ALL ABOUT THYRISTORS-1

ALL ABOUT THYRISTORS-1

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'.

There are two ways of learning anything. First one: 'Thyristor' is a 4 layer power semiconductor device having 3 terminals namely anode,cathode & gate.And it has a PNPN structure i.e., it has 3 p-n junctions.There are 13 types of Thyristor.... This is certainly what is given in the textbooks you refer or your lecturers expect you to write in examination! Well,this is one way of learning and this doesn't seem like learning to me. One can possibly learn anything only if he/she can understand particular topic.
Now, I am going the second way which is easier. I have tried to provide a step by step explaination of 'thyristors' followed by its vast applications(in part 2).

HISTORY: The cocept of thyristor was first proposed by William Shockley in the year 1950. With the rapid increase in Power Electronics, commercial thyristors were soon introduced in 1950's.Then,it was popularly called as 'silicon controlled rectifier(SCR)'.The reason for this is in the later part of the post(in part 2).

Why the name 'thyristor'?: 'Thyristor' is a combination of two words- 'Thyratron' & 'transistor'. A 'transistor' is again a combination of two words- transfer and resistance. 'Thyratron' is a type of gas filled tube. It is basically a controlled gas rectifier.Thus 'thyristors' have applications of thyratron in addition to what transistors have.

Construction: 'Thyristors' are manufactured by the process of diffusion. It has three p-n junctions forming four layers. Hence it is said to have PNPN structure. It has three terminals namely anode,cathode and gate.It acts as a bistable switch i.e., a 'thyristor' can be either in conducting state or non-conducting state.Since its ON & OFF conditions can be controlled, it can be used as a switch. The symbolic representation and view of p-n junctions is shown below:

  

        

Operation: It can be observed in the figure that 'thyristor' has 3 terminals and 3 p-n junctions. Once anode terminal is made more positive compared to cathode terminal,junctions J1 and J3 will be forward biased.Whereas junction J2 will be reverse biased. As a result,only small amount of current i.e., leakage current flows.Under this condition,'thyristor' is said to be in "forward blocking" or "off state" condition. In order to bring the 'thyristor' in ON state, anode to cathode voltage needs to be increased. When we keep increasing the anode-cathode voltage, at particular value of anode-cathode voltage 'avalanche breakdown' occurs. (Avanlanche Breakdown: Eventhough reverse current does not depend on reverse voltage, if reverse voltage is increased to sufficiently large value,velocity of minority carriers increases.Due to kinetic energy associated with minority carriers,some more of them get generated on colliding with other atoms.In this way carrier multiplication takes place and minority carriers get accelerated due to high reverse voltage.Thus a large reverse current flows. This effect is called 'Avalanche effect').Here since junction J2 is reverse biased,anode to cathode voltage acts as reverse voltage to J2.Hence junction J2 breaks when anode-cathode voltage is increased to particular value. Now the carriers can move freely in all the 3 junctions and large anode current will flow. In this condition,'thyristor' is said to be in "on state" or "conducting state". The value of anode-cathode voltage at which avalanche breakdown occurs is called "forward breakdown voltage". The operation of a 'thyristor' can be understood by looking at its v-i characteristics.

It can be analysed from the v-i characteristics that when anode-cathode voltage is below 'forward breakdown voltage',only a small leakage current flows. And 'thyristor' enters to conducting mode once anode-cathode voltage reaches 'breakdown voltage'.

'Latching current' and 'Holding current' are the two new terms that can be observed in the figure above.

Latching current(IL): A minimum amount of current is required to bring the 'thyristor' from non-conducting state to conducting state. This current is defined as Latching current.

Holding current(IH): Once the 'thyristor' is in conducting mode, if the forward current decreases below certain value of current called 'holding current', then the 'thyristor' stops conducting.In other words, 'Holding current' is the minimum current required to maintain the 'thyristor' in ON state condition.

Many of them confuse the 'latching current' with the 'holding current'. But its very important ot know the difference between these two.


-Pooja.