- [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.
