Thyristor or Silicon Controlled Rectifier (SCR)

A Silicon Controlled Rectifier (SCR) is a silicon p-n-p-n switch that conducts in one direction. The same four-layer family is a thyristor. General Electric coined the name SCR for the commercial part. It has three terminals and four alternating p and n layers, like a solid-state thyratron.

That stack makes three junctions J₁, J₂ and J₃. The figures show the p-n-p-n layers, the anode (A), cathode (K) and gate (G). The gate sits on the p-layer next to the cathode.

The SCR / thyristor symbol is below.

The two-transistor model is below.

One SCR is a pnp (Q₁) plus an npn transistor (Q₂). The Q₁ emitter is the anode. The Q₂ emitter is the cathode. The Q₁ base ties to the Q₂ collector, and the Q₁ collector ties to the Q₂ base. The gate is also the Q₂ base.

The working of SCR can be understood by analyzing its behavior in the following modes:

Reverse Blocking Mode of SCR

In this mode, the SCR is reverse biased by connecting its anode terminal (A) to negative end and the cathode terminal (K) to the positive end of the battery. This leads to the reverse biasing of the junctions J₁ and J₃, which in turn prohibits the flow of current through the device, in spite of the fact that the junction J₂ remains in forward biased condition.In this state, the SCR behaves as a typical diode. In this reverse biased condition, only reverse saturation current flows through the device as in the case of the reverse biased diode which is shown in the characteristic curve by blue line. The device also exhibits the reverse breakdown phenomenon beyond a reverse safe voltage limit just like a diode.

Forward Blocking Mode of SCR

Here a positive bias is applied to the SCR by connecting anode terminal (A) to the positive and cathode terminal (K) to the negative terminal of the battery, as shown in the figure below. Under this condition, the junction J₁ and J₃ get forward biased while junction J₂ gets reverse biased.

Here also current cannot pass through the thyristor except the tiny current flowing as saturation current as shown by the blue curve in the characteristics curve below.

Forward Conduction Mode of SCR

The SCR can be made to conduct either
(i) By increasing the positive voltage applied at anode terminal (A) beyond the Break Over Voltage, V_B or
(ii) By applying positive voltage at the gate terminal (G) as shown in the figure below.

In the first case, increasing the applied bias causes the initially reverse-biased junction J₂ to break down at the forward Break Over Voltage (V_B). This breakdown results in a sudden increase in current through the SCR, even though the gate terminal remains unbiased.

However, an SCR can also turn on at a lower voltage by applying a small positive voltage to the gate terminal. This can be understood by examining the transistor equivalent circuit of the SCR.

Here it is seen that on applying a positive voltage at the gate terminal, transistor Q₂ switches ON and its collector current flows into the base of transistor Q₁. This causes Q₁ to turn ON which in turn results in the flow of its collector current into the base of Q₂.

Both transistors then saturate. The gate can be removed and the SCR stays on if anode current is still above the latching current. Latching current is that just-after-turn-on minimum. Later, a lower holding current is enough to keep the device on.

In such state, the SCR is said to be latched and there will be no means to limit the current through the device, unless by using an external impedance in the circuit. This necessitates one to resort for different techniques like Natural Commutation, Forced Commutation or Reverse Bias Turn Off and Gate Turn-Off to switch OFF a conducting SCR.

These techniques aim to reduce the anode current below the holding current. Holding current is defined as the minimum current needed to keep the SCR in its conducting state.

Similar to the turn off techniques, there are also different turn-on techniques for the SCR like Triggering by DC Gate Signal, Triggering by AC Gate Signal and Triggering by Pulsed Gate Signal, Forward-Voltage Triggering, Gate Triggering, dv/dt Triggering, Temperature Triggering and Light Triggering.

There are many variations of SCR devices viz., Reverse Conducting Thyristor (RCT), Gate Turn-Off Thyristor (GTO), Gate Assisted Turn-Off Thyristor (GATT), Asymmetric Thyristor, Static Induction Thyristors (SITH), MOS Controlled Thyristors (MCT), Light Activated Thyristors (LASCR) etc. Normally SCRs have high switching speed and can handle heavy current flow. This makes the thyristor (SCR) ideal for many applications like

1. Power switching circuits (for both AC and DC)
2. Zero-voltage switching circuits
3. Over voltage protection circuits
4. Controlled Rectifiers
5. Inverters
6. AC Power Control (including lights, motors, etc.)
7. Pulse Circuits
8. Battery Charging Regulator
9. Latching Relays
10. Computer Logic Circuits
11. Remote Switching Units
12. Phase Angle Triggered Controllers
13. Timing Circuits
14. IC Triggering Circuits
15. Welding Machine Control
16. Temperature Control Systems