ALL ABOUT THYRISTORS-2

ALL ABOUT THYRISTORS-2

(I will be taking a slight deviation from book review blogposts.In this post,will be trying to explain an electronic topic "Thyristor" in a simple way. Hope all of you who are interested read and learn about the topic.Enjoy!
I will be posting it in 2 parts. The first part contains History,construction and operation of 'thyristor'.)

In part 2 of this post,you will find concept of thyristor turn ON,Effect of gate current,Types and Applications.

Turning ON of Thyristor:

So inorder to turn on a 'thyristor', we need to increase the anode-cathode voltage above 'breakdown voltage'.But practically if you do so, you will end up burning the 'thyristor'!Huh..!You don't want to do that right?
Manufacturers specify the voltage limits(dv/dt limitations) and care should be taken not to exceed this limit. So to avoid risks of burning the 'thyristor' a different method should be opted which involves the use of gate terminal.Have you noticed that I haven't used the gate terminal after mentioning it in the 3 terminals of 'thyristor'? Gate terminal has a very important role in the process of turning the 'thyristor' on. So all you need to do is,apply a small voltage(mostly pulses) at the gate terminal and now start increasing anode-cathode voltage.Guess what happens!? When you apply gate voltage, 'forward breakdown voltage' decreases. In other words, gate voltage is inversely proportional to breakdown voltage. This avoids the risk of burning the 'thyristor'. Eventhough you remove the voltage applied to gate terminal now,'thyristor' continues to conduct.It can be brought back to non-conducting state only by reducing the forward current below holding current.

Effect of gate current on breakdown voltage:

As gate current increases from IG1 to IG2,forward breakdown voltage decreases
from v1 to v2.

Thus by increasing the gate voltage, 'breakdown voltage' can be decreased. The applied signal to the 'gate' terminal should be removed once the 'thyristor' is turned on. If not,it results in excess power loss.Hence it is better to prevent the losses.

NOTE: From the v-i characteristics of a 'thyristor', note that a 'thyristor' allows the current to flow only in forward direction i.e., thyristor is an unidirectional conductor.But a type of 'thyristor' named 'TRIAC' is birectional which conducts in 2 directions.

Types of Thyristors: There are a total of 13 types of 'thyristors' which are as follows:
1)Gate Turn Off Thyristors(GTO's)
2)Fast switching thyristors
3)Phase Controlled Thyristor(or SCR's)
4)Light-activated Silicon Controlled Rectifiers(LASCR's)
5)FET-controlled thyristors(FET-CTHs)
6)MOS controlled thyristor(MCTs)
7)MOS-turn off thyristors(MTOs)
8)Birectional triode thyristor(TRIAC)
9)Bidirectional phase-controlled thyristors(BCTs)
10)Emitter turn-off thyristor(ETOs)
11)Integrated gate-commutated thyristor(IGCT)
12)Static Induction thyristor(SITH)
13)Reverse-conducting thyristors(RCTs)

Applications of Thyristor:
'Thyristor' has abundant applications in today's rapidly growing Power Electronics field.It is used in most of the Power circuits.

• Thyristor is widely used as a switch as its ON and OFF conditions can be controlled.
• Thyristors are used in AC voltage controllers which inturn are employed in Induction motors to vary the speed by varying output voltage.
• Thyristors can handle high power.In other words they have good power handling capability because of which they are used to control power in many circuits.
• Commonly thyristors are used in light dimmers,fans etc.

Thyristors are majorly used by soldering them into IC's(Integrated Chips).

How Thyristors are superior to Transistors?
Though 'thyristors' and 'transistors' have their own applications,'thyristors' are more useful in quite a few ways compared to 'transistors'.

• The basic advantage of thyristors is that,unlike transistors, their ON-OFF can be controlled.
• Thyristors have high power handling capability than transistors.

Also there is low power loss in thyristors compared to transistors.

Why thyristor is also called SCR(Silicon controlled Rectifier)?
The reason thyristor was earlier called as SCR is this- 'silicon' simply because it is made up of semi-conductor silicon and it basically acts as rectifier whose operation can be controlled.Hence the name 'Silicon Controlled Rectifier'.

-Pooja.