18-01-2016, 04:20 PM
Protection of switching devices and circuits: Switching devices and circuit components
may fail due to the following reasons.
1. Overheating – thermal failure
2. Overcurrent
3. Overvoltage – usually happens during turn-off
4. Excessive
dt
di
5. Excessive
dt
dv
6. Switching loss –excessive switching loss is a major contributing factor of
overheating.
Power electronic circuits and their switching devices and components can be
protected from overcurrent by placing fuses at suitable locations. Heat sinks, fins and
fans are used to take the excess heat away from switching devices and other components.
Snubber circuits are required to limit
dt
di
,
dt
dv and overvoltage during turn-on and turnoff.
Some typical snubber circuits are described below.
RC Snubber Circuits
RC snubber circuits are normally connected across a switching device to limit the
dt
dv . An RC snubber circuit can be polarized or unpolarized. A forward-polarized RC
snubber circuit shown in Figure 1 is appropriate when a thyristor or a transistor is
connected with an anti-parallel diode. R limits the forward
dt
dv and R1 limits the
discharge current of the capacitor when Q1 is turned on.