SCR (Silicon Controlled Rectifier)

In this article, we will discuss about what is SCR? Working of SCR.

Symbol:

Structure:

SCR is known as Silicon Controlled Rectifier. SCR has three terminals; anode, cathode and gate. It is unidirectional device, which means it allows flow of current in only one direction. SCR is a bipolar device, which means it can block positive and negative voltage. It four layers (p,n-,p,n+) and three junction (J1, J2, J3). In layer positive (+) and negative (-) is denoted for heavily and lightly doped like n+ is heavily doped and n- is lightly doped and with sign is denoted as normally doped. 

Working of SCR:

SCR is a semi controlled device means it can control only ON state of SCR but we cannot control the OFF state of SCR. ON state can be controlled by gate current. 

SCR works in three modes;

1. Reverse blocking mode
2. Forward blocking mode
3. Forward conducting mode

1) Reverse blocking mode:

In reverse blocking mode, the negative terminal of supply connected with anode terminal of SCR and the positive terminal of the supply is connected with cathode terminal of SCR. Now for junction J1, as shown in below figure, p terminal is connected with negative of battery and n terminal of SCR is connected with positive terminal of battery, So, junction J1 is in reverse bias. Same this way, J2 is forward bias and J3 is in reverse bias. It means that, whatever negative voltage is given by supply that will drop in J1 and J3. But here, J3 is heavily doped and any semi conductor device which is heavily doped has lower voltage withstand capability and it will break quickly when applying some amount of voltage. So, if you increase supply voltage more, than J3 will break and start conducting. In this condition entire voltage drop will occur across junction J1. 

Reverse blocking mode of SCR is same as diode. When supply voltage is increased to breakdown voltage, infinite current will flow through the SCR and in this condition junction J1 is break and start conducting. But if this voltage is increased to break over voltage, SCR may damage.

2) Forward blocking mode:

In forward blocking mode, positive terminal of supply is connected with anode terminal of SCR and negative terminal of supply is connected with cathode terminal if SCR. It means Vak>0 and Ig=0. In this mode, junction J1 is in forward bias, J2 is in reverse bias and J3 is in forward bias. Here, in this mode also, current following through the SCR is zero, beacuse junction J2 is in reverse bias and entire voltage drop will occur in junction J2. So, in this mode, if you supply positive supply voltage than also SCR will not allow the current to flow. But if you increase positive voltage junction J2 will break, this voltage is denoted by Vbo (Voltage break over). When Vin > Vbo, Junction J2 will break and start conducting and the current will flow through the SCR. But this is not a safe for SCR, if you will do the same for 4 to 5 times, SCR may get damage. So, this is not safe way to conducting SCR. 

3) Forward conducting mode:

In forward conducting mode, connection is same as forward blocking mode, but in this mode, we are supplying gate current. If you supply gate current to the junction J2, it will neutralise the charge stored across the depletion region and junction J2 will in forward bias, even when supply voltage is less than break over voltage. So, in this mode, current will flow through the SCR and all junctions remains in forward bias. 

Now, if the gate current is increase, more the value of gate current, fastest the charge neutralisation. Once the SCR enters in conduction, it means once the junction J2 is in forward bias and start conducting by supping gate current, even after this if you will remove the gate current than also SCR start conducting. 

So, in SCR we can only control the ON state of SCR by gate current, once it enters in to ON state you cannot control the OFF state, that's why this device is known as semi controlled device. To control the OFF state of the SCR, we have to use separate commutation circuit or change the input supply, so it will convert into reverse blocking mode. So, turn OFF state is depends upon the supply voltage and the commutation circuit.

Latching current: The latching current is define as the minimum amount of the anode current is required to turn ON the SCR and the latching current is denoted as IL.

Holding current: To turn OFF SCR, we need to decrease the anode current up to some desired value and this value is known as holding current. The holding current is denotes as IH. It means holding current is defined as the minimum amount of anode current below which SCR turned OFF.

So, for ON state of SCR, IA > IL,

and for OFF state pf SCR, IA < IH

Here we can derive that the latching current is always grater than the holding current, IL> IH