# Thyristor Theory and Operation

### In this section

The thyristor theory and operation can be viewed from a number of levels. To look at the thyristor theory and operation, the use of equivalent circuit helps describe the operation

In order to be able to utilise a thyristors, a knowledge of their operation and theory is required.

## Thyristor theory and operation basics

The thyristor has three basic states:

• Reverse blocking:   In this mode or state the thyristor blocks the current in the same way as that of a reverse biased diode.
• Forward blocking:   In this mode or state the thyristor operation is such that it blocks forward current conduction that would normally be carried by a forward biased diode.
• Forward conducting:   In this mode the thyristor has been triggered into conduction. It will remain conducting until the forward current drops below a threshold value known as the "holding current."

The thyristor consists of four semiconductor regions - p-n-p-n. The outer p region forming the anode, and the outer n region forming the cathode as shown below.

Thyristor theoretical structure

For the thyristor operation,and looking at the simplified block structure it can be seen that the device may be considered as two back to back transistors. The transistor with its emitter connected to the cathode of the thyristor is a n-p-n device whereas the transistor with its emitter connected to the anode of the SCR is a p-n-p variety. The gate is connected to the base of the n-p-n transistor.

Equivalent circuit of a thyristor or silicon controlled rectifier (SCR)

This arrangement forms a positive feedback loop within the thyristor. The output of one transistor fed to the input of the second. In turn the output of the second transistor is fed back to the input of the first. As a result it can be seen that the total current gain of the device exceeds one. This means that when a current starts to flow, it quickly builds up until both transistors are fully turned on or saturated.

When a voltage is applied across a thyristor no current flows because neither transistor is conducting. As a result there is no complete path across the device. If a small current is passed through the gate electrode, this will turn "on" the transistor TR2. When this occurs it will cause the collector of TR2 to fall towards the voltage on the emitter, i.e. the cathode of the whole device. When this occurs it will cause current to flow through the base of TR1 and turn this transistor "on". Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage. This will cause current to flow in the emitter of TR2, causing its "on" state to be maintained. In this way it only requires a small trigger pulse on the gate to turn the thyristor on. Once switched on, the thyristor can only be turned off by removing the supply voltage. . . . . . . .

By Ian Poole

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