Silicon Controlled Rectifiers (SCRs) are the fundamental components in modern power electronics. These are the solid state current controlled devices that allow the conduction of current in one direction only. SCR is the very first and most popular member of the thyristor family in Power Electronics and commonly known as Thyristor.

SCRs are most popularly used in modern power control applications such as motor speed control, switch mode power supplies (SMPS), UPS (uninterruptible power supplies) in dimmers for lighting control, battery chargers for Electric Vehicle etc.

Understanding the VI characteristics of SCR is important to utilize it in specific applications efficiently.

But before moving on to the main topic of this article, it is recommended that you should have the understanding about the foundational concepts of SCR such as What is SCR? Construction and Working of the SCR, Latching and Holding current of the SCR.

We have created a dedicated article that covers these concepts in detail. If you’re interested then check it out by visiting Silicon controlled Rectifier.

We’ll also give you a quick overview of these concepts in this article at the time of concept needed, so that you can understand the VI characteristics of SCR without any confusion.

What is the VI Characteristics of SCR?

The VI Characteristics of the SCR is the graphical representation of the relationship between voltage applied across the terminals of the SCR and current flowing through the SCR under different conditions. This characteristic gives us information about how a SCR works during different biasing conditions.

Importance of the VI Characteristics of SCR

As we have discussed above, the comprehensive study of the VI characteristics of SCR is important to understand its efficient use in specific applications. Moreover, it gives us a broader idea about the working modes of the SCR, forward and reverse voltage blocking capabilities of the SCR, latching current and holding current of the SCR etc.

VI Characteristics of SCR

In our discussion about how an SCR works, we mentioned that it operates in three modes i.e. Forward Blocking Mode, Reverse Blocking Mode and Forward Conduction Mode. Out of these three modes of operation, the SCR conducts current only in forward conduction mode.

To understand the VI characteristics of SCR. Let us first explore the VI characteristics of SCR in these three modes of operation separately. Alongside, we briefly discuss the working of SCR in these three modes.

VI Characteristics of SCR during Reverse Blocking Mode

In reverse blocking mode of the SCR, a reverse voltage is applied across the terminal of SCR (discussed in working of SCR). Reverse voltage means the anode is connected to the negative terminal of the supply and the cathode is connected to the positive terminal of the supply and keeps the gate terminal open that means V_AKis negative.

The circuit diagram for obtaining the VI characteristics of SCR in this mode of operation is shown in the given figure.

This negative voltage will make the junction J₁ and J₃ of the SCR reverse biased and J₂ is forward biased. As the junction J₁ and J₂ is reverse biased, these junctions will not conduct the current. Therefore, the SCR does not conduct current in this mode.

Ideally, no current will flow in this condition. But practically, a small amount of current called reverse leakage current will flow through SCR.

The VI Characteristics during this condition is shown in the given figure.

The term V_BRin the given characteristics stands for Reverse Breakdown Voltage, it is the maximum amount of reverse voltage that can be applied across the terminal of the SCR beyond this voltage avalanche breakdown occurs at junctions J₁ and J₂.

VI Characteristics of SCR during Forward Blocking Mode and Forward Conduction Mode

The reason behind explaining the VI Characteristics of SCR during these two modes combinedly is that these two modes of the operation of the SCR are inter-connected.

During these two modes of operation, the forward voltage is applied across the terminal of the SCR. Forward voltage means the anode of the SCR is connected to the positive terminal of the supply and the cathode of the SCR is connected to the negative terminal of the supply. This means V_AKis positive.

However, there is a discrepancy between these two modes i.e. during forward blocking mode the gate terminal of the SCR is kept open while during forward conduction mode a positive voltage is applied across the gate and cathode terminal of the SCR.

The circuit diagram for obtaining the VI characteristics of SCR in these modes of operation is shown in the given figure. In this figure, we see that a positive voltage source E_gis connected across the gate and cathode terminals of the SCR through a switch S and resistance R_g. By turning the switch on and off, we can simply apply or remove the gate voltage. Additionally, by varying the resistance R_g, we can vary the gate current.

