Snubber circuit

What is a snubber circuit?

Snubber circuits are used for semiconductor switching devices such as transistors, SCRs, etc.
When a switching device changes its state from ON-state to OFF-state, the impedandce of the device abruptly jumps to a very high level, blocking the current. But the current still tends to keep flowing through the switch, which induces a high voltage across the switch. The faster the current decreases, the higher the induced voltage becomes. It may reach to sufficiently high level to destroy the switch. If the switch is unable to withstand the high induced voltage, it will be destroyed, and can no more block the current as an OFF-state switch. To avoid this, an auxiliary network is connected across the switch that prevents the induced voltage from going too high. The network is called a snubber.
Conversely, when a switching device changes its state from OFF-state to ON-state, the impedance of the device abruptly goes down to nearly zero, allowing current to flow freely. The problem in this case is that the current does not start to flow uniformly through the switch. This is because the switch does not recover its conductivity over the whole its cross-sectional area simultaneously. Some part of the area may be first open, and the current flow is concentrated to this relatively small portion of the switch, which may cause overheating and eventually failure. Thus some semiconductor switches should be protected from too fast increase of current. This is another function of a snubber.
In view of above discussion, the snubber circuit is someting like a "bumper." It alleviates rise of voltage when the switch is turning OFF, and alleviates rise of current when the switch is turning ON, to a safe level.