Turn on methods of SCR or Triggering Methods of SCR

SCR stands for “Silicon Controlled Rectifier”. It is the fundamental and the most widely used device among all the thyristor family devices and it is commonly known as thyristor. 

SCR is a three terminal, current controlled, unidirectional solid-state device. It consists of four alternating layers of p-type semiconductor and n-type semiconductor therefore it is called a solid-state device. The three terminals of SCR are Anode, Cathode, and Gate. The term current controlled derived from its controlling characteristics, we can control its on state or conducting state by controlling the gate current and the term unidirectional shows its one-way conducting nature i.e. conduction during forward bias condition.

Before moving onto the main concept of this article we recommend that first go through some basic concepts of SCR such as What is SCR? How does SCR work? Working modes of SCR, Latching and Holding current in SCR etc. Understanding these fundamentals will facilitate you to understand this article, as these concepts are frequently referenced throughout in this article.

We have created a dedicated article on these concepts, to read this article visit “Silicon Controlled Rectifier”.

Although, we will also provide a quick overview of these concepts at the time of concept used, ensuring that you can easily grasp the concept of the article.

What is Triggering of SCR?

Triggering of SCR means turning it on, or in other words switching the SCR from forward blocking mode (off -state) to forward conduction mode (on-state).

A SCR works in three different modes of operation i.e. forward blocking mode, reverse blocking mode and forward conduction mode.

A SCR is said to be in reverse blocking mode, when the SCR is connected to reverse biased voltage (cathode is connected to positive terminal and anode is connected to negative terminal of supply) with the gate terminal kept open. In this condition, the junction J₁ and J₃ of the SCR is reverse biased (does not conduct current) and junction J₂ is forward biased (conducts current). Since junctions J₁ and J₃ do not support current conduction, therefore, the SCR does not conduct the current (discussed in details in working of SCR ).

A SCR is said to be in forward blocking mode, when the SCR is connected to forward biased voltage (anode  -> positive and cathode -> negative terminal of supply) with the gate terminal kept open. In this condition, the junction J₁ and J₃ of the SCR is forward biased (conducts current) and junction J₂ is reverse biased (does not conduct current). Due to this reverse biased junction J₂, SCR does not conduct the current. 

Since, the SCR does not conduct the current by applying either forward biased voltage or reverse biased voltage alone, it must be triggered to move from forward blocking mode to forward conduction mode. To achieve this, various triggering methods are employed. 

 

Triggering Methods of SCR

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

 

Forward Voltage Triggering of SCR

As we discussed previously, when the forward biased voltage is applied across the SCR then the junction J₁ and J₃ of the SCR are forward biased thus supporting the conduction and junction J₂ is reverse biased and does not support conduction of current due to the formation of depletion layer at junction J₂. 

So, if we increase the forward biased voltage beyond the breakdown voltage of reverse biased junction J₂, avalanche breakdown occurs at junction J₂. This breakdown causes a large amount of current to flow through the reverse bias junction J₂ that supports the conduction near this junction. Since the junction J₁ and J₃ are already conducting the current, and increasing the voltage causes the junction J₂ conducts as well, therefore the SCR will start conduction.

 

This method is not so popularly used to turn on the SCR because increasing the voltage will cause the permanent damage of junction J₂ as well as the SCR.

 

The increased forward biased voltage beyond which SCR starts conduction is called forward breakover voltage.

 

dv/dt triggering of SCR

As we discussed previously in forward biased SCR, junctions J₁ and J₃ are forward biased and conducting current and junction J₂ is reversed biased and does conduct current due to the formation of depletion layer at junction reverse bias junction J₂.

In this condition the SCR behaves as a capacitor with J₁ and J₂ being the conducting plates and J₂ being the dielectric of the capacitor. This happens due to the uncovered position of electrons and holes at reverse bias junction J₂ (refer working of the P-N junction diode during reverse biased condition).

Let us say C is the capacitance of the SCR, and Ia is the anode current that is flowing through anode to cathode of the SCR. This anode current Ia behaves as the charging current of the capacitor.

The expression for the current flowing through the capacitor is 

So, from the above equation we can see if we increase dv/dt, the anode current of the SCR is also increased. 

So, increasing dv/dt to a value at which anode current becomes greater than the latching current of SCR, tends to turn on the SCR.

Latching Current in SCR

It is the minimum amount of anode current below which if we remove the triggering circuit then SCR fails to turn on. It is represented by I_L.

Anode current (I_a) of the SCR must be greater than the latching current (I_L) of the SCR for turning it on.

Light triggering of SCR

In this method of triggering of SCR, a high intensity light is protected on the gate junction of the SCR. In this condition, the depletion layer at the reverse bias junction J₂ absorbs the light energy and produces more electrons and holes pairs. These excess numbers of electrons initiate the conduction near the depletion reason and thus support the conduction near this region as well as in the SCR. 

The SCR which is turned on by this method is called Light Activated Silicon Controlled Rectifier (LASCR).

This triggering method is more efficient and reliable, where the multiple numbers of SCRs are triggered simultaneously.

 

Gate triggering of SCR

The gate triggering method of SCR is the most efficient and reliable method for turning it on. In this method a positive voltage is applied across the gate and cathode terminal of the SCR. This terminal voltage injects current into the depletion layer, attracting more electrons from the n type layer. This results in a flood of electrons flowing from n type layer to p-type layer. This flood of electrons increases the reverse leakage current flowing through the reverse biased junction J₂, causing breakdown of junction J₂. This breakdown of junction J₂ supports the conduction at junction J₂.

As junctions J₁ and J₃ are already conducting the current and after breakdown junction J₂ conducts as well, therefore SCR starts conduction.

There are three different types of gate triggering of the SCR, these are DC gate triggering, AC gate triggering and high frequency pulse gate triggering.

DC Gate Triggering of SCR

In this triggering method of SCR, a positive DC voltage is applied across the gate and cathode terminal of the SCR. This DC voltage injects the continuous current into the gate terminal, which drives the SCR from forward blocking mode to forward conduction mode or we can say turn it on.

This type of gate triggering has two major drawbacks. First is, it has more gate power losses and second is, there is no isolation between the main circuit (power circuit) and the controlling circuit (gate circuit).

 

AC Triggering of SCR

As the name implies, AC supply is used to trigger the SCR. In this method AC voltage is applied across the gate and cathode terminal of SCR with proper isolation between power and control circuit. This Ac voltage injects the pulses of gate current into the gate terminal which turns on the SCR.

There are two advantages of using AC triggering of SCR. First is there is proper isolation between Power circuit and control circuit, “pulse transformer is used” and second is power losses are less as compared to DC gate triggering of SCR.

However there is one major disadvantage as well i.e. only positive pulses of the AC supply are available for the gate circuit to turn on the SCR. 

Pulse triggering of SCR

In this method of gate triggering,  a train of high frequency pulses of current are injected into the gate terminal by using different firing circuits like synchronized UJT firing circuits. 

The use of high frequency current pulses reduces the gate power losses up to a great extent and also the size of the pulse transformer is reduced.

Related Posts :-

#_Silicon Controlled Rectifier (SCR)

#_VI Characteristics Of SCR