Silicon controlled rectifier function and application

Introduction to silicon controlled rectifier (SCR)

You all must familiar with the diode. It is a device that allows current only in the forward direction. A silicon controlled rectifier or SCR in short is a device with a function to control its forward current also. It has one extra terminal to control the current. So, it has three terminals Anode, Cathode, and Gate.  It is also called “Thyristor”.

This can handle high voltage and current. It is a power electronics component.

Silicon controlled rectifier Symbol

It has three terminals anode, cathode, and gate.

Function of silicon controlled rectifier

 We can turn it on by applying a positive voltage to its gate. The gate voltage on which it starts conducting is called gate threshold voltage. If anode to cathode current is greater than or equal to its latching current, it stays conductive even after we remove the gate voltage. It will turn off if the anode to cathode current reaches below its holding current. As a diode, it blocks current in reverse bias (polarity).

Modes of operations of SCR

There are three modes in which a silicon controlled rectifier can function.

1. Forward Blocking Mode – In the mode, anode is connected to positive voltage, cathode is connected to negative (zero) voltage and cathode is connected to zero voltage. SCR is in forward bias but voltage on gate terminal is zero so, it will block any current through anode to cathode. So, it is called forward blocking mode of SCR.
2. Forward conducting mode – In this mode, anode is connected to the positive voltage, cathode is connected to negative voltage. To operate the SCR in this mode a positive gate pulse is applied. Then SCR starts conducting immediately. After then we can remove the gate voltage because we no longer need this.
3. Reverse blocking mode- As diode if we connect negative voltage to the anode of the SCR and positive voltage to the cathode terminal. SCR won’t conduct even after we apply the nay voltage to the gate terminal. This mode is called reverse blocking mode of SCR.

VI characteristics of SCR

VI characteristics are shown by the graph. As you see at different gate currents (below threshold voltage), forward triggering voltage is different. After once reaching forward triggering voltage (a method to turn SCR ON), silicon controlled rectifier can conduct (function) even at lower voltages than its forward blocking voltages. SCR will not conduct in reverse voltages. If we increase the reverse voltage up to “reverse breakover voltage”, a huge reverse current will flow and SCR will get damaged.

Silicon controlled rectifier structure

It has four layers of semiconductor materials. So, it forms two types of SCR: NPNP and PNPN. They together form three p-n junctions J₁, J_2, and J₃.

Working of silicon controlled rectifier

When we connect a positive voltage to the anode and a negative voltage to the cathode. Junctions j₁ and J₃ will be in forward bias and junction J₂ will be in reverse bias. In this case, SCR will not conduct.

But, when we connect positive threshold voltage to its gate terminal, junction J₂ also comes in forward bias. SCR then starts conducting. Once a current is established then we can remove the gate voltage.

When we connect SCR in reverse, junction J₁ and J₃ are in reverse bias. So, it doesn’t conduct.

Thyristor turn ON methods

1. By raising temperature (thermal triggering) – If we increase the temperature of device then we can trigger it without gate pulse.
2.  By high voltage (forward voltage triggering) – As described before that we can turn a SCR on by giving it high enough forward voltage. (See graph)
3. By gate current (gate triggering)- We can turn it on using a positive gate pulse.
4. By light (optical triggering) – A special type of SCR called LASCR (light activated SCR) can be triggered using light.

Thyristor turn OFF method

The basic method to turn off an SCR is to take an anode current below the holding current. But, there are some other methods too like “natural commutation of SCR”, “reverse bias commutation of SCR”, “by gate voltage” etc.

Thyristor (SCR) protection

1.  Protection from Changing current (di/dt) – If SCR is in forward direction and we give a gate pulse to then there will be a flow of anode current through device. This current needs some time to spread across device. If the current is fluctuating at the higher rate than its spread time. Device will start heating. To avoid that an inductor is connected in series to the SCR.

• Protection from changing voltage (dv/dt)- High rate of change of voltage can cause false triggering of SCR. To avoid that a capacitor is connected in parallel of SCR. Function of capacitor in this circuit is to bypass high rate of change of voltage. A resistor is connected in series to discharge the capacitor. This RC circuit is called Snubber circuit.

Two transistor analogy of SCR

An SCR can be thought of as two transistors connected as shown.

Thyristor triggering circuits

In the triggering circuit, we trigger the SCR using some additional circuits.

Resistor trigger circuit of SCR

The circuit and graph for resistor triggering are given below.

Here VR is used to define the gate triggering angle, R_min is used to limit the current, and R_B is used to stabilize the gate resistance.

If VR is less firing angle will be more and if VR is more firing angle will be less. The firing angle is between 0° to 90°.

RC triggering of SCR

In the RC trigger circuit, we use a resistor and capacitor to trigger the SCR. There are two types of the RC trigger circuits.

1.      Half wave RC triggering circuit of SCR

Here firing angle depends on the value of the capacitor and resistor. Let’s see its graph.

Here pink graph shows the voltage across the capacitor (V_C).

2.      Full wave RC triggering circuit of SCR

Here a full bridge rectifier is added to the circuit for the full-wave triggering of SCR.

Application of SCR

• For control high power as switch
• Lamp dimming
• Motor control
• High power regulation
• Widely used in transmission line to rectify high power AC into high voltage DC (DC transmission line)
• To control welding machine