 30V and 250V and a
relatively high slope resistance. Figure 5.3(e) shows a circuit which uses a
varistor across the a.c. lines and a suppressor across each component.
Figure 5.4 illustrates a simple crowbar circuit, in which the overvoltage is
converted into an overcurrent, which is then protected by the fuse Fs. An
overvoltage appearing at the main thyristor TH2 would cause zener diode
D1 to start to conduct, which would apply gate current to protection
thyristor TH1, turning it on. This thyristor applies a short circuit across the
supply lines, blowing the protection fuse. The circuit can act very quickly,
being limited primarily by the turn-on time of thyristor "HI.
An R-C network, called a snubber network, is also commonly used to
protect thyristors and triacs against dv/dt effects, which cause spurious