Let us first understand the VI Characteristics of SCR when the switch S is turned off.

Case(i) :- When the switch S is turned off (Forward Blocking Mode)

When the switch S is turned off, the voltage source E_g is not connected between the gate and cathode terminals of the SCR. In this condition above circuit can be redrawn as show in given figure.

As the voltage source E_g is not connected between gate and cathode terminal the SCR, the gate current is zero and the SCR is forward biased (discussed previously in this section), during this condition, the junctions J₁ and J₃ of the SCR are forward biased and they will conduct the current, while the junction J₂ is reversed biased and will not conduct the current. Therefore, the SCR will not conduct the current and it is said to be in forward blocking mode.

Ideally, during this period the SCR will not conduct the current. But practically, a small amount of current called forward leakage current will flow through SCR.

During this period the vi characteristics of SCR is shown in the given figure.

In above characteristics, V_BO stands for forward breakover voltage, it is the maximum amount of the forward voltage up to which the SCR is in forward blocking mode.

Beyond this voltage, avalanche breakdown occurs at junction J₂ of the SCR. This breakdown will cause the SCR to switch from forward blocking mode to forward conduction mode. Because, this breakdown results in a large amount of current flowing through the junction J₂_, and this current will support the conduction near junction J₂.

Since, junctions J₁and J₃ already conduct the current and after the forward breakover voltage junction J₂ will also conduct the current. Therefore, the SCR will move to the forward conduction state from forward blocking state.

But this method of turning on or driving the SCR from forward blocking mode to forward conduction mode will not be preferred because this will cause the permanent damage of junction J₂ and make the SCR useless.

There is an efficient alternative to turn on the SCR i.e. applying the positive gate voltage between gate and cathode terminal of the SCR.

Case (ii) :- When the switch S is turned on (Forward Conduction Mode)

When the switch S is turned on, the voltage source E_g is connected between the gate and cathode terminals of the SCR and the gate current is no longer zero. In this condition, the above circuit diagram can be redrawn as shown in the given figure below.

As gate current is no longer zero, it initiates the conduction in the depletion layer of the reverse biased junction J₂. This initial conduction in the depletion layer supports the flow of current near the reversed biased junction J₂.

Since, junctions J₁ and J₃ are already conducting the current, applying the gate current causes the reverse biased junction J₂ conducts as well. Therefore, SCR will conduct the current.

Increasing the gate current will enhance the rate of this initial conduction in the depletion region, resulting in the SCR turning on even at lower voltages. We can increase the gate current by varying the resistance R_g.

In this condition the VI characteristics of SCR is drawn similar to the previous case, but with the inclusion of the gate current indications, which shows the SCR can turn at lower voltages by applying the gate current.

There are two more quantities that are included in the above characteristics graph i.e I_H and I_L. These are the notations for Holding Current and Latching Current in SCR.

What is Latching Current in SCR?

Latching current in SCR is the minimum amount of the anode current Ia that flows through the SCR, below which if we remove the gate current pulse then SCR fails to turn on.

As we discussed previously the SCR can turn on by applying the gate current. Once the SCR starts, the conduction gate loses control on it. Therefore in order to avoid gate power losses we have to remove gate current.

To efficiently turn on the SCR it is necessary that the anode current must be greater than the latching current before removing the gate current.

What is Holding Current in SCR?

Holding current is the minimum amount of anode current Ia that keeps the SCR in on state i.e conduction state. If the anode current falls below the holding current then SCR will turn off.

It is associated with the turn off process of the SCR. As we know that SCR is a semi-controlled switch means we can control its on state by controlling the gate current but the gate has no control on the turn off process of the SCR. To turn off the SCR it is necessary that anode current of the SCR is less than Holding Current. If it is not then SCR will not turn off and it will start the conduction again.

VI Characteristics of SCR

So, after this detailed discussion on the VI characteristics of SCR VI characteristics of SCR, we can successfully conclude that the VI characteristics of the SCR is shown as the given figure.