TRIAC

A TRIAC (triode for alternating current; also bidirectional triode thyristor or bilateral triode thyristor) is a three-terminal electronic component that conducts current in either direction when triggered. The term TRIAC is a genericized trademark.

TRIACs are a subset of thyristors (analogous to a relay in that a small voltage and current can control a much larger voltage and current) and are related to silicon controlled rectifiers (SCRs). TRIACs differ from SCRs in that they allow current flow in both directions, whereas an SCR can only conduct current in a single direction. Most TRIACs can be triggered by applying either a positive or negative voltage to the gate (an SCR requires a positive voltage). Once triggered, SCRs and TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current.

Gate turn-off thyristors (GTOs) are similar to TRIACs but provide more control by turning off when the gate signal ceases.

The bidirectionality of TRIACs makes them convenient switches for alternating-current (AC). In addition, applying a trigger at a controlled phase angle of the AC in the main circuit allows control of the average current flowing into a load (phase control). This is commonly used for controlling the speed of a universal motor, dimming lamps, and controlling electric heaters. TRIACs are bipolar devices.

Operation

To understand how TRIACs work, consider the triggering in each of the four possible combinations of gate and MT2 voltages with respect to MT1. The four separate cases (quadrants) are illustrated in Figure 1. Main Terminal 1 (MT1) and Main Terminal 2 (MT2) are also referred to as Anode 1 (A1) and Anode 2 (A2) respectively.

Quadrant 2

thumb|Figure 5: Operation in quadrant 2

Quadrant 2 operation occurs when the gate is negative and MT2 is positive with respect to MT1.<sup>Figure 1</sup>

Figure 5 shows the triggering process. The turn-on of the device is three-fold and starts when the current from MT1 flows into the gate through the p-n junction under the gate. This switches on a structure composed by an NPN transistor and a PNP transistor, which has the gate as cathode (the turn-on of this structure is indicated by "1" in the figure). As current into the gate increases, the potential of the left side of the p-silicon under the gate rises towards MT1, since the difference in potential between the gate and MT2 tends to lower: this establishes a current between the left side and the right side of the p-silicon (indicated by "2" in the figure), which in turn switches on the NPN transistor under the MT1 terminal and as a consequence also the pnp transistor between MT2 and the right side of the upper p-silicon. So, in the end, the structure which is crossed by the major portion of the current is the same as quadrant-I operation ("3" in Figure 5). already have such a resistor built in to safeguard against spurious dv/dt triggering. This will mask the gate's supposed diode-type behaviour when testing a TRIAC with a multimeter.

In datasheets, the static dv/dt is usually indicated as <math> \left (\frac{\operatorname{d}v}{\operatorname{d}t}\right )_s </math> and, as mentioned before, is in relation to the tendency of a TRIAC to turn on from the off state after a large voltage rate of rise even without applying any current in the gate.

Critical di/dt

A high rate of rise of the current between MT1 and MT2 (in either direction) when the device is turning on can damage or destroy the TRIAC even if the pulse duration is very short. The reason is that during the commutation, the power dissipation is not uniformly distributed across the device. When switching on, the device starts to conduct current before the conduction finishes to spread across the entire junction. The device typically starts to conduct the current imposed by the external circuitry after some nanoseconds or microseconds but the complete switch on of the whole junction takes a much longer time, so too swift a current rise may cause local hot spots that can permanently damage the TRIAC.

In datasheets, this parameter is usually indicated as <math>\frac{\operatorname{d}i}{\operatorname{d}t}</math> and is typically in the order of the tens of ampere per microsecond.

Example data

{| class="wikitable" style=text-align:center

|+ Some typical TRIAC specifications

! Variable <br/>name

! Parameter

! Typical <br/>value

! Unit

| <math>V_\text{gt}</math>

|align=left| Gate threshold voltage

|align=right| 0.7–1.5

| V

| <math>I_\text{gt}</math>

|align=left| Gate threshold current

|align=right|

| mA

| <math>V_\text{drm}</math>

|align=left| Repetitive peak off-state forward voltage

|align=right|

| V

| <math>V_\text{rrm}</math>

|align=left| Repetitive peak off-state reverse voltage

|align=right|

| V

| <math>I_\text{t}</math>

|align=left| RMS on-state current

|align=right|

| A

| <math>I_\text{tsm}</math>

