All About Thyristors--Power Electronics

Basic Symbol for Thyristor.

Thyristor is a four Layer and three Junction Silicon Controlled Device. After it is turned on it acts as normal diode, Thyristor has two stable Swtching states namely:

i- ON State

As you can see it is made from connecting to 
PN Junction or it can
Also be made from a PNP and A NPN transistor

ii- OFF State


Types of Thyristor: (In terms of Direction):

There are two types of Thyristors in terms of Direction of current flowing through them

1. Unidirectional
2. Bidirectional (TRIAC)

Unidirectional Thyristors are Further Divided into two types:

1. SCR (Silicon Controlled Rectifier)
2. LASCR (Light activated Silicon Controlled Rectifier)

We will only be discussing SCRs and their Turn ON and Turn OFF Techniques in this post.

Sillicon Controlled Rectifier:

SCR is Three Terminal device namely Anode, Cathode and Gate. When Anode voltage is More Positive with respect to  Cathode then J1 and J3 Junctions will be forward Biased where J2 will be Reverse Biased.

There will be slight Leakage Current, This stage of SCR is called Forward Blocking State or Leakage State.

If Anode to Cathode voltage is increased significantly to Larger value J2 will break down and Current will start flowing, This State of SCR is called Forward Breakdown State.

On Negative x-axis is called Reverse Characteristics
Positive X-axis are Forward Characteristics

As we can see the V I Characteristics of Thyristor
Vbo is Forward Breakdown Voltage
I'h= Holding Current
I L= Latching current


The Anode Current must be more than the value of I L i.e Latching current otherwise device will revert to Blocking Condition, Once Thyristor starts conducting it behaves like a P-N Junction then there will be no control over the device, it will continue to conduct because there is no J2 junction, however if forward anode current is reduced below the value of Holding current than J2 will devolp again and thyristor will be back in blocking mode.

Since a thyristor can be turn by increasing forward voltage beyond Vbo , but this type of turning on would be destructive, In practice Forward voltage is not given above Vbo and is turned on by applying a positive voltage between gate and cathode.



Thyristors Turn ON Techniques:

There are following techniques through which thyristor can be turned on.

1. Applying Gate voltages
2. Anode Voltage > Vbo
3. Thermal Energy
4. Large dV/dt.
5. Optically( Specifically in LASCR)

Large dV/dt:

Large dV/dt will cause very High Displacement Current from Junction as we know they act as Capacitor.

Commutation(Turning OFF):

It is a process of turning off a thyristor and it is normally done in such a way that Load Current transfers to another part of circuit.

Types of Commutation:

There are many types of commutation it is broadly classified into two types

1. Natural Commutaion
2. Forced commutation

               Forced is further divided into two types

•                Resonant
•                Impulse Commutation

1- Natural Commutation:

Natural commutaion is process in which device turn off itself by natural means like when Negative peak of AC and Thyristor is reverse biased then there is no current flowing in circuit so thyristor is turned off  and we have to turn it on again by firing at Gate.

2- Forced Commutation:

In some Application the input voltage is DC and hence there is no Zero Crossing of the current is possible. In these types of circuits the forward current of thyristor is forced to zero by an additional circuitry , forced commutation can be acheived by following methods:


Resonant/ Self Commutation:

In this circuit value of L and C must be chosen such that we acheive an underdamped circuit, due to this underdamped circuit the oscillating current will start flowing and SCR will be turned off at zero Crossing.



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