The SCR (silicon-controlled rectifier) is a power semiconductor device belonging to the thyristor family. It is similar to a diode that allows the flow of current only in one direction. The major difference between SCR and diode is, the SCR has a built-in feature to switch ON and OFF of the power flow. An additional input signal called gate signal is given to the SCR to control its conduction. Thus by controlling the gate signal we can control the power delivered to the load.

SCR can be used in controlled rectifiers, inverters, choppers, cyclo converters, switching circuits, etc. They are available from a range of a few voltages to several kilovolts and can handle power up to 10MW.

It is basically a four-layered, three junction, and three-terminal device. The four layers are pnpn, the three junctions are namely J₁, J₂, and J₃, and the three terminals are anode (A), cathode (K), and gate (G). The below shows the construction and symbol of silicon controlled rectifier.

V-I Characteristics or Modes of Operation of SCR :

The V-I characteristics of SCR is a graph between the anode current I_a and the anode-cathode voltage V_a, for different values of gate current I_g. It can be drawn by considering the basic operation of an SCR. The figure below shows the V-I characteristics of SCR which are also called static anode-cathode characteristics.

The V-I characteristics basically consist of three regions or modes of operation, they are

• Reverse blocking mode
• Forward blocking mode
• Forward conduction mode.

Reverse Blocking Mode :

When the positive terminal of supply is connected to the cathode and the negative terminal is connected to anode with gate circuit open then SCR operates in the reverse blocking mode. In this mode, junction J₁ and J₃ become reverse biased whereas, junction J₂ becomes forward biased.

The reverse-biased junctions (J₁ and J₃) act as an open-circuit and the forward-biased junction (J₂) acts as a short circuit, as shown above. It can be seen that the SCR does not conduct any current and it is said to be in the reverse blocking state or OFF state. However, a very small amount of leakage current flows through it.

Now, if we increase the reverse voltage to a value called reverse breakdown voltage, then an avalanche will occur and breaks the junction J₁ and J₃. Due to this a very huge amount of current flows through the circuit and hence the SCR starts conducting.

But this method of conducting SCR is not recommended as it may damage the SCR or the circuit in which it is working. Hence, we must ensure that the reverse voltage does not exceed the reverse breakdown voltage V_BR.

Forward Blocking Mode :

In the forward blocking mode of operation of SCR, the anode is connected to the positive terminal, and cathode is connected to the negative terminal of the power source with the gate circuit open. In this mode, the junction J₁ and J₃ get forward biased whereas, the junction J₂ gets reverse biased.

The forward biased junctions (J₁ and J₃) acts as short-circuit whereas the reverse-biased junction J₂ acts as an open circuit, as shown in the above figure. Even in this mode, the SCR does not conduct any current except a very small value of leakage current. This mode of operation of SCR is called forward blocking mode and it is the most wide operated mode.

Just as in the reverse blocking mode, the SCR can be made to conduct in the forward blocking mode by increasing the anode-cathode voltage to a value called forward breakdown voltage V_BO. Even this method is also not recommended as it may also damage the SCR. Hence, the SCR does not conduct even in this mode and is treated as an open switch.

Forward Conduction Mode :

In the forward conduction mode of SCR, the anode is connected to the positive terminal and the cathode is connected to the negative terminal with the gate circuit closed (i.e., application of small gate voltage). In this mode, all the three junctions (J₁, J₂, and J₃) act as the short circuit as shown in the figure below, and hence the SCR conducts current.

This method of conducting the SCR is most efficient, as it requires a voltage that is very much less than V_BO. The only extra thing we require is a gate signal for a small period of time for which the anode current becomes equal to the latching current. The minimum value of anode current is compulsory in order to maintain conduction.

Once the anode current attains this value, the gate loses control and hence can be removed. The removal of the gate signal will not have any effect on the SCR conduction.

However, if the anode current decreases to a value called holding current, the SCR will once again go back to the forward blocking state. Hence, care must be taken that, the anode current should not drop below the holding current after the gate signal is removed.

Important Terms Related to V-I Characteristics of SCR :

Forward Breakdown Voltage V_BO :

It is the voltage above which the SCR starts conducting i.e., ON state. The forward breakdown voltage depends upon gate current. The typical value of V_BO lies between 50V to 2500V.

Latching Current I_L :

Latching current is defined as the minimum value of current above which the anode current should rise for the reliable turn ON of the SCR. The width of the gate pulse should be so chosen that the anode current rise above the latching current I_L.

In other words, for the proper turn ON of SCR, the anode current must build up rapidly such that, the latching current of SCR is reached before the removal of the gate pulse.

Holding Current I_H :

Holding current is the minimum value of anode current below which the SCR will be in an OFF state. For an anode current of I_H and above I_H, it holds the SCR in ON state. It is useful to decide the instant for the application of the reverse voltage to completely turn OFF the SCR.

Usually, the holding current value is less than the latching current value (i.e., I_H < I_L). The holding current of an SCR is associated with the turn OFF process while the latching current is associated with the turn ON process.

Reverse Breakdown Voltage V_BR :

The reverse voltage above which the breakdown of SCR takes place i.e., the breakdown of junctions J₁ and J₃. It is also called the avalanche breakdown voltage. The V_BR is less than the V_BO with a typical value of a few kilovolts.