What are Snubber Capacitors?

Snubber circuits are used in electrical and electronic systems having inductive loads, where sudden breaking of a current causes a high voltage to appear across switching device as an induced reaction. This can cause deterioration of switching devices or other components, and lead to their premature failure. Additionally, a high frequency Electromagnetic Interference is generated which can affect other equipment in vicinity. Snubber provides a short-term alternative path to current around the switching device, thus protecting it and keeping the circuit and surrounding safe.

Switching circuits are everywhere, and snubbers are needed when large currents are broken, such as power supply circuits. They are also used for arc suppression across contacts (switches, relays, contactors), or switching semiconductor devices like IGBT, GTO etc. Snubbers are used for reducing current / voltage spikes, limiting dV/dt, or di/dt, reducing losses caused by switching operations.

Main purpose of snubber capacitor is to prevent unwanted sudden voltage rise across SCR at the moment of switching on and by keeping dV/dt within the design limits of SCR. If dV/dt is high, it may lead to false triggering of SCR. A device is needed to prevent this false triggering.

In the circuit below, capacitor is in parallel with thyristor. When switch is closed, voltage across capacitor and thyristor is zero (capacitor being a short-circuit at the instant of switching), and the voltage slowly build up across thyristor as the capacitor charges. This keeps the rate dV/dt low enough within the specifies ratings of SCR.

This solves the problem of dV/dt, but creates another issue. Consider what happens when SCR is triggered on. At the instant it is triggered, SCR becomes a short-circuit. Capacitor C, which is charged, then discharges suddenly through the SCR. The discharge current is very heavy (being short-circuit path to capacitor, the path resistance being very low), and this will damage the SCR.

Solution to this issue is to add a resistor in series with capacitor, so as to keep the discharge current within safe limits for SCR. The RC combination reduces the di/dt of the SCR within its safe limits. Thus the following circuit emerges.

This is the practical snubber circuit, which essentially is a series combination of resistance and capacitor. R, C and load together form an under-damped circuit, which keeps the di/dt within the ratings of SCR.  Most common snubber circuit is series RC combination across the switching device. Capacitors are specially designed for this duty to be able to carry high current pulses and high frequency currents that are involved.

The SCR is triggered at high frequency, depending on source frequency and other circuit parameters. High frequency components can go to several thousand Hertz while absorbing the noise and disturbances. Invariably these capacitors carry very high currents, even though supply voltages or frequency may not be high. For example, a capacitor rated 1 μF 440 V may be rated for as high as 10 Amps. Effect of snubber on voltage across thyristor is shown in representative figure above.

Snubber capacitors are therefore characterized by:

• High permissible operating frequencies (100 KHz or more)
• Low self-heating
• Low loss
• Low ESR
• High current and power capacity

Snubber capacitors are today found everywhere on power switching devices in supply system, as also in electric vehicles. EVs have several circuits using snubbers, and all motor control systems use them.

Snubber capacitors may be from several types, depending on power levels and application requirements. They could be ceramic capacitors, or plastic film capacitors. They vary in size and ratings from pf range ceramic capacitors to few hundred KVARs and frequencies up to hundreds of KHz. The figure above gives an idea of variety of sizes and types of snubber capacitors. These are only representative, and they could have large variety of shapes, sizes, ratings from different dielectric type capacitors.

Plastic film capacitors may be made using film/foil, MPP, or double sided MPP film. Large MPP snubber capacitors are capable of extremely large current densities, at high frequencies, and their design is made with this in view. Their self-inductance must be very low to be able to operate at high frequencies. Hence their diameters are large in relation to height, and end spray, as also external connections, are quite heavy. Small width also helps keep surface current densities on film small enough, and reduces ESR.

Values of RC snubber capacitors may range from 0.1μF to 1 μF, while for low frequency applications, they may range from 0.1 μF to 10μF. The value is decided by switching device frequency, and the load inductance. Its current rating is governed by the maximum ripple current of the device. Its ESR must be sufficiently low so that heat generation in capacitor is kept within limits. Voltage rating of snubber capacitor must be more than peak voltage of the circuit.