TRIAC is a short form of TRIode for Alternating Current. The other power electronics switches like IGBT, MOSFET, are used to control the DC power. The TRIAC is a device that operates in the AC application.
A
TRIAC is a bidirectional device belonging to the thyristor family. The TRIAC
allows the current to flow in both directions. Therefore, the TRIAC is turned
ON with the AC power, and it can operate in both directions (positive and
negative).
The SCR is a
unidirectional device. After the SCR, TRIAC is the most widely used device of
the thyristor family. And therefore, TRIAC is available in a wide range of
voltage and current ratings.
Construction of TRIAC
TRIAC is a
three-terminal device. Two terminals are main terminals (MT1 and MT2); one
terminal is a gate terminal. The gate terminal is a control terminal.
The main
terminals are names MT1 and MT2. The MT1 terminal is near the gate terminal.
The TRIAC is a four-layer device
and consists of six doping regions. Gate terminal helps TRIAC to get triggered
in positive and negative directions.
Gate
terminal is connected with N4 and P2 regions by metallic contact. Terminal MT1
is associated with regions P2 and N2. Terminal MT2 is connected with P1 and N4
regions.
Characteristics
of TRIAC
The TRIACs is a bi-directional
device. Therefore, the characteristic of TRIAC will be in the first and third
quadrant. And characteristic of TRIAC is similar to the characteristic of SCR.
In TRIACs, gate current can be
positive or negative. But in the case of SCR, the gate current always positive.
TRIAC characteristics classified into three
regions of operation;
- Blocking state (OFF state)
- Transition state (Unstable state)
- Conduction state (ON state)
When the applied voltage is less than the
breakdown voltage, a small leakage current will flow through the device. When
voltage increases than the breakdown voltage, it will turn ON, and a high
current will flow.
Even if the applied voltage is less than the
breakdown voltage, the TRIAC can be turned ON by the gate pulse.
The
operation of TRIAC is the same in the negative direction. It will create a
mirror image in the third quadrant.
The gate current decides the
supply voltage at which the TRIAC starts conducting. The below figure shows the
characteristics of TRIAC.
Modes
of Operation
If
the applied voltage is more than the breakdown voltage, TRIAC operates in a
conduction state. But TRIACs can be served by giving positive or negative gate
pulse.
The
greater the gate current results small amount of voltage to turn ON the TRIAC.
The TRIAC is a bi-directional device. Therefore, it is possible
to connect a different combination of positive and negative voltages.
Four possible combinations make a different operating mode of
TRIAC, as listed below.
- MT2 is positive with respect to MT1, with gate polarity positive with respect to MT1
- MT2 is positive with respect to MT1 with gate polarity negative with respect to MT1
- MT2 is negative with respect to MT1, with gate polarity negative with respect to MT1
- MT2 is negative with respect to MT1, with gate polarity positive with respect to MT1
