Silicon Controlled Rectifier (SCR) – Thyristor

Silicon Controlled Rectifier (SCR) - Thyristor

Before the invention of thyristor, semiconductor diodes are used for rectification. But the diodes are not helpful for high voltage applications. A thyristor is invented to overcome this disadvantage that can operate on a high voltage and is used to make high voltage rectifiers.

The revolution of power electronics truly started after the invention of a thyristor. The first device invented in the thyristor family is Silicon Controlled Rectifier.

The thyristor was introduced by JL Moll and other scientists of Bell laboratory in 1956. The commercial selling of Silicon Controlled Rectifier started in 1957. After the invention of SCR, many devices are added to the thyristor family.

SCR is a short form of Silicon Controlled Rectifier. As the name suggests, It is used in power conversion applications like a rectifier. It is also used for power regulation.

Construction of SCR

Like diode, SCR is also a unidirectional power electronics switch that allows current flow in one direction. It is three terminals, four layers, and two junction devices.

It has three terminals;

  • Anode (A)
  • Cathode (K)
  • Gate (G)

The construction of SCR is as shown in the below figure.

Construction of SCR
Construction of SCR

There are three junctions J1, J2, and J3. Four layers form these junctions.

  • First layer – P+
  • Second layer – N-
  • Third layer – P
  • Fourth layer – N+

The structure of the Silicon Controlled Rectifier forms the PNPN type structure. Out of these four layers, the first layer (P+) and Last layer (N+) are heavily doped layers. The second layer is lightly doped, and the third layer is moderately doped.

Junction J1 is formed by the P+ layer and N- layer. Junction J2 is formed by the N- layer and P layer. The P layer and N+ layer form junction J3.

The Anode terminal is taken from the P+ layer. The Cathode terminal is taken from the N+ layer. The Gate terminal is taken from the middle P-layer. Sometimes, the Gate terminal is known as the control terminal.

In Silicon Controlled Rectifier, Silicon is used as an intrinsic semiconductor because leakage current in silicon is minimal compared to germanium.

When the anode terminal of SCR is connected with a positive terminal of the battery and the cathode is connected with a battery’s negative terminal. This condition is known as a forward bias condition.

When the anode terminal is connected with a negative terminal, and the cathode is connected with a positive terminal of the battery, the SCR is connected in reverse bias.

Working of SCR

According to biasing type, the working of SCR is divided into three modes;

  • Forward Blocking Mode
  • Forward Conducting Mode
  • Reverse Blocking Mode

Forward Blocking Mode

In this mode of operation, the battery’s positive terminal is connected with Anode, and the negative terminal of the battery is connected with a cathode. Hence, It is connected to forward bias.

But the Gate terminal is controlling the terminal of SCR, which is open in this mode. The connection diagram of this mode is as shown in the below figure.

Forward Blocking Mode of Operation
Forward Blocking Mode of Operation

Junctions J1 and J3 are in forward bias in this mode of operation, and junction J2 is in reverse bias. Due to junction, J2 is in reverse bias. When the voltage is applied to SCR, a tiny amount of leakage current will flow through the device until the applied voltage reaches breakdown voltage. And it will remain in the OFF state.

When the applied voltage is more than the breakdown voltage, avalanche breakdown occurs, and current will flow through the SCR.

Here, it is connected in forward bias then also current cannot flow through the device. Hence, the name of this mode is Forward Blocking Mode.

Forward Conducting Mode

In this mode of operation, the connection is the same as the previous mode. The SCR is connected to forward bias. In the previous mode, the current cannot flow because the junction J2 is in reverse bias.

This problem can be solved by applying a small positive voltage to the gate terminal. Hence, junction J2 becomes in forward bias. The connection diagram of this mode is as shown in the below figure.

Forward Conducting Mode of Operation
Forward Conducting Mode of Operation

If we apply a small forward voltage between anode and cathode, a current will flow through the SCR, and it operates as a close switch.

In this condition, we need not apply a large voltage. A small voltage can create a current pass through the SCR, and it becomes in ON state.

Reverse Blocking Mode

In this mode of operation, the battery’s negative terminal is connected to the anode terminal, and the positive terminal of the battery is connected with the cathode terminal of SCR. Hence, the SCR is connected in reverse bias. The connection diagram is as shown in the below figure.

Reverse Blocking Mode of Operation
Reverse Blocking Mode of Operation

In this condition, junction J1 and J3 are connected in reverse bias, and junction J2 is in forward bias.

The current cannot flow through the device because of the reverse bias of junction J1 and J3. So, the SCR remains open, and it operates as an OFF switch.

V-I Characteristic of SCR

Similar to the working, the characteristics of SCR can be divided into three regions;

  • Forward Blocking Region
  • Forward Conducting Region
  • Reverse Blocking Region

The below figure shows the graph between applied voltage (Anode Voltage) and Current passing through SCR (Anode Current).

