SCR (Silicon controlled rectifier or semiconductor controlled rectifier) is an electric device which comprises four pnpn layers with three major terminals anode, cathode and gate. The device has usage among hundreds of industrial products.
Mainly the internal structure of SCR is similar to the most basic type of thyristor i.e Shockley diode. If you want to understand SCR deeply, first you need to understand Shockley diode.
In fact, to understand other thyristors like DIAC and TRIAC, we must have to grasp basic concepts regarding the function of shockley diode. That is why I will start this lecture with the working of Shockley diode.
So to get concrete info continue reading this lesson. You can also take the lecture in the following video.
The contents of this lecture are:
- Internal Structure and Equivalent Circuit of Shockley Diode
- Characteristic Curves of Shockley Diode
- Internal Structure and Equivalent Circuit of SCR
- Characteristic Curves of SCR
- Characteristics of SCR
- Turning OFF Methods of SCR
Internal Structure and Equivalent Circuit of Shockley Diode
The following figure-1 shows the internal structure of a Shockley diode. You can see there are four layers of P and N-type semiconductor materials. Two layers are of P type material and two layers are of N type material. There are three PN junctions J1, J2 and J3. The terminal connected to the P1 layer is called Anode and the terminal connected to N2 layer is called cathode.
Fig 1: Shockley Diode Structure
Shockley diode symbol is given in Figure-2
Fig 2: Shockley Diode Symbol
You can separate these 4 layers in the form of two BJT transistors. One is PNP starting from anode and other is NPN starting from cathode. See figure-3
Fig-3: Shockley diode in the form of two BJTs
By following this we can make equivalent circuit of Shockley diode which you can see in the following figure-4
Fig-4: Equivalent Circuit of Shockley Diode
Working of Shockley Diode
Working of a Shockley diode can only be understood with the help of its characteristic curves. When we connect the anode with the positive electrode and cathode with negative electrode, the shockley diode should forward bias.
But this doesn’t happen because J2 is reverse biased which doesn’t allow the flow of current through the diode. Due to this reason there is a huge loss of voltage. this behavior is shown in the following fig-5 from 0 to until VBRF. VBRF is known as forward break over voltage.
The forward break over voltage is the voltage below which the Shockley diode doesn’t work and remains in the forward blocking region. The voltage drop is very large in this region.
Fig 5: Characteristic Curves of Shockley Diode
At the VBRF the diode moves from forward blocking region to the forward conduction region. In the forward conduction region, the thyristor is fully ON and working.
In the forward conduction region, the thyristor continues to turn ON until the anode current drops below the holding current (IH).
Why does it continue to turn ON about the holding current? I explained this point in the video above.
Holding current is the minimum amount of anode current at which the thyristor remains in forward conduction region. Below the holding current it switches OFF and goes again back to the forward blocking region.
Switching current (IS) is the current at which thyristor goes from forward blocking region to the forward conduction region. Switching current is always less than holding current.
Shockley Diode is Unidirectional
If we connect anode with the negative terminal of the battery and cathode positive terminal of the battery, the shockley diode will be reverse biased and will not work. This is shown in the third quadrant of the graph.
Now I think you have fully understood the working of Shockley diode. Its time to move towards the working and construction of SCR.
Internal structure and working of SCR
The solid state devices with name Silicon controlled rectifiers are very popular electronic devices because they have a wide range of applications in power electronics circuits. Their most famous applications in the industry are dimmers due to their ability to control voltage.
Normally SCRs are available in different packages. See the following pictures of just two SCRs from the whole family. The manufacturer is NTE.
The internal structure of SCR entirely similar to the shockley diode we have seen above the only difference is the gate terminal. See figure-6 below
Fig 6: SCR Structure
You will learn the purpose of gate terminal when we study the characteristic curves of SCR below.
