Silicon Controlled Rectifier (SCR) or Thyristor

What is the Thyristor?


The name thyristor is derived from the combination of the first four letters of Thyratron and the last five letters of Transistor.  

 

Thyratron is an electrical switch that is used for high power applications. Construction wise it is a gas filled tube.

 

Transistors are low power and high switching speed electronic devices. Construction wise these are formed by the combination of P-type semiconductor and N-type semiconductor. Therefore, these are categorized as solid-state devices.

 

In a similar context, Thyristor is a solid-state semiconductor device that has the capabilities of both thyratron and transistor. Just like a thyratron, a thyristor has high power withstand capabilities and from the construction point of view it belongs to the transistor family. As it is formed by the combination of alternating layers of P-type semiconductor and N-type semiconductor, just like a transistor. Mainly it constitutes of four or more alternating layers of P-type semiconductor and N-type semiconductor. 

 

So, a Thyristor can be defined as a three terminal (anode, cathode and gate) and four layered i.e. p-n-p-n semiconductor switching device. 

 


Thyristor Family

 

Thyristor Family is the class of power semiconductor devices which has the properties of both thyratron and transistor as stated above.


Some members of thyristor family are listed as follows.

 

SCR         :-     Silicon Controlled Rectifier

LASCR    :-     Light Activated SCR

RCT         :-     Reverse Conducting Thyristor

GTO         :-     Gate Turn Off Thyristor

TRIAC     :-      Triode for AC

DIAC       :-      Diode for AC

SCS         :-      Silicon Controlled Switch

UJT         :-      Unijunction Transistor      etc.





Silicon Controlled Rectifier (SCR)


SCR

SCR stands for Silicon Control Rectifier. It is the very first and most popular member of the thyristor family in Power Electronics and it is commonly known as thyristor. 

 

As its name implies, it is a controlled switch or power semiconductor device which is constructed by silicon semiconductor and it works as a rectifier in a circuit, means it allows the conduction of current in one direction only that means it is a unidirectional switch.

 

The term, controlled switch drives from its controlling characteristics. In SCR we can control its on-state by controlling the gate current (discussed in details further in this article). As its on-state is controlled by the gate current, it is categorized as a current controlled device.

 

A SCR has high power and least switching frequency among all the power semiconductor devices.



Construction of SCR

 

A SCR has three terminals named as Anode (positive terminal), Cathode (negative terminal), and Gate (controlling terminal) and four alternating layers of P-type and N-type semiconductor materials (Silicon is used for its construction as its name indicates) i.e. (p-n-p-n) shown in the given figure. 

Construction of SCR


  • The terminal connected to the outer p-region is called Anode.
  • The terminal connected to the outer n-region is called Cathode.
  • The terminal connected to the inner p-region is called Gate.

 

These four alternating layers of P-type and N-type semiconductor materials formed three junctions J1, J2 and J3, each of these junctions itself referred to as a PN junction diode (This practice is only theoretical, to get the better understanding about working of SCR) shown in the given figure below.

SCR (Silicon Controlled Rectifier)



SCR Working


The working of SCR depends on the biasing in conjunction with triggering. 

 

Biasing means applying the voltage to the anode and cathode terminals of the SCR.

 

Triggering of SCR means turning on the SCR. As we discussed above, SCR is a control switch, in which we can control its on state or conduction state by controlling the gate current (discussed in detail further in this article). 

 

 

There are two ways of biasing of the SCR or applying the voltage across the terminal of SCR. 

 

In the first method i.e. forward biasing, the anode is connected to the positive terminal of the supply, the cathode is connected to the negative terminal of the supply and the gate terminal of the SCR is kept open. In this condition SCR does not conduct current and is said to be in forward blocking mode

 

In the second method i.e. reverse biasing, the cathode is connected to the positive terminal of the supply, the anode is connected to the negative terminal of the supply and the gate terminal of the SCR is kept open. In this condition SCR does not conduct current and is said to be in reverse blocking mode

 

There is an another condition where the voltage applied to the SCR terminals is the same as the first case i.e. forward biasing but with a triggering circuit connected to the gate and cathode terminal of the SCR. In this condition, SCR conducts current and is said to be in forward conduction mode.

 

Since SCR does not conduct current in both forward biasing condition as well as reverse biasing condition, it has bipolar voltage withstand capacity and is categorized as a bipolar device.

