Understanding How to Turn Off a Conducting SCR

A Silicon Controlled Rectifier (SCR) is a type of thyristor, which is a four-layer semiconductor device used for controlling high power. Once an SCR is turned ON (made to conduct), it stays ON even if the gate signal is removed, provided the anode current remains above a certain minimum value called the holding current (\(I_{holding}\)). The process of turning off a conducting SCR is known as commutation or opening the SCR.

SCR Turn-Off Mechanism Explained

Unlike a transistor which can be turned OFF by simply removing the base current, an SCR is a latching device. Once it starts conducting, the gate terminal loses control. To turn OFF a conducting SCR, the forward current flowing through it (the anode current) must be reduced below its holding current value. When the anode current falls below \(I_{holding}\), the internal regenerative action that keeps the SCR ON can no longer be sustained, and the SCR returns to its blocking state.

Let's examine the given options in the context of turning off a conducting SCR:

• Supply voltage: Reducing the supply voltage connected across the anode and cathode of the SCR can eventually cause the anode current to decrease. If the supply voltage is reduced significantly, perhaps even to zero, the anode current will naturally drop below the holding current, thus turning off the SCR. While reducing the supply voltage is a *method* to reduce the anode current, the anode current reduction is the direct condition that causes turn-off.
• Gate voltage: Once an SCR is conducting, the gate terminal loses control. Reducing the gate voltage or removing the gate signal completely will have no effect on a conducting SCR. It will continue to conduct as long as the anode current is above \(I_{holding}\).
• Gate current: Similar to gate voltage, the gate current pulse is only required to trigger the SCR into conduction when it is forward biased. Once the SCR is conducting, removing or reducing the gate current has no impact on its ON state.
• Anode current: This is the current flowing from the anode to the cathode through the SCR. Reducing this current below the holding current (\(I_{holding}\)) is the fundamental requirement to turn off a conducting SCR. Reducing the anode current all the way to zero is a sure way to ensure it falls below \(I_{holding}\).

Based on the operating principle of an SCR, the device turns off when the forward current through it (anode current) is reduced below the holding current. Reducing the anode current to zero certainly fulfills this condition.

Therefore, a conducting SCR can be opened (turned off) by reducing the anode current to zero.

Revision Table: SCR ON and OFF Conditions

Additional Information: Holding vs. Latching Current

It's useful to understand two related terms:

• Latching Current (\(I_L\)): The minimum anode current required to maintain the SCR in the ON state immediately after the gate signal is removed. The SCR will latch ON if the anode current reaches \(I_L\) after triggering.
• Holding Current (\(I_{holding}\)): The minimum anode current below which the SCR turns OFF from the ON state. \(I_{holding}\) is typically less than \(I_L\). To ensure turn-off, the anode current must be reduced below \(I_{holding}\). Reducing it to zero is a common way to achieve this.

Methods to reduce anode current include reducing the supply voltage, increasing the circuit resistance, or diverting the anode current using external commutation circuits.