Working Principle of Silicon Controlled Rectifier (SCR)

The Silicon Controlled Rectifier (SCR) is the most important and commonly used member of the thyristor family. SCR can use for a variety of applications such as optimization, power regulation, and reversal. Like a diode, an SCR is a non-directional device that passes current in one direction and opposes it in the other.

Working Principle of Silicon Controlled Rectifier (SCR)

Silicon Controlled Rectifier (SCR) are four-layer semiconductor devices with three terminals known as anode, cathode, and gate. Depending on the activation of the shutter, the device can be viewed as a switch or used as a rectifier. These are not suitable for SCR implementation. SCRs are responsible for current flow in one direction.

Therefore, it is also an indirect device. It has three nodes. These are the devices that run on the currents. Therefore, they are called regulated current devices. When the voltage in the equipment supply is considered too high to provide control to the lamps and provide phase control for the AC motors, they are beneficial for controlling the equipment.

A diode carries an electric current in one direction and stops an electric current in the other. In other words, the diode converts AC to DC. This unique behavior allows the construction of different rectifiers such as half-wave, full-wave, and bridge rectifiers. These rectifiers alternately convert direct current.

In half-wave, full-wave, and bridge rectifiers, conventional PN junction diodes (double layer diodes) were used. Therefore, if the voltage across these diodes is too high, the diodes can be damaged. Thus, rectifiers cannot operate at high voltages.

Mods of Working Operation in Silicon Controlled Rectifier

The Principle of operation or working of the Silicon Controlled Rectifier(SCR) that we have is in the different ways it works. Depending on the polarity of the voltage applied and the gate pulse of the SCR, the SCR operation is divided into three modes, which is explained below:

1: Forward Blocking Mode
2: Forward Conduction Mode
3: Reverse Blocking Mode

1: Forward Blocking Mode

In Forward Blocking Mode, a positive voltage is applied to the anode side and a negative voltage to the cathode side. And no pulse was applied to the gate while the gate terminal remained open. J1 junction and J3 junction are forward biased, and J2 junction is reverse biased.

Since J2 is reverse biased, the width of the depletion region increases, and it acts as an obstacle to conduction, so only a small amount of current will flow from J1 to J3. A small leakage current flows through the SCR.

As long as the voltage applied to the SCR is not greater than the cut-off voltage, the SCR will have a very high resistance to current flow. Therefore, the SCR acts as an open switch in this mode, blocking the forward current flowing through the SCR.

2: Forward Conduction Mode

In forward conduction mode, the SCR or enters conduction mode from blocking mode. This is the only mode in which the SCR will be ON and conduct. In this mode, we can make the SCR behave in two different ways. The first is to apply a positive pulse to pin A of the gate. And the second is by increasing the forward voltage (or voltage at the anode and cathode) above the SCR cut-off voltage.

When the forward voltage between the anode and cathode increases with the gate open circuit. The reverse junction J2 will have an avalanche breakdown with the forward breakdown in voltage VBO, which will cause the thyristor to turn on.

As soon as the thyristor is turned on, we can see from the thyristor characteristic diagram that point M immediately shifts towards N, and then anywhere between N and K. NK is the forward conduction mode of the SCR. In this mode of operation, the SCR conducts maximum current with minimum voltage drop. This is known as forward conduction or thyristor turn-on mode.

When we increase the applied forward bias voltage between the anode and cathode, junction J2 will be depleted due to avalanche breakdown, and the SCR will begin to conduct. We cannot do this for all applications, and this method of activating SCR will ultimately shorten the lifespan of the SCR.

3: Reverse Blocking Mode

In reverse blocking mode, the cathode becomes positive with respect to the anode. In this mode, a negative voltage (-) is applied to the anode (+), a positive voltage (+) is applied to the cathode (-), and the gate opens.

Then the transitions J1 and J3 are shifted in the opposite direction, and J2 – in the forward direction. This drives the reverse voltage SCR to the reverse blocking region results so that little leakage flows through it and acts as an open switch.

This is the reverse blocking mode or the off state of the thyristor. If the reverse voltage now increases, then at a specific voltage, known as the breakdown voltage VBR, avalanches occur in J1 and J3, and the reverse current increases rapidly.

The high current associated with the VBR causes further damage to the SCR, resulting in heat generation. This can damage the thyristor, as the junction temperature may rise due to an increase in the allowable temperature.

Therefore, care must be taken to ensure that the maximum operating reverse voltage across the thyristor does not exceed VBR. When the reverse voltage applied to the thyristor is less than VBR, the device provides a very high reverse impedance. Therefore, SCR in reverse blocking mode can be viewed as an open circuit.

You can also read about the Difference between IGBT and SCR

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