Introduction to Thyristors

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Introduction to Thyristors
What is a Thyristor?
Thyristor is a family of semiconductor devices. These includes the SCR, Triac, Diac,
MCT, IGCT PUT, RCT, GTO among others. In basic terms, it is a controlled
electronic switch.
The word comes from the its gas tube equivalent, the “thyratron”. It is a four
layered device with three terminals (particularly the SCR and Triac).
The oldest member of the thyristor family is the SCR (Silicon Controlled Rectifier)
and has become synonymous with the word thyristor because of its vast use.
Compares to transistors, thyristors have lower on-state conduction losses and
higher power handling capability. On the other hand, transistor generally have
superior switching performances in terms of faster switching speed and lower
switching losses.

Applications
A Thyristor is typically used for power control or power disruption system crowbar
circuits that are designed to remove a load from a malfunctioning power supply.
It may also be used in power control to vary the power going to a load

Switching Characteristics (for an SCR)

The thyristor has 3 main terminals: Anode, Cathode, and the Gate Terminal.
Trigger signals are applied at the gate terminal to turn it on or off.

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 The SCR conducts in first quadrant only. When a trigerring signal is applied at the
gate terminal and the SCR is made sufficiently forward biased to cross holding
current limit, the SCR is now able to conduct. Even if the triggering signal is
removed the SCR is still able to conduct.
Special techniques are requires to turn the SCR off, which is known as
communtation.

Semiconductor Model

A Thyristor is actually a four layered PN-Junction device with two P and two N
junctions. Without the application of any voltage, by default it has three diffusion
regions. When we apply positive voltage to the anode with respect to the cathode

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 the junctions j1 and j3 become forward biased, while the middle junction j2 is
reversed biased.
When a positive signal is applied to the gate terminal, j2 becomes forward-biased
allowing current to flow. On the removal of the positive gate signal, the current
continues to flow as charge is drifted from anode to cathode.

Two Transistor Model

A Thyristor is actually made up of a PNP and an NPN transistor wherein the

collector of the PNP transistor is connected to the base of the NPN transistor. The
gate is connected to the base of the NPN transistor. On the application of signal at
the gate terminal, a signal is sent to the base of the NPN transistor which in turn
conducts and send another signal to the PNP trannsistor

Type of Thyristors
Shockley Diode (Silicon Unilateral Switch)

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 The first transistor (Q1) is a PNP structure while the second transistor (Q2) is a
NPN transistor. The collector of Q1 is connected to the base of Q2 while the
collector of Q2 is connected to the base of Q1.

When insufficient voltage is applied, the device is operating in the blocking region
until a sufficient current goes through the diode and it switches over. Since
everything is blocked, there is no substantial current flow.

A forward-bias is observed at Q2 between the emitter and the base, as well as at

the emitter and base of Q2. Little current flow is caused by the forward-biasing of
these two junctions (j1 and j3).

When a voltage gets more positive at the anode terminal or more negative at the
cathode terminal, the device switches into full conduction mode as the current gets
to a high enough value. This current is referred to as the "switching current" denoted
by Is.
When the diode switches into full conduction and goes up slightly more, it turns
into a "holding current" denoted by Ih where the diode switches on completely and
the two forward-biased junctions are now in saturation.
Since the two transistors are operating in saturation, the voltage drop across
the entire Shockley is going to be less than 1 volt. Also, because of this very
low voltage drop, power dissipation will also be very small, which makes it
suitable for high power applications.

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 The voltage wherein the device goes into conduction is calles as the "break over
voltage" denoted as Vbr. On a Shockley diode, this is a fixed value.

When the device is operating in the blocking region, it acts as an open switch
wherein no current can flow. Once we get past the blocking region and the current
increases into the holding current values, the device now acts as a closed switch.
In AC operation, the device will only produce an output during the positive half of
the waveform after the break over voltage is reached. After the break over voltage is
reached more current will go to the output. In the negative half cycle of the
waveform, the device acts as an open switch since the pn junctions will be reverse-
based. In summary, the device acts as a half-wave rectifier with a break over voltage
that eliminates part of the positive waveform.

The Shockley diode is usually used in conjunction with a triac as it triggers the triac
device.

SCR

The SCR is considered to be an improved version of the Shockley diode. The main
difference between the Shockley diode and the SCR is that the SCR now has a gate
terminal which is attached to the p-type material between what was Q1 and Q2.

When there is no voltage at the gate terminal, the SCR is operating in the blocking
region until the current builds up until it reaches the switching current region.
Over time, as the current builds up a little more, it will reach the holding current
region, and the SCR will now be operating in full-conduction mode.

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 💡 The advantage of the gate terminal is that we can start adding current to
the device using the gate terminal, causing the device to reach the break
over voltage earlier.

Because of the gate terminal, it makes it possible to change the break over voltage
at different points by changing the current that will go through the device, unlike
in the Shockley diode where we have to wait for the current to build up naturally.

While in full conduction mode, the devices are operating in saturation, the voltage
drop would be roughly equal to less than or equal to one.

The AC operation for the SCR is similar to that of the Shockley. The only difference
is that since the SCR contains a gate terminal, the break over voltage may be
changed, and it can occur earlier or later depending on the specification, design, or
the usage of the device.
The SCR may be used to control the phase of a circuit with the help of the gate
terminal. This ability is called phase control. This works by causing the break over
voltage to be triggered at different phases.

Like the Shockley diode, this device will not turn off once it meets the break over
voltage. There are two ways to turn off the SCR.

1. Anode Current Interruption: Simply means putting a switch on the

anode, and when you need to turn off the device, or decrease the current
below the holding current threshold, this switch will be turned on, which
will shut the SCR off.

2. Forced Commutation: This means reversing the current path by adding a

capacitor, resistor, and a switch in parallel to the SCR. When the switch is
open, it charges up the capacitor. As the switch closes, the positive voltage
from the capacitor is connected to the cathode while the negative voltage is
connected to the anode, dropping the current through the SCR and turning
it off.

DIAC (DIode for Alternating Current)

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 Similar to a Shockley Diode (SUS) in a way that it will only switch at a fixed break
over voltage, and it has no gate terminal.
The main advantage of the DIAC is that since it is optimized for AC, we can now
use both parts of the AC waveform.

Looking at the structure, both anode 1 and 2 are sharing a connection with an n
type material and a p type material
Basically a DIAC is nothing more than two Shockley diodes placed in parallel, one
for the positive alternation and the other for the negative alternation.

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 Device Configuration

The DIAC functions like a regular Shockley

TRIAC (TRIode for Alternating Current)

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 A Triac is essentially two SCRs that are constructed in a complementary
configuration where the gates are joined. This is commonly used for best control of
an AC waveform as far as power control is concerned.
This type of configuration is often preferable to using just a single triac device.

In a single triac device the waves may not be symmetrical because of the difference
between the break over voltages of the two areas. This is solved by making use of 2
SCRs instead of one triac device because you can adjust the break over voltage of
the individual SCR.

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