Impulse Commutation in Thyristors

Before Starting to study Impulse study first we revise the concept of Commutation, that what is commutation and its different types names and then we will move on to Impulse Commutation and its circuit and working, Ners can skip this introduction part about commutation.

Commutation (Turning OFF):

It is a process of turning off a thyristor and it is normally done in such a way that Load Current transfers to another part of circuit.

Types of Commutation:

There are many types of commutation it is broadly classified into two types

1. Natural Commutaion
2. Forced commutation

               Forced is further divided into two types

•                Resonant
•                Impulse Commutation

So now you have an idea about commutation as a whole process, now we will discuss the working of Impulse Commutation circuit.

Impulse Commutation:

First talk about circuit of impulse commutation, circuit contains three thyristor T1, T2 and T3

One freewheeling diode

One Inductor

and obviously source and load.



Working principle of impulse commutation circuit starts with an assumption that capacitor is initially charged to a voltage negative Vout with polarity shown across Capacitor, let us assume that main Thyristor T1 is initially conducting and carrying the load current when auxillary thyristor T2 is fired, then main thyristor T1 is reverse biased due to negativ voltages is devolped across its anode and cathode due to negatively charged Capacitor, T2 will carry a load current which in turn duscharges the capacitor from negative to zero and then charge it from zero to positive voltages.

Therefore T2 will be turned off on its own once capacitor is fully charged from zero to positive voltages, now charge reversal of capacitor is done by third Thyristor T3 which is placed in resonant Loop. Once T3 is fired and circuit will resonate at particular frequency and capacitor will be charged negatively. Discharging time of Capacitor depends on the load current and by assuming a constant load current discharge time  tq an be calculated by means of capacitor voltage equation.


         ic= C dV/dt


         Vc= C ∫ i dt

        V= 1/C ∫ Im dt

          = Im/C x tq

            tq= V xC /Im

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