What are the various Triggering Methods of Thyristor or SCR

Friday, August 24, 2012 ,

Thyristor-Power Electronics
A thyristor or SCR is a semi-conductor device having three junctions, three terminals and four layers. For full details about SCR, refer to this article:

What is an SCR:

Now come to our main topic that is triggering methods of an SCR or thyristor.

What is triggering:

Triggering is a process by which thyristor or SCR is brought to the conducting state or switched ON.

The most used methods for triggering an SCR or thyristor are mentioned below:

  • Voltage Triggering
  • Thermal Triggering
  • Radiation Triggering
  • dv/dt triggering
  • Gate triggering

Also Read: What is Commutation and Its Various Methods

1. Voltage triggering:
The method of triggering in which the triggering of SCR or thyristor is caused by the applied voltage across anode and cathode terminals, is known as Voltage triggering.

When a thyristor is in forward biased and applied voltage across anpde and cathode is increased, then the depletion layer of reverse biased junction decreased. At breakdown voltage, the depletion layer is totally destroyed and as a result the thyristor triggers and comes to ON state and start conducting heavily due to increase in number of charge carriers.

Because this triggering is caused by Voltage, thats why its called Voltage triggering.

2. Thermal Triggering:

The triggering method in which triggering is caused by the thermal effect i.e. by increasing the junction temperature, is called Thermal triggering.

We know that conductivity of semi-conductor materials increases with increase in temperature. Here we make use of this property. When the junction is reverse biased and the applied voltage across anode and cathode is near to breakdown voltage then we increases the temperature of the junction as a result the due to formation of electron-hole pairs, the thyristor starts conducting.
Also Read: What is Silicon Controlled Rectifier (SCR) - Power Electronics

In this case we should not increase the temperature to a high value, because this may cause damage to device. So, in this method the SCR or thyristor is triggered by thermal properties, therefore it is called thermal Triggering.

3. Radiation Triggering :

The method in which the triggering of a SCR or thyristor is caused by radiation i.e. by the bombardment of energy particles , for example Neutrons, photons etc.

In this method the triggering is done with the help of Radiation or by bombardment of energy particles. The thyristor is bombarded by energy particles like neutrons or photons. Due to the radiation, the electron-hole pairs are generated in the device. These pairs causes the flow of current in the device. Thus thyristor comes to ON state and hence triggered.

4. dv/dt triggering:

The triggering method in which we make use of high rate of increase of voltage(or dv/dt i.e. rate of change of voltage with respect to time). When a thyristor is optimised for a critical rate of rise of voltage, then if the rate of rise of voltage increases, the thyristor will be triggered and start conducting.
Also Read: Important Terms About SCR (Silicon Controlled Rectifier)

5. Gate Triggering:

The triggering method in which triggering of SCR or thyristor is caused by applying a signal between gate and cathode, is called Gate triggering.

Gate triggering is mostly used method for triggering an SCR or thyristor. This method is used in almost all industries and laboratories to trigger thyristor. In this method a positive signal is applied in between gate and cathode terminal. By using this method we can trigger the device much before its breakdown voltage. Hence, we can also control the firing angle(α) and the conduction angle(β=180-α) of SCR or thyristor.