 

 

Let us understand these three modes of operation of SCR in detail



Forward Blocking Mode of SCR

When the forward voltage is applied to the SCR i.e. anode is made positive with respect to the cathode and leaving the gate terminal open means the gate current is zero. In this biasing condition, SCR does not conduct the current and it is said to be in forward blocking mode (discussed previously).

In this condition, the direction of current flows in such a way that it enters to the P side of the junctions J1 and J3 therefore, these junctions are forward biased and it enters to the n-side of the junction J2 therefore, it is reverse biased.

The direction of the current flows through these junctions indicated by the blue arrow in the given figure. These indications are only theoretical assumptions so that we can understand these modes of operation of SCR easily.

SCR Silicon Controlled Rectifier

As the junction J1 is forward biased, it allows the conduction of current therefore, the current flows from the p region to n region through junction J1. (refer working of p-n junction diode)

 


But the junction J2 is reverse biased and due to this the width of the depletion layer as junction J2 increases immediately after applying the voltage across the SCR terminal (reverse voltage across junction J2). Therefore, junction J2 offers a very high resistance (theoretically infinite resistance) to the conduction of current, consequently the current will not flow further through the junction J2. However, practically a small amount of current called leakage current will flow from anode to cathode.

 

Therefore, in this condition SCR is said to be in off state or blocking state because it does not allow the conduction of current.



Reverse Blocking Mode of SCR

In this mode of operation, SCR is reverse biased, means anode is connected to the negative terminal of the supply and cathode is connected to the positive terminal of the supply, keeping the gate voltage zero. 

SCR (Silicon Controlled Rectifier)
 

In this condition the current enters to the n-side of the junctions J1 and J3 therefore, these are reverse biased and junction J2 is forward biased because the current enters to the p-side of the junction J2.

 

As junctions J1 and J3 are reverse biased, they do not allow the conduction of current because the width of the depletion layer at junctions J1 and J3 increases immediately after applying the reverse voltage across the terminal of the SCR. Consequently, these junctions offer very high resistance to conduction of current.

 

As these junctions do not allow the conduction of current the SCR is said to be in off state or blocking state and this mode of operation is called reverse blocking mode of SCR because in this mode SCR is reverse biased and it does not allow the conduction of current. 


The maximum amount of voltage up to which the SCR is said to be in reverse blocking mode is called reverse breakdown voltage. Beyond this voltage breakdown of junction J1 and J2 occurs, simultaneously breakdown of SCR occurs.




Forward Conduction Mode of SCR

In this mode of operation, the voltage applied across the SCR terminals is the same as that of the case of forward blocking mode, i.e. anode is made positive with respect to cathode.

 

As discussed previously, in this case junctions J1 and J3 are forward biased and they allow the conduction of current and junction J2 is reverse biased and it does not allow the conduction of current. Therefore, initially the SCR is in forward blocking mode. 

 


To drive the SCR from forward blocking mode to forward conduction mode there are two ways. 

 

First way is increasing the forward voltage applied across the SCR terminal beyond the reverse breakdown voltage (VBR) of junction J2 while keeping the gate terminal open or at zero potential. This will result in the occurrence of avalanche breakdown at junction J2, consequently the large amount of current will flow through the junction J2.

SCR Working

As junctions J1 and J3 are forward biased and already conducting current, and increasing the voltage will cause junction J2 conduct as well. Consequently, the SCR will conduct current.

 

Practically, this method to turn on the SCR will not be preferred because the voltage applied across SCR terminals cannot be increased beyond the safe rated value.

 

In the second method, SCR is brought from forward blocking mode to forward conduction mode by applying the positive gate current pulses between gate and cathode terminals. These gate current pulses initiate conduction near the depletion region. Increasing the gate current pulse width will expand the initial conduction area near the depletion region. This will result in the current flowing through the depletion layer at junction J2.

SCR Working

As junctions J1 and J3 are forward biased and already conducting current and by applying gate pulse between gate and cathode terminals of the SCR junction J2 conducts as well. Consequently, the SCR will start conducting current.

 

Once SCR starts conduction no more gate pulses are required to keep it on. So, we can safely remove the gate pulse to avoid the power losses at the gate terminal. But we have to take care about the latching current of the SCR while removing the gate pulses so that SCR cannot fails to turn on.



