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Structure, Working Principle, Packaging Form And Application Characteristics Of TRIAC

Structure, Working Principle, Packaging Form And Application Characteristics Of TRIAC

October 23 , 2023 0

A three-terminal bidirectional thyristor, often referred to as a triac, is a semiconductor device that consists of three main terminals: the main terminal 1 (MT1), the main terminal 2 (MT2), and the gate terminal (G). Unlike the standard thyristor, which has only two terminals, the triac allows for bidirectional current flow by utilizing two thyristor structures connected in parallel. The MT1 and MT2 terminals serve as the main current-carrying terminals, while the gate terminal controls the triggering and conduction of the device. This three-terminal configuration enables the bidirectional thyristor to control AC power in both positive and negative half-cycles, making it suitable for various applications requiring bi-directional power control, such as motor speed control, lighting dimming, and AC power switching. The gate terminal receives a triggering signal to initiate conduction, and once triggered, the triac remains conducting until the current falls below a certain threshold or the polarity of the applied voltage changes.

What Is TriAC?

End-directional thyristor, or AC triode, is a semiconductor device widely used in power control and switching applications. It is in switching, phase control, chopper design, lamp brightness control. Fan, motor speed control and other aspects have applications. The power control system is designed to control the distribution level of AC or DC. This power control system can be used to switch the power supply of an appliance manually or when the temperature or light level exceeds a preset level.
The end bidirectional thyristor is equivalent to two SCRS in reverse parallel, whose gates are connected together. Thus, once the gate is triggered, it acts as a bidirectional switch to transfer current in both directions. It is a three-terminal device, primary terminal 1 (MT1), primary terminal 2(MT2), and Gate. The MT1 and MT2 terminals are used to connect the phase and neutral lines, while the Gate is used to feed the trigger pulse, and the gate can be triggered by positive or negative voltage
When the MT2 terminal gets a positive voltage relative to the MT1 terminal and the Gate gets a positive trigger, the SCR on the left side of the TRIAC is completed. However, if the MT2 and MT1 terminal voltage is reversed and a negative pulse is applied to the gate, the right SCR of the three-terminal bidirectional thyristor switch is switched on. When the grid current is removed, the TRIAC turns off. Therefore, a minimum holding current must be maintained at the gate to keep the TRIAC on.

Basic Structure

The structure of the end-bidirectional thyristor is shown in the figure below, which includes four layers and six doping regions. The design of its gate terminals can be accomplished by using ohmic contacts in two regions, the P-zone and the N-zone, so that the device can be activated by both polarity. Although it is a bidirectional device, its current and voltage can be specified with MT1

In the case of SCR, the terminals of the TRIAC can be represented by MT1 and MT2, just like the anode and cathode, and the gate terminals can be represented by a "G", just like a thyristor. The gate terminal "G" is connected to the P2 and N4 areas via metal contacts and is close to the MT1 terminal
MT1 can connect to regions of P2 and N2, while MT2 can connect to regions of P1 and N3. Thus, two terminals like MT1 and MT2 are connected to the P and N regions of the device. Therefore, the current between these two terminals can be determined by the layers in the device
In contrast to MT1 of the forward-biased connected TRIAC, MT2 is connected to the positive terminal by gate opening. Therefore, the TRIAC operates in forward block mode until the voltage at both ends of the TRIAC is lower than the forward disconnect overvoltage. Similarly, when the TRIA is reversely biased with respect to the MT1 terminal when opened through the gate, the MT2 terminal becomes negative, then the device operates in reverse block mode. The TRIAC can be switched on by a +ve or -ve voltage on the gate terminal.

Operating Process

When the applied voltage in the TRIAC is equal to the breakdown voltage, the TRIAC will enter the on-state. However, the most common way to turn on TRIAC is to provide a positive or negative grid signal
If the current at the gate terminal is higher, less voltage is required to open the TRIAC, and it is able to open to the gate signal through both polarity. TRIAC works in the following four modes:
By positive polarity with respect to the MT1 terminal, the MT2 terminal is +ve with respect to the MT1 terminal
With a negative gate polarity relative to MT1, the MT2 terminal is +ve relative to the MT1 terminal.
By negative gate polarity with respect to the MT1 terminal, the MT2 terminal is negative with respect to the MT1 terminal.
The MT2 terminal is negative with respect to the MT1 terminal, through a positive gate polarity with respect to the MT1 terminal.

