Why snubber circuit is used

In a simple power switching network with a power semiconductor switch and a resistive load, the device voltage and current are large when the power semiconductor is switching between on or off.

In essence, this results in high power dissipation across the device and a consequential high loss of energy. When the switch is open to interrupt the current flow in resistive load, a linear variation of switch voltage and current will be visible in the waveform. The resultant peak power dissipation will be one-quarter of a product of maximum voltage and current, which is very high in high voltage switching circuits.

In an inductive load, the dissipation will be larger still. Using a snubber that is connected across the semiconductor, however, these turn off losses can be minimized. In this setup, the current is diverted into the snubber - which is initially uncharged - when the switch opens. This diversion slows the build-up of switch voltage and, as a result, the current drops to a low value before the switch voltage has a chance to significantly increase.

The effect of this is a slow increase in switch voltage, which produces a reduction in switching energy loss. With this arrangement, snubbers in power electronics can attain a high operation frequency and low switching losses. These are placed across the various switching devices like transistors, thyristors, etc. Switching from ON to OFF state results the impedance of the device suddenly changes to the high value.

But this allows a small current to flow through the switch. This induces a large voltage across the device. If this current reduced at faster rate more is the induced voltage across the device and also if the switch is not capable of withstanding this voltage the switch becomes burn out. So auxiliary path is needed to prevent this high induced voltage. Similarly when the transition is from OFF to ON state, due to uneven distribution of the current through the area of the switch overheating will takes place and eventually it will be burned.

Here also snubber is necessary to reduce the current at starting by making an alternate path. There are many kinds of snubbers like RC, diode and solid state snubbers but the most commonly used one is RC snubber circuit.

This is applicable for both the rate of rise control and damping. This circuit is a capacitor and series resistor connected across a switch. For designing the Snubber circuits. The amount of energy is to dissipate in the snubber resistance is equal to the amount of energy is stored in the capacitors.

An RC Snubber placed across the switch can be used to reduce the peak voltage at turn-off and to lamp the ring. An RC snubber circuit can be polarized or non-polarized. Since the value of resistance of this local path is quite small, the magnitude of discharge current will be quite higher. In order to limit the magnitude of the discharge current, a resistance should be connected in series with the capacitor C. This is shown below. The above circuit is the actual Snubber Circuit. Thus, a snubber circuit comprises of series combination of resistance and capacitance in parallel with SCR or thyristor.

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