Explain the role of snubber circuit to protect SCR against high di/dt and dv/dt.
1 Answer


di/dt protection:

  1. When VAK > 0 and I, > 0, conduction of anode current begins near gate cathode junction. If the rate of rise of anode current (di/dt) is greater than spread velocity of carriers, local hot spots will be formed near the gate due to high current density which may destroy the thyristor.

  2. The di/dt is limited by using inductor in series with anode circuit. IES 2005

Overcurrent Protection:

  1. A fuse is attached in series with SCR so that when current go above rated current, the fuse get blow off.

Thermal Protection:

  1. To keep temperature within limit, a suitable heat sink is provided in SCR.

Gate Protection:

  1. We place a Zenar diode parallel to SCRAwhich maintain constant voltage across SCR and protect it from overvoltage. A resistance is also placed which limits gate current and protect SCR from overcurrent. Capacitor and a resistor combination is also placed to increase noise immunity of SCR. IES-200

Heat sink:

  1. It is mounted on thyristor. In order to get best results per unit cost, heat sinks are made of aluminium. IES-2004

dv/dt protection:

  1. If the rate of rise of VAK is high then thyristor may get turn ON (explained earlier in dv/dt triggering). If we don’t want to turn ON the thyristor then we have to reduce dv/dt. So, accidental turn ON due to dv/dt can be check by using snubber circuit shown in figure.

    IES 2007, IES 2001, IES 2004, IES 2002


Overvoltage Protection:

A varistor is attached parallel to SCR. Varistor is metal oxide non- linear resistor whose v-i characteristics is nonlinear. So as voltage increases, the slope of v-i characteristics of varistor decreases or resistance decreases or it offers less resistance. So it offers high resistance at low voltage and low resistance at high voltage.

Please log in to add an answer.