SCR | Structure | Modes of Operation | Static V-I Characteristics | Power Electronics | Lecture 12

SCR stands for Silicon Controlled Rectifier, named for the silicon material used in its construction and its ability to be controlled. The 'controlled' aspect refers to the gate terminal, an additional terminal compared to a normal diode, which allows for controlling the device. Rectifier indicates its use in rectifier applications and high voltage DC applications. SCRs are also known as thyristors and were first prototyped in 1957, seeking to modify diodes and transistors for control.

The basic structure of an SCR includes three terminals: anode, cathode, and gate. It consists of four layers (p-n-p-n) and three junctions (J1, J2, J3). The gate terminal is typically placed closer to the cathode for practical reasons. A crucial point for SCR operation is that all three junctions (J1, J2, and J3) must be forward-biased for the SCR to turn on and conduct current from cathode to anode. SCR is a unidirectional device, similar to a diode, but can block current in the forward direction with the gate terminal.

In reverse blocking mode, the gate is unconnected. The anode is connected to a negative terminal and the cathode to a positive terminal. This reverse biases junctions J1 and J3, while J2 is forward-biased. As a result, no current flows, and the SCR acts as an open circuit, blocking current flow. A small reverse leakage current flows due to minority carriers. If the reverse voltage exceeds the reverse breakover voltage (VBO), the device will be permanently damaged. Applying gate voltage in this mode is pointless and increases power losses as J1 remains open-circuited.

In forward blocking mode, the positive terminal is connected to the anode and the negative to the cathode, with the gate still open. Junctions J1 and J3 are forward-biased, while junction J2 is reverse-biased. Similar to the reverse blocking mode, the SCR acts as an open circuit and does not conduct, even with forward biasing, due to the reverse-biased J2. A small forward leakage current flows. If the forward voltage exceeds the forward breakover voltage (VBO), avalanche breakdown occurs at J2, causing it to become short-circuited and the SCR to turn on. However, this method for turning on the SCR is generally avoided due to potentially dangerously high currents.

In forward conduction mode, the anode and cathode are forward-biased, and a gate voltage is applied. This causes all three junctions (J1, J2, J3) to become forward-biased, leading the SCR to conduct. The gate terminal allows for controlled turn-on of the SCR. Once the SCR is conducting, the gate loses control, and the gate supply can be removed. Before removing the gate supply, the anode current must reach a minimum value called the latching current (IL), which is the minimum current required to turn on the SCR. To turn off the SCR, a reverse voltage is applied, reducing the anode current below a minimum value called the holding current (IH). Latching current is always greater than holding current (IL > IH).

The static V-I characteristics illustrate the relationship between anode current and anode-to-cathode voltage. In the forward blocking region, a small forward leakage current flows until the forward breakover voltage (VBO) is reached, at which point the SCR transitions to the forward conduction mode. As the gate current (Ig) increases, the VBO decreases, allowing the SCR to turn on at lower anode-to-cathode voltages. In the forward conduction mode, the current rapidly increases after the latching current (IL) is reached. In the reverse blocking region, the SCR blocks current until the reverse breakover voltage is reached, beyond which the device can be damaged.