Silicon controlled rectifier or SCR is a Thyristor.  Usually the term, Thyristor itself is used to describe SCR. SCR is a unidirectional four layer PNPN power semiconductor device. Structure and symbol is as shown in figure.

SCR is a three terminal device with anode connected to the P region at one end, cathode to the N region at the other end and gate to the P region nearer to the cathode. Current flowing through the device depends on the control signal at the gate terminal. But like in transistors, it is not possible to vary the output voltage in proportion to gate signal, in SCR. You can turn it ON and OFF depending on the control signal. Power through the device is controlled by varying the switching time of this device. 

SCR is a power switch and it is extensively used in switching DC and AC, rectifying AC to give controlled DC output, regulation of power flow etc. It can be used in high power applications. It can handle up to thousand amperes and voltages more than 1 kV.                                      

SCR Operation

There are only two output states for an SCR, fully ON and fully OFF. But output power can be controlled using SCR, by varying the frequency of the control signal. That is, it determines how much time the SCR should remain ON or OFF. Based on the biasing conditions, SCR can be operated in three modes of operation.

(a). Reverse blocking mode

In reverse blocking mode, anode is at a negative voltage with respect to cathode. Junctions J1 and J3 are reverse-biased and so no current flows through the SCR.

(b). Forward blocking mode

In forward blocking mode, anode is positive with respect to cathode. Junctions J1 and J3 are forward biased. But, reverse biased junction J2 blocks the flow of current through it. However, at a stage, the breakdown of reverse biased junction J2 happens as the applied voltage is gradually increased and SCR starts conducting heavily. The voltage across anode and cathode at which SCR turns on without gate voltage is called forward breakover voltage. SCR configured to work in forward blocking mode is shown in the figure. 

 Forward Blocking Mode

(c). Forward conducting mode

In forward blocking mode, SCR starts to conduct at breakover voltage. Under normal conditions, breakover voltage is much higher than normal operating voltage. Turn ON can be reduced by applying a gate trigger.  When a positive trigger is applied at the gate terminal, junction J2 is forward biased. Now the depletion width is narrowed by letting charge carriers to drift across the junction.

In other words, in forward blocking mode, junction J2 is reverse biased. Therefore width of the junction depletion region, where no free charge carriers are present, is very large. When a positive gate trigger is given near this region, more charge carriers are introduced into this junction, creating a continuous path for current to flow and the device comes to forward conduction mode.

Even if gate voltage is removed, the anode current is not decreased. The only way to stop conduction is to reduce the applied anode voltage to zero. Therefore, usually SCRs are used in AC circuits, where a zero crossing point is available in every half cycle. While used in DC circuits, indirect methods are employed for commutation. Biasing of SCR to operate it in forward conduction mode is as shown in the figure.

Forward conducting mode


SCR Two Transistor Model

Operation of an SCR

Operation of an SCR can be explained easily by considering it as a tightly coupled pair of bipolar transistors as shown in the figure.

When a positive trigger is applied at the gate of transistor T2, it starts to conduct. Current through transistor T2 will increase and drive the base of transistor T1, now, turning it ON. Now, current through T1 will drive transistor T2. It is continued so and the transistor T2 will continue to turn ON even if the gate trigger is removed.

SCR Characteristics

(a). Forward Characteristics

In forward mode, positive voltage is applied at the anode of SCR. Junction J2 is now reverse biased, such that, only leakage current flows through the SCR. If the voltage is increased beyond breakover voltage, breakdown occurs at the junction J2 and SCR starts to conduct. Large current flows through the device with a sudden drop in voltage across the device. Voltage at which breakover occurs for an SCR can be decreased by applying a positive voltage at the gate.

When gate is triggered, more charge carriers reach the junction, initiating early occurrence of junction breakdown. SCR in forward blocking mode is switched to forward conduction mode with a gate trigger. Current through SCR is continued even after gate trigger is removed.

(b). Reverse Characteristics

In reverse mode, V-I characteristics is similar to that of a diode, anode voltage is negative with respect to cathode. Only leakage current flows through the SCR and its voltage is less than breakdown voltage. When applied voltage is greater than breakdown voltage, a sudden increase in current will occur with a slight increase in the applied voltage.

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