- [Presenter] Do you know that a semiconductor device
called a thyristor solves the huge issue of power transfer
from a generating station to consumers located far away?
Traditional AC power transmissions
face huge power losses and also suffer
from the issue of stability and controllability.
For long-distance power transmission,
HVDC technology is the right choice.
In HVDC, bulk amounts of AC power have to be converted
to DC with the help of converting stations.
After that, the DC power is transmitted to the consumers.
This important task of a conversion is performed by a unique
semiconductor switching device called a thyristor,
more specifically, by silicon-controlled rectifiers.
Let's explore how a thyristor works.
You may have seen different semiconductor
switching devices such as diodes and transistors,
similarly, a thyristor is also a switch.
All of these switching devices are made up
of the well-known semiconducting material of silicon.
A thyristor is made of four alternating layers of n-
and p-regions, to understand why the thyristor is used,
let us look at a normal transistor, a BJT.
When we connect a primary power source, we observe one
of the junctions of the transistor is always reverse-biased.
To turn on the transistor, we just connect
a secondary voltage supply between
the emitter and the base terminal.
This will turn the transistor on.
However, if we remove the secondary voltage supply,
the transistor will turn off as it needs
a continuous secondary voltage supply.
The need for a continuous base current supply
causes a huge power loss,
especially during high-power applications.
To overcome this problem, in 1950 William Shockley proposed
a very interesting power switch, known as a thyristor.
In thyristors, unlike with transistors, no such
continuous secondary supply is needed.
After the triggering, even if you remove
the secondary supply, the thyristor will keep on working.
To understand the workings of a thyristor properly,
first we need to understand what a depletion region is
and the basic workings of a diode.
A pure silicon structure is shown here.
Pure silicon has very low conductivity.
We can increase its conductivity by injecting
n-type or p-type impurities, a process known as doping.
If part of the silicon is doped with p-type
and the other is part with n-type,
we will get a p-n junction, or put simply, a diode.
One interesting phenomenon happens
at the junction of the p-n intersection,
the natural migration of electrons.
This will cause the p-side to be slightly negatively charged
and the n-side slightly positively charged, in short,
a depletion region where there are no free electrons
or holes formed at the p-n junction.
The slight negative and positive charges across
the depletion region will produce an electric field
in between them, this electric field causes
a barrier potential, because of the barrier potential
further natural migration of electrons will not happen.
This p-n junction is nothing but a diode.
To see how it works, let's connect
a forward-voltage supply to the diode,
with a voltage value greater than the barrier potential.
You can see that the electrons
will be pushed away by the negative terminal,
and they will cross the p-n junction.
After crossing, they will occupy
the holes available in the p-region.
Due to the attraction of the n-region,
these electrons will jump to the nearby holes
and the flow will continue.
Here, the diode is working in a forward-biased condition.
However, if we reverse the supply voltage, the electrons
and holes will simply move away and the diode will not work.
In the p-layer, holes are the majority charger carriers.
However, it should be noted that there are
a few electrons in the p-region as well.
We call them minority carriers.
It is the same case with the n-region.
With this basic knowledge, let us learn
about the workings of the thyristor.
If a silicon structure wafer is doped with four
alternate forms of p- and n-types, a thyristor is born.
Here also, the formation of depletion regions
occurs at the junctions, whichever way you apply
a voltage in a thyristor there will be always
at least one reverse-biased junction.
In the second case, there is
only one reverse-biased junction.
Let's try to make a working
thyristor from this configuration.
In order to make the thyristor conduct,
we have to break this depletion region.
In thyristors, an efficient and popular method
called gate triggering is used for this.
Gate triggering is the process
of the injection of electrons.
For this, let's connect the secondary voltage supply
to the gate and cathode terminal.
This secondary supply injects a lot
of electrons into the p-region.
As this process continues the p-region
becomes over-flooded with the electrons.
The electrons have now become
majority charged carriers in this region.
In short, the p-region eventually becomes an n-region.
This new n-region will cause
the depletion region to automatically diminish.
As the p-region has become a new n-region,
due to the gate triggering, the three regions
on the bottom side, collectively become a big n-region.
Now, the structure of a thyristor
looks like a p-n junction diode.
As we have seen earlier, when we apply
a forward-biased voltage supply
to the p-n junction diode, it starts conducting.
At this stage, even if you remove the secondary
voltage supply, the thyristor will keep on working,
since the injected electrons in the p-region
have already made it into an n-region.
This way in a thyristor, the secondary supply voltage
is needed only for the triggering.
Now, let's see how we can turn off a thyristor.
The only way to turn off a thyristor
is by applying a reverse voltage across it.
The most efficient way to achieve this
is by the use of an LC oscillator.
In an LC oscillator an energy exchange happens
between a capacitor and an inductor.
You can see a fluctuating electron flow occurs
in the circuit, this means the voltage
to the circuit will also oscillate as shown.
Assume the peak voltage of the LC circuit is more than
the voltage applied across the thyristor.
If we insert the thyristor circuit into this LC circuit,
the thyristor will be subjected
to fluctuating voltage instead of a steady voltage.
In the reverse-biased voltage mode,
the thyristor will definitely turn off.
Without the need for secondary power, thyristors help
HDVC technology to save a huge amount of electric power.
We hope this video gave you a good insight
about the working of thyristors.
Thank you.