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Guide: SCR has controllable rectification, inverter, frequency conversion, voltage regulation, and can be used as a non-contact switch. What is the working principle of thyristor? Let’s explore it together.

First, the Principle of Thyristor – Introduction

SCR stands for Silicon Controlled Rectifier, commonly known as thyristor. It is a high-power semiconductor device with four layers and three PN junctions. The main feature of thyristors is that they can only be turned on and off, not continuously controlled like transistors. They are widely used due to their fast response, high efficiency, and low noise characteristics.

Second, the Principle of Thyristor – Structure

A thyristor consists of four layers: P1N1P2N2. It has three terminals: anode (A), cathode (K), and gate (G). The anode is connected to the P1 layer, the cathode to the N2 layer, and the gate to the P2 layer. Inside, there are three PN junctions: J1, J2, and J3.

When analyzing the thyristor's operation, it can be considered as a combination of a PNP transistor and an NPN transistor, which helps explain its switching behavior.

Third, the Principle of Thyristor

In the diagram, when a forward voltage is applied to the anode, both transistors BG1 and BG2 are in an active state. When a trigger signal is applied to the gate, BG2 starts conducting. This current flows through BG2 and is amplified by β2, then passes to the base of BG1, causing it to conduct. This creates a positive feedback loop, leading to saturation and full conduction of the thyristor.

The thyristor operates in two states: on and off. To turn on, the anode must be at a higher potential than the cathode, and the gate must receive sufficient voltage and current. Once on, the anode current must remain above the holding current. To turn off, the anode voltage must drop below the cathode, or the current must fall below the holding level.

Understanding the SCR principle is essential for anyone working with power electronics, especially in industrial control systems and motor drives. Its simplicity and reliability make it a fundamental component in modern electrical engineering.