BASIC KNOWLEDGE - SNUBBER CIRCUIT What are snubber circuits and why are they used in power electronics?

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A snubber is a circuit that is used in semiconductor devices for protection and performance enhancements. They have many different purposes, namely the reduction of power dissipation in power electronic switching networks.

A snubber circuit limits or stops (snubs) switching voltage amplitude and its rate of rise, therefore reducing power dissipation. In its simplest form, a snubber circuit basically consists of a resistor and capacitor connected across the thyristor.

They are capable of doing many things, including:

• Reducing or eliminating voltage and/or current spikes;

• Limiting dl/dt or dV/dt;

• Shaping the load line to keep it within the safe operating area;

• Reducing total switching losses;

• Reducing EMI by damping voltage; and

• Transferring power dissipation from the switch to a resistor (or a useful load).

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Snubber circuits in power electronics

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.

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The energy in the charged snubber needs to go somewhere, though. When the switch is closed at the start of its conduction angle, the snubber discharges through it, and a small resistor is typically used with the snubber to limit the discharge current.

Snubbers are not always desired in power electronics

Snubbers are better for some applications than others. Often, snubbers will induce more losses than the losses that they try to avoid.

Many engineers will tell you that it is easier to remove losses from a transistor than from a resistor or use solutions that induce less losses, such as placing a small capacitor in parallel to a transistor without using a diode or resistor. At inductive load, this capacitor increases turn on losses but reduces turn off losses.

Due to the availability of alternative solutions, snubbers are less common today than they once were. In modern power electronics, they are commonly found to reduce ringing.

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