WHAT ARE THYRISTORS?
“Thyristors are three terminal semiconductor devices with 4 layers of alternating p and n regions used for high switching,rectification and control of electric signal.”
Types of thyristors:
- The first and the most common type of thyristor in the industry is SCR(Silicon control rectifier)
- GTO(Gate turn off SCR)
- LASCR(Light activated SCR)
Let’s try and understand the basic mechanism of the most basic and common type of thyristor:-
SCR(Silicon controlled rectifier)
As you may see,an SCR has 4 layers of alternating p and n regions that actively form three junctions j1,j2 & j3.
An SCR’s region of operation mainly depends upon the biasing across the device.It can be categorized as:
1.Reverse conduction:When cathode is made positive with respect to anode,the junctions j1 and j3 become reverse biased and j2 is in forward bias.To conduct the SCR in this region,a lot of voltage will be required for breakdown of two reverse biased junctions j1 and j3 which is not practically desired characteristic.
2.Forward conduction:When anode is made positive with respect to cathode,the junctions j1 and j3 become forward biased and j2 is in reverse bias.Now conduction of SCR in forward conduction mode is quiet easy.We have to just break one reverse biased junction j2.
Let me introduce a technique in which this reverse biased junction can be breaked with substantially less amount of Vak(anode to cathode voltage).Positive voltage is applied at p region through a gate terminal.As soon as sufficient voltage is applied through gate,the junction breaks and conduction starts.
Lets understand it better with the help of two transistor analogy.
Now i want you to observe carefully..
We conclude that an SCR is nothing but a combination of PNP and NPN transistors.
Now as soon as positive gate voltage is applied,it reaches the base of transistor Q2 and transistor Q2 turns on.Collector current starts flowing from transistor Q2 and this collector current acts as base current for transistor Q1 which turns the transistor Q1 on.Collector current of Q1 becomes base current for Q2.Check out this article to know how a transistor works?
This process continues and current multiplication takes place.The current cycle is in a continuous loop and now even if we remove the external gate voltage the transistor remains latched in on state.Gate loses its control over the device and the device is now in infinite saturation.
Now since we know exactly how the SCR works,let us observe the behavior of the device in reverse conduction,forward conduction and the role of gate current in turning on of the SCR with the help of VI characteristics.
Some important factors to be understood:-
1.Holding current(IH):It is the current that flows through the SCR to keep the SCR in conducting state when gate current is yet to be applied(approx 0).
NOTE: SCR turns off once the anode current falls below holding current.
2.Latching current(IL):It is the amount of current required to latch the SCR in on state.
3.Forward breakover voltage(Vbo):The maximum voltage in forward biased condition between anode and cathode at zero gate current when current conduction starts is nothing but forward breakover voltage.
4.Reverse breakover voltage(Vbr):The voltage in reverse biased condition at which the junctions break and huge current flows due to avalanche breakdown is known as reverse breakover voltage.This condition is normally avoided and hence SCR is operated only in forward bias to avoid any damage due to overheating due to excess current flow.
Notice that in forward conduction,the anode current rises sharply after reaching the latching current level.This is the region where SCR moves into saturation region and current multiplication takes place as discussed above with the help of two transistor model.
- High speed switching.
- Controlled rectifiers.
- Battery chargers.
- Light dimmers.
- Fan speed regulators.
- Voltage stabilizers for protection against excess voltages.