2 October 2018

Thyristor chopper circuit

Actually, a chopper consists of a main power semiconductor types of device together with their turn on as well as turn off mechanisms. Here we have already learned about different types of chopper circuit like type A, type B, type C, type E etc. Now let us check it out the information about thyristor chopper circuit to know more details about chopper circuitry design.

In chopper definition, the low power chopper circuit; power transistors and also a GTOs terms. are being used widely. In high power level, however thyristor definition are in most commonly use. So here we have to learn the object of this section is to study the thyristor chopper circuit along with their commutation circuitry.

Forced commutation: 

In the method of forced commutation called, the external elements L and C which do not carry the load current continuously, are used to turn off a conducting Thyristor. Forced commutation can be achieved in the following ways :

1. Voltage commutation : 
  • In this scheme, a conducting thyristor is commutated by the application of a pulse of a large reverse type of voltage. This reverse voltage is applied by switching a previously charged capacitor of the circuit device. 
  • The sudden application of the reverse voltage across the conducting Thyristor reduces the anode to zero rapidly.
  • Then the presence of a reverse voltage across the SCR stands aids in the completion of its turn off process.
2. Current commutation :
  • In this scheme, an external pulse of current greater than the load current is passed in the reversed direction through the conducting SCR. When the current pulse attains a value equal to the load current, net pulse current through thyristor becomes zero and the device turn off. So the current pulse is usually generated by an initially charged capacitor.
  • An important feature of current commutation is the connection of a diode in anti-parallel with the main thyrostor so that voltage drop across the diode reverse biases the main SCR. Since this voltage drop is of the order of is 1 volt only, the commutation time in current commutation is more as compared to that in voltage commutation.
  • In both voltage and current commutation schemes, commutation is initiated by gating an auxiliary SCR called thyristor.
Load commutation : 

In load commutation, a conducting thyristor is turned off when load current flowing through a thyristor either :

1. It becomes to zero due to the nature of load circuit parameters 
2.Transferred to another device from the conducting thyrostor