V-I Characteristic of SCR
V-I Characteristic of SCR

SCR is connected in forward bias in the forward blocking region, but the Gate terminal is open. Hence, a tiny amount of current will flow through the SCR. This current is known as forward leakage current. The region OP represents the forward blocking region.

SCR is connected in forward bias in the forward conducting region, and the gate terminal is positive. Hence, all three junctions are in forward bias, and the current can pass through the SCR. The region QR represents the forward conducting region.

When the gate terminal is open, but the anode voltage is more than forward break-over voltage (VBF), the current passes through the SCR due to avalanche breakdown. This region is also known as a forward conducting region.

In the reverse blocking region, the SCR is connected in reverse bias. And junction J1 and J3 are in reverse bias. Hence, the current cannot flow through the SCR. But in this condition also, due to minority charge carriers, a small amount of current will flow through the SCR. This current is known as reverse leakage current, and this current is not enough to turn on the SCR. This region is represented as OX.

When the applied voltage is more than the reverse break overvoltage (VBR), the current will flow to the device due to the avalanche breakdown. This current may damage the SCR. This region is represented as XY.

Two Transistor Analogy of SCR

Two transistor models can explain the working of SCR. The below figure shows the model of two transistor combination which works similar to the SCR.

Two Transistor Analogy
Two Transistor Analogy

Two transistors are named T1 (PNP) and T2 (NPN). The collector T1 is connected with the base of T2, and the Collector of T2 is connected with the base of T1.

The emitter of T1 is the anode terminal, and the emitter of T2 is the cathode terminal of SCR. The load is connected between the anode and cathode terminal.

Now, if the gate terminal opens, it means no current will flow through the base of T2 and collector of T1. Therefore, both transistors will behave as an open circuit.

If a particular voltage is given between the base and cathode terminal, the small base current will flow. Hence, the current will also flow through the collector is T1 which allows a base current of T1 to flow, and current will pass through the anode to cathode.

So, the load current will flow through both transistors, and it behaves as a close circuit.

Turn-On Methods of SCR

As we have discussed modes of SCR, in forward bias condition, It can be in forward blocking mode or forward conducting mode. Turn ON or Trigger the SCR means to switch ACR from forward blocking mode to forward conducting mode.

There are several methods to turn ON Silicon Controlled Rectifier. Here enlist the techniques used to Turn-On SCR.

  • Forward Voltage Triggering
  • Temperature Triggering
  • dv/dt Triggering
  • Light Triggering
  • Gate Triggering

Turn-off Methods of SCR

The main disadvantage of this device is that, once it is triggered, if you remove the gate current, then also it remains in conducting mode. So, it is not turn-off directly by removing gate pulses. These methods are known as Thyristor commutation methods.

There are two main methods for commutation or turn-off methods of SCR.

  • Natural Commutation
  • Forced Commutation

The forced commutation is further classified according to the class of commutation. And that is;

  • Class-A Commutation – Self commutated by resonating load
  • Class-B Commutation – Self commutated by an LC circuit
  • Class-C Commutation – C or LC switched by another load-carrying SCR
  • Class-D Commutation – C or LC switched by an auxiliary SCR
  • Class-E Commutation – External pulse source for commutation
  • Class-F Commutation – AC line commutation

Advantages of SCR

The advantages of Silicon Controlled Rectifier are listed as below;

  • The size of the SCR is small. Hence it is easy to mount on PCB.
  • The main advantage of SCR is that it can operate at high voltage and current rating with a small gate current compared to the diode.
  • It is a unidirectional device. So, it allows the current to flow in only one direction and oppose the current flow in another direction.
  • It can be used as an oscillator in the digital circuit.
  • It is suitable for AC operation.
  • In structure, there is no moving part. So, there is no electromechanical loss occurs.
  • The efficiency of this device is high as an on-state loss is less.
  • The cost of the Silicon Controlled Rectifier is less

Disadvantages of SCR

The disadvantages of Silicon Controlled Rectifier are listed as below;

  • It causes a delay in operation as turn ON time and turns OFF time is high. So, it is not helpful in the application, which is sensitive to the switching period.
  • It is not helpful in high-frequency applications.
  • The primary current must be interrupted to turn off the SCR.
  • It needs an extra circuit to turn it OFF. The cost of SCR is less, but the price of turn-OFF the circuit is high, and it is bulky.
  • It cannot use in DC applications.
  • The response rate of this switch is less.

Applications of SCR

The Silicon Controlled Rectifier is used in the following applications;

  • It is used in power-switching circuit
  • It is used in applications where the primary current is naturally zero, i.e., AC to DC.
  • Used in circuit breaker
  • Over-voltage protection
  • Inverters
  • Controlled rectifier
  • Timing circuits
  • Welding machine control
  • Battery charging regulator

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