Triggering of Thyristors or SCR Triggering

The SCR or thyristor is one kind of semiconductor device and it is specially designed to utilize in high-power switching applications. The operating of this device can be done in a switching mode only and acts as a switch. When the SCR is triggered by its gate terminal into the transmission, then it will supply the current constantly. When designing an SCR or Thyristor circuit, special concentration should require for activating the circuit. The working of the entire region of the SCR circuit mainly depends on the way of its triggering. This article discusses different methods of SCR triggering or SCR turn ON methods or triggering of Thyristors. There are different triggering methods are available based on various entities which include temperature, voltage, etc. We will discuss some of them which are frequently used in SCR triggering.

What is SCR Triggering?

We know that silicon controlled rectifier (SCR) or thyristor includes two stable states namely forward conduction and forward blocking. SCR triggering method can be defined as, when the SCR is switching in forward blocking state to forward conduction state which means OFF state to ON state, then it is termed as SCR turn ON methods or SCR triggering.

SCR Triggering Methods

The SCR triggering mainly depends on different variables such as temperature, voltage supply, gate current, etc. When the voltage is applied to the silicon controlled rectifier, if the anode terminal can be made +ve relating to the cathode, then the SCR turn into forwarding biased. Therefore this thyristor enters into the forward blocking state.

This can be made to activate into conduction mode and it performs by using any type of the SCR Turn ON methods. There are different methods for activating the SCR which include the following.

• Forward Voltage Triggering
• Temperature Triggering
• dv/dt Triggering
• Light Triggering
• Gate Triggering

Forward Voltage Triggering

This kind of triggering method is mainly used to increase the voltage among the anode and cathode. So that the width of the depletion layer can be increased and makes to increase the accelerating voltage of minority charge carriers at J2 junction. Further, this can be lead to an avalanche breakdown of J2- junction at a forward break over-voltage.

In this stage, silicon controlled rectifier can change into conduction mode & therefore a huge flow of current with a less drop of voltage will be there. Throughout the triggering state in the SCR, the range of forwarding voltage drop is 1 to 1.5 volts across the SCR. This may be amplified using the load current.

In practical, this method cannot be used as it requires an extremely large anode voltage to the cathode. Once the voltage is high than the break over-voltage, then it offers extremely huge currents. This may cause harm to the thyristor. So, in most of the situations, this kind of SCR triggering method cannot be used.

Temperature Triggering

This type of triggering mainly occurs because of some circumstances. It can increase sudden responses & then its results must be noted down while the element of any design method.

Temperature triggering of thyristors mainly occurs when the voltage across the J2 junction as well as leakage current can increase the junction’s temperature. When the temperature increases then it will increase the leakage current.

This increasing method can be adequate to activate the thyristor, even though it tends to simply happen as the temperature of the device is high.

dv/dt Triggering

In this type of triggering, whenever the SCR is in forwarding bias, then two junctions like J1 & J3 are in forwarding bias and J2 junction will be in reverse bias. Here, J2 junction performs like a capacitor because of the existing charge across the junction. If the ‘V’ is the voltage across the SCR, then the charge (Q) and capacitance can be written as

ic = dQ/dt

Q = CV

ic = d (CV) /dt = C. dV/dt + V.dC/dt

When dC/dt = 0

ic = C. dV/dt

Thus, as the change of voltage rate across the SCR turns into high or low, then the SCR may trigger.

Light Triggering

When the SCR is triggered with the radiation of light is named as LASCR or Light Activated SCR. This kind of triggering is used for converters which are controlled by phase within HVDC systems. In this technique, intensity and light emissions with suitable wavelength are permitted to hit the J2 junction.

These kinds of thyristors include a position within the P-layer. Thus, as the light strike on this position, the pairs of electron-hole can be produced at the J2 junction to give extra charge carriers at the leads of the junction to trigger the thyristor.

Gate Triggering

Gate triggering is an efficient and most commonly used method to trigger the thyristor or SCR. As the thyristor is in forward biased, then an ample voltage on the gate terminal adds some electrons to the J2 junction.  This affects to amplify reverse outflow current & therefore the J2 junction breakdown still at the voltage will be less than the VBO.

Based on the thyristor size, the gate current will change from a few mA to 200 mA. If the current which is applied to the gate terminal is high, then additional electrons will be inserted into the J2 junction & consequences to approach into the conduction position at less applied voltage.

In this technique, a positive voltage can be applied among the two terminals like the gate & the cathode. So, we can employ 3- kinds of gate signals for SCR triggering namely pulse signal, DC signal, and AC signal.

While designing the gate SCR triggering circuit, the following important points must be kept in your mind.

• When the SCR is triggered, then the gate signal must be detached instantly, otherwise, the power loss will be there within the gate junction.
• As an SCR is in reverse biased, then gate signal shouldn’t be applied to this.
• The gate signal’s pulse width must longer than the required time used for the anode current for increasing to the value of holding current.

Thus, this is all about an overview of SCR triggering methods. From the above information finally, we can conclude that changing the thyristor from forward blocking state to forward condition state is known as triggering. Here is a question for you,