Similarly, the Equivalent circuit of SCR is almost the same except gate terminal. See figure-7
Fig-7: SCR Equivalent Circuit
The symbol of SCR is given in following figure-8
Fig-8: SCR Symbol
Silicon Controlled Rectifier Working
The function of SCR can be explained by doing the analysis of the following characteristic curves. The curves also explain the operating regions of the device. The operation of SCR is symmetric to Shockley diode if any potential is not applied at the gate terminal. This behavior is represented by the green curve in the following figure-9.
Fig-9: SCR Characteristic Curves
In this state SCR will only be turned ON after the forward breakover voltage (VBRF3). Below VBRF3, the SCR is in the forward blocking region. After that it goes into the forward conduction region. As I told you above this high voltage drop happens due to the reverse bias junction J2.
Now if we trigger the gate by applying a positive pulse, the SCR will go into the forward conduction region before VBRF3. The positive potential at the gate breaks the junction J2. Hence SCR goes into forward conduction region and voltage drop is less in this case. In this state forward breakover voltage is VBRF2 which is less than as compared to the voltage drop if we don’t apply any potential at the gate. This behavior is shown by the blue curve in the above figure.
If we apply more positive potential at the gate SCR will go into forward conduction region before VBRF3. This behavior shown by the pink curve in the above figure.
Once the SCR goes into forward conduction region it will continue to conduct even if we remove the gate pulse. This state is above the holding current. If you want to turn off SCR again we have to drop the anode current below the holding current.
One important thing to notice is that by using a gate pulse we can switch ON SCR at our required voltage before the VBRF. This characteristic is very important especially when we want to use SCR as a phase controller.
Characteristics of SCR
- SCR can be used as a switch. It means we can turn it on and off as per our requirement.
- SCR is a unidirectional device. It means if we reverse the terminals of the battery, it will not work even if a pulse is given at the gate.
- Because it’s a unidirectional device. So SCR fulfils basic rectifier definition and can be used as AC to DC converter
Turning OFF Methods of SCR
As we have already learned above, the SCR cannot be turned off by just removing the pulse from the gate. This is because internal feedback system is being generated due to NPN and PNP transistors.
From the equivalent circuit, you can see the collector of NPN transistor keeps ON PNP transistor because it’s connected to the base of PNP transistor. Whereas collector of PNP transistor provides the necessary current to the base of NPN transistor to keep it ON.
Now we have to use another way to shutdown SCR. There are many methods to do this but we will discuss the most basic two methods. Whatever external circuit used to switch off SCR the end result is we have to bring the anode current below the holding current These methods are:
- Anode current interruption
- Forced commutation
1. Anode Current Interruption
In this method we momentarily bring the anode current zero. Another name of this method is natural commutation. There are further two ways to do this
A. Series Switch
In this method we connect a switch in series to the SCR to switch it off. This is not an ordinary switch but an electronically controlled switch usually BJT or MOSFET. See in figure-10 below we used a PNP transistor as a switch
Fig-10: Series Anode Current Interruption
You can see in (b), When we turn on BJT current flows through SCR. And to turn off SCR we simply turn off BJT as in (c) above. By doing this anode current goes below the holding current or ultimately goes to zero.
B. Parallel Switch
In this method we simply connect the same switch in parallel to the SCR. Now the switch is working as a shunt resistance or in other words it will shunt all the current across the SCR. See figure-11.
Fig-11: Parallel Anode Current Interruption
You can see in (b) above when we switch on the transistor all the current flows through it. And eventually the anode current of SCR goes below the holding current and SCR turns off.
To turn ON SCR again. Simply turn off the transistor and give pulse at the gate of SCR as shown in (c) above.
2. Forced Commutation
In this method we use some external circuit to run the current opposite to the anode current. A polarized capacitor is being used across SCR and also a switch is being placed in parallel to SCR. See figure-12
Fig-12: Forced Commutation
When SCR is working, the capacitor charges. Now to switch off SCR we simply switch ON the transistor. In this situation capacitor continues to discharge and current flows opposite to the anode current. This current is shown by the red arrows in (c).
As both currents are opposite to each other. The resultant current should be less than holding current of SCR .
I hope you got all necessary concepts about SCR. Take care Subscribe to stay updated