Latching Current in SCR

Latching current is associated with the turn on process of the SCR. It is the minimum amount of the anode to cathode current of the SCR below which if the gate pulses are removed then SCR fails to turn on.

 

As we discussed above the gate signal initiates the turn on process of the SCR. Once the SCR starts the conduction gate loses control on it. Therefore, we can remove the gate signal in order to avoid the continuous gate power losses. 

 

If we remove the gate pulses when the anode to cathode current of the SCR is below the latching current then the SCR will fail to turn on. Therefore, we have to maintain the gate pulse width at least for a period until the anode to cathode current reaches slightly greater than the latching current, to turn on the SCR.




Triggering Methods of SCR

Triggering methods of SCR also known as turn on method or SCR. In the previous section on working of SCR, we studied that both biasing and triggering are essential for the proper operation of SCR. 

There we discussed that only applying the voltage across the terminal of the SCR is insufficient to turn it on. To turn on the SCR, there we discussed two ways: first is by increasing the forward voltage beyond the reverse breakdown voltage of junction J2 and second is by applying the positive gate voltage between gate and cathode terminals of the SCR.

Apart from these two methods to turn on the SCR, there are three more methods to turn on the SCR. So combinedly there are total five different turn on methods of SCR  

 

  • Forward Voltage Triggering
  • Gate Triggering
  • dv/dt triggering
  • Light Triggering

 

We discussed Forward Voltage Triggering and Gate Triggering in the above section. You can simply visit these sections by clicking  their respective names.




dv/dt triggering of SCR


In the construction of SCR, we discussed that the four layers of the SCR make three junctions J1, J2 and J3 each of these junctions have their own junction capacitance. The current flowing through the SCR will pass through these junction capacitance and act as the charging current of these junction capacitance. 

While the anode current passes through the junction J2 that is reverse biased the expression for the current.






So, from the above expression we can clearly see that if we increase dv/dt then the charging current (anode current Ia) across the junction capacitance Cj2 will increase.  

So, if we increase the dv/dt sufficiently high, so that the charging current will be greater than the latching current of the SCR then SCR will turn on.

 


Light Triggering of SCR


When a high intensity light is incident near the gate junction of the SCR then the depletion layer absorbs the light and produces more electrons and holes. These electrons and holes initiate the conduction in the depletion layer and simultaneously support the conduction of SCR. The SCR which is turned on by this method is known as Light activated Silicon Controlled Rectifier (LASCR).




VI Characteristics of SCR

The VI characteristics of SCR illustrate the relationship between the voltage applied across the anode and cathode terminals of the SCR (biasing voltage) and current passing through it under different operating modes. It gives us the understanding of different parameters of SCR, so that we utilize the SCR in any circuit efficiently.  


SCR VI Characteristics


Above given figure shows the VI characteristics of SCR. We have created a separate article that offers a detailed insight into these Characteristics, including how to draw them, the information they provided and much more. To read that article please visitVI Characteristics of SCR”.   



In above figure

  • VBR  is the Reverse Breakdown Voltage, it is the maximum amount of reverse biased voltage that the SCR can withstand in the reverse blocking state. Beyond this voltage breakdown of the junctions J1 and J2 of the SCR occurs, this will lead to the breakdown of the SCR (discussed previously in this article in Reverse Blocking Mode of SCR Section). This breakdown will result in the flow of a large amount of reverse current through the SCR. This flow of current is shown by the sharp decline line denoted by BC after the VBR

  • VBO is the Forward Breakover Voltage, it is the maximum amount of forward biased voltage upto which SCR is in forward blocking state. Beyond this voltage, breakdown occurs at junction J2. This breakdown will results the flow of large amount of current through junction J2. This current supports the conduction of current near junction J2 as well as in the entire SCR.

  • IL is the latching current of SCR discussed above in separate section

  • IH is the Holding Current of the SCR.

 

             

Holding Current in SCR

 

Holding current is associated with the turn off process of the SCR. It is the minimum amount of the anode to cathode current below which SCR will turn off.

 

As we have discussed above, SCR is a semi controlled or half-controlled device that means gate signal only controls the turn on process of the SCR it has no control on the turn off process of the SCR. SCR will turn off only when the anode current of the SCR will become less then the holding current.








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