How Triac Works and its Applications
The word TRIAC can be expanded as a TRIode for Alternating Current. While other power electronic switches like MOSFET, IGBT, etc are used for switching/controlling DC power, the TRIAC is used to control AC power because once turned on TRIAC can conduct in both the direction allowing AC voltage to passes completely in both the positive and negative cycle.
The TRIAC is a three-terminal semiconductor switching device that is used for controlling current flow in a circuit. It is one of the most important members of the thyristor family; it is a bidirectional device that can pass the current in both forward and reverse direction, which means that they can conduct in both the conditions of the gate signal, positive and negative.
TRIAC Construction
The below shows the structure of the TRIAC, it is a four-layer device that consists of six doping regions. The gate terminal is designed in a way to have ohmic contact with both N and P regions, which helps the device to get triggered with both positive and negative polarities.
Although TRIAC is a bidirectional device, everyone prefers to specify voltage and current using MT1 as the reference in order to reduce confusion.
TRIAC Working Principle and Operation:
TRIAC can go to conduction state if the applied voltage is equal to the breakdown voltage, but the most preferred way of turning on a TRIAC is by providing a gate pulse, either positive or negative. If the gate current is high, a very small amount of voltage is enough to turn on the TRIAC. As the TRIAC is bidirectional and has an ability to get turned on with both the polarities to the gate pulse it can operate in four different types of modes of operation as listed below
1. MT2 is positive with respect to MT1 with a gate polarity positive with respect to MT1.
2. MT2 is positive with respect to MT1 with a gate polarity negative with respect to MT1.
3. MT2 is negative with respect to MT1 with a gate polarity negative with respect to MT1.
4. MT2 is negative with respect to MT1 with a gate polarity positive with respect to MT1.
5. MT2 is positive with respect to MT1 with a gate polarity positive with respect to MT1
When the terminal MT2 is positive with respect to the Terminal MT1 the current will be flowing in the path of P1-N1-P2-N2. During this operation, the junction between the layers P1-N1 and P2-N2 are forward biased whereas the Junction between N1-P2 is reverse biased. When the positive signal is applied to the gate the junction between P2-N2 is forward biased and breakdown occurs.
When the MT2 is positive and the gate pulse is negative, the current flow will be in the same path as the first mode which is P1-N1-P2-N2, but here the junction between the P2-N2 is forward biased and the current carriers are injected into the P2 layer.
When the terminal MT2 is positive and negative pulse is provided to the gate terminal the current will be flowing in the path of P2-N1-P2-N2. During the operation the junction between the layers P2-N1 and P1-N4 are forward biased whereas the junction between the layers N1-P1 is reverse biased, hence the TRIAC is said to operate in the negatively biased region.
MT2 is negative with respect to MT1 with a gate polarity positive with respect to MT1
When the terminal MT2 is negative and the gate is triggered with a positive pulse the junction between P2-N2 is forward biased and the current carriers are injected, hence the TRIAC is turned on.
The TRIAC doesn’t usually operate in mode 4 because it carries a disadvantage that it should not be used for circuits with high di/dt. The sensitivity of triggering of TRIAC with mode 2 and 3 is high and negative gate pulse is used in case of a marginal triggering capability. The triggering of mode 1 is even more sensitive than mode 2 and 3 triggering but it requires a positive gate pulse for triggering. In most of the cases, the triggering mode 2 and 3 are preferred.
VI characteristics of TRIAC
Since the TRIAC is a bidirectional device it’s the VI characteristics curve of TRIAC will be on the first and third quadrant of the graph, which is similar to the VI characteristics of a Thyristor. If you are completely news to Thyristors like SCR you can check the Introduction to SCR article. When the terminal MT2 is set to be positive with respect to the terminal MT1 the TRIAC will be operating in the forward blocking mode.
During the initial stage due to the resistance of the TRIAC, there will be a small leakage current flowing through the device as the applied voltage is less than the breakdown voltage. When the voltage is increased and it reaches the breakdown voltage the TRIAC is turned on and high current starts flowing through the device.
TRIAC Application:
As mentioned earlier, TRIACs are commonly used to switch AC voltages. A sample application TRIAC circuit for AC switching is shown below.
TRIACs can be used in various applications such as
• Control circuits like electric fan speed control and smaller motor controls
• High Power lamp switching and light dimmers
• AC power control domestic appliances
Different types of TRIAC Packages
Pin/standard Type: The Standard Type TRIACs will be looking like a small IC with three Pins which are the MT1, MT2 and gate (G) and a heat sink on the top. These types of TRIACS are used in Domestic electronic appliances. Some of the common packages are TMA36S-L, TMA54S-L, TMA84S-L, TMA124S-L, TMA126S-L, TMA206S-L, TMA106S-L, etc.