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THYRISTOR AS A CONTROLLED CONVERTERS

The controlled rectifier circuit is divided into three main circuits,

(1) POWER CIRCUIT

This is the circuit contains voltage source, load and switches as diodes, thyristors or IGBTs.

(2) CONTROL CIRCUIT

This circuit is the circuit, which contains the logic of the firing of switches that may, contains amplifiers, logic gates and sensors.

(3) TRIGGERING CIRCUIT

This circuit lies between the control circuit and power thyristors. Sometimes this circuit called switch drivers circuit. This circuit contains buffers, opt coupler or pulse transformers. The main purpose of this circuit is to separate between the power circuit and control circuit.

The thyristor is normally switched on by applying a pulse to its gate.  The forward drop voltage is so small with respect to the power circuit voltage, which can be neglected. When the anode voltage is greater than the cathode voltage and there is positive pulse applied to its gate, the thyristor will turn on. The thyristor can be naturally turned off if the voltage of its anode becomes less than its cathode voltage or it can be turned off by using commutation circuit. If the voltage of its anode is become positive again with respect to its cathode voltage the thyristor will not turn on again until gets a triggering pulse to its gate.

The method of switching off the thyristor is known as Thyristor commutation. The thyristor can be turned off by reducing its forward current below its holding current or by applying a reverse voltage across it. The commutation of thyristor is classified into two types,

1- NATURAL COMMUTATION

If the input voltage is AC, the thyristor current passes through a natural zero, and a reverse voltage appear across the thyristor, which in turn automatically turned off the device due to the natural behavior of AC voltage source. This is known as natural commutation or line commutation. This type of commutation is applied in AC voltage controller rectifiers and cycloconverters. In case of DC circuits, this technique does not work as the DC current is unidirectional and does not change its direction. Thus the reverse polarity voltage does not appear across the thyristor. The following technique work with DC circuits:

2- FORCED COMMUTATION

In DC thyristor circuits, if the input voltage is DC, the forward current of the thyristor is forced to zero by an additional circuit called commutation circuit to turn off the thyristor. This technique is called forced commutation. Normally this method for turning off the thyristor is applied in choppers.

There are many thyristor circuits we cannot present all of them. In the following items we are going to present and analyze the most famous thyristor circuits. By studying the following circuits you will be able to analyze any other circuit.

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