Mode 1

In this mode, once the MT2 terminal is +ve with respect to the MT1 terminal, the current will flow in the direction of P1-N1-P2-N2. Throughout the process, the junctions between layers such as P1-N1 and P2-N2 are connected with forward bias, while the junctions between N1-P2 are connected with reverse bias. Once the +ve signal is provided to the gate terminal, the junction between P2-N2 is connected with a forward bias and breakdown occurs.

Mode 2

If the -MT2 terminal is +ve and the gate signal is -ve, the flow of current will be similar to the first mode of P1-N1-P2-N2, but here the junction between P2-N2 can be connected in a 1-direction bias, and current carriers are added to the P2 layer

Mode 3

- And the MT2 terminals are +ve and -VE signals, which can be supplied to the gate terminals, and then the current will flow in the direction of P2-N1-P2-N2. In the whole process, P2-N1 and P1.
The junctions between layers such as N4 are connected with a forward bias, while the junctions between layers N1 and P1 are connected with a reverse bias. Therefore, the TRIAC will operate in the negative bias region.

Mode 4

If the MT2 terminal is -ve and the gate terminal is activated by a positive signal, the junction between P2-N2 is connected with a forward bias and current carriers are added, so the TRIAC is switched on. Normally, TRIAC does not work in this mode, as it should not be used in high di/dt circuits.

Note that the sensitivity of using modes 2 and 3 to trigger TRIAC is high. A negative grid signal can be used when the activation capacity is small. The activation of mode 1 is more sensitive compared to other modes such as 2 and 3, but it uses the +ve gate signal for activation. The most common patterns are 2 and 3.

Working Principle

The following diagram shows a simple TRIAC application circuit. Typically, the TRIAC has three terminals, M1, M2, and grid. TRIAC, lamp load and power supply voltage in series. When the power supply is turned on during the positive cycle, the current flows through the lamp, resistor and bidirectional trigger diode DIAC(if a trigger pulse is provided at pin 1 of the photosynthesator, causing pins 4 and 6 to start conducting) gate and reaches the power supply, and then only the lamp emits light for this half cycle directly through the M2 and M1 terminals of the TRIAC.

The TRIAC waveform is shown as follows:

In the negative half cycle, the same thing will repeat. Thus, the lamp emits light in a controlled manner for two cycles, depending on the trigger pulse at the optical isolator, as shown in the image below. If it is provided to a motor instead of a lamp, the power is controlled and thus the speed is controlled.

TRIAC Package Form

For ease of use and different applications, TRIAC is designed in different packages such as needle/standard, glue/disc and stud.

Pin/ Standard Type

The TRIAC looks like a miniature integrated circuit, with three terminals - MT1, MT2 and Gate - and a heat sink on top. These TRIacs are mainly used in household appliances. Standard SCR commonly used packages are TMA36S-L, TMA54S-L, TMA124S-L, TMA84S-L, TMA126S-L, TMA106S-L, TMA206S-- and so on.

Gum/disc Type

The capsule or disc TRIAC will be in the shape of a disc through a wire extending towards the terminal. These types of TRIAC have a high current capacity and are designed through ceramic seals.
Applications of this type include fast control of motors as well as AC switches. Common capsule type packages are KS200A, KS100A, KS500A, KS300A, KS600A.
KS1000A, and KS800A.

Stud Type

Stud type Triacs are primarily used for high-power applications because they include a threaded bottom that works like the main terminal, and on top of it include two terminals, which are the other main terminal and the gate terminal.
These are mainly used in phase control applications such as lighting circuits, converters, RPS, speed control of circuits, and temperature. The stud TRIAC package includes TO-93 and T0.
118, TO-94, TO-48, TO-48, RSD7, and TO-65.