# Thyristor Turn OFF Methods in Chopper Circuits

### Chopper Circuit SCR Turn OFF Methods

In this short post, let us discuss about various methods to turn OFF/openning the conducting thyristors in the DC chopper circuits.

Before proceeding further is recommened to refresh about the basics choppers.

In high power choppers, SCRs are used as switching devices. In DC choppers, a conducting SCR has to be turned OFF at the end of TON time. The process of opening or turning off a conducting SCR is called commutation.  In DC choppers, a commutation circuit has to be provided to turn off the main power SCR.

A conducting SCR can be truned OFF by reducing its anode current below the holding current value and then applying a reverse voltage across the SCR to enable it to regain its forward blocking capability.
There are several ways of turning off of an SCR. All these methods differ from one another in the manner in which commutation is achieved.

Commutation circuit of DC choppers can be broadly classified into 2 groups as given below:

[a] Forced commutation

• In forced commutation scheme, external elements like  Inductor (L) and capacitor (C), which do not carry the load current continuously are used to trun off a conducting SCR.
• The Forced commutation can be achieved in the following 2 ways.

(i) Voltage commutation

• In this method, a conducting SCR is commutated by applying a large pulse of reverse voltage.
• Usually this reverse voltage is applied by switchining a perviously charged capacitor.
• This reverse voltage reduces  the anode curent to zero repidly and aids in the completion of turn off process of the thyristor.

(ii) Current commutation

• In this method, an external current pulse which is greater than the load current is passed in the reverse direction through the conducting SCR.
• When the current pulse value reaches (equal to) the load current value, net pulse current through SCR becomes zero.
• Thus the conducting  SCR is turned OFF.
• Usually the current pulse is generated by an initially charged capacitor.

In this method, a diode is connected in anti parallel with main SCR so that voltage drop across the diode reverse biases the main SCR.
Since the voltage drop across the diode is about one volt, the commutation time in current commutation is more as compared to that in voltage commutation.
In both voltage and current commutation schmes, commutation is initiated by gating an auxiliary SCR .

In load commutaion a conducting SCR is turned off when load current flowing through the SCR either

1. Becomes zero due to the nature of load circuit parameters or
2. Is transferred to another device from the conducting SCR