Full form of SCR

Full form of SCR is Silicon Controlled Rectifier. It is the most important and widely used member of the thyristor family. SCRs can be used for a variety of purposes such as rectification, power regulation, and inversion, among others. SCRs, like diodes, are unidirectional devices that allow current in one direction while opposing it in the other.

As illustrated in the figure, an SCR has three terminals: anode, cathode, and gate. SCRs have the ability to switch on and off, and their switching is controlled by biassing conditions and the gate input terminal.

As a result of varying the ON periods of the SCR, the average power delivered at the load varies. It is capable of handling thousands of voltages and currents.

Silicon Controlled Rectifier Construction

The SCR has four layers and three terminals. The four layers of P and N layers are alternately arranged to form three junctions, J1, J2, and J3. Depending on the type of construction, these junctions are either alloyed or diffused.

The outer layers (P and N) are heavily doped, while the middle P and N layers are lightly doped. The gate terminal is taken from the middle P-layer, the anode from the outer P-layer, and the cathode from the N-layer terminals. The SCR is made of silicon because the leakage current in silicon is very low when compared to germanium.

This design is primarily used in high-power Silicon Controlled Rectifiers. The SCR is braced with plates made of molybdenum or tungsten to provide high mechanical strength. One of these plates is soldered to a copper stud, which is threaded to connect to the heat sink.

SCR Working Principles or Modes of Operation

The operation of the SCR is classified into three modes based on the biassing applied to it.

Mode of Forward Conduction

In this mode, an SCR or thyristor transitions from blocking to conduction mode. It can be accomplished in two ways: by applying a positive pulse to the gate terminal or by increasing the forward voltage (or voltage across the anode and cathode) beyond the SCR’s break over voltage.

When any of these methods is used, the avalanche breaks down at junction J2. As a result, the SCR enters conduction mode and acts as a closed switch, allowing current to flow through it.

It is worth noting that in the VI characteristic figure, if the gate current value is high, the time to come in conduction mode will be the shortest, as Ig3 > Ig2 > Ig1. Maximum current flows through the SCR in this mode, and its value is determined by the load resistance or impedance.

It is also worth noting that as gate current increases, the voltage required to turn on the SCR decreases if gate biassing is used. The current at which the SCR switches from blocking to conduction mode is referred to as latching current (IL).

Furthermore, holding current occurs when the forward current reaches the level at which the SCR returns to the blocking state (IH). Around junction J2, a depletion region begins to form at this holding current level. As a result, the holding current is marginally less than the latching current.

Mode of Reverse Conduction

It is also worth noting that as gate current increases, the voltage required to turn on the SCR decreases if gate biassing is used. The current at which the SCR switches from blocking to conduction mode is referred to as latching current (IL).

Furthermore, holding current occurs when the forward current reaches the level at which the SCR returns to the blocking state (IH). Around junction J2, a depletion region begins to form at this holding current level. As a result, the holding current is marginally less than the latching current.

This reverse current causes more losses in the SCR and even raises its temperature. As a result, if the reverse voltage is greater than VBR, the SCR will suffer significant damage.

The Benefits of a Silicon-Controlled Rectifier

• SCR has no moving parts when compared to electromechanical or mechanical switches. As a result, with a high efficiency, it can provide noiseless operation.
• It has a very fast switching speed, capable of performing 1 nano operation per second.
• With a small gate current, these can operate at high voltage and current ratings.
• More suitable for AC operations because the SCR will automatically turn off at every zero position of the AC cycle.
• Because they are small in size, they are simple to install and provide trouble-free service.

Wrapping Up

The Silicon Controlled Rectifier works like a switch, with two states: non-conducting and conducting. SCR operates in three different modes. There are three of them: forward blocking, forward conduction, and reverse blocking. The Silicon Controlled Rectifier can be used for both AC and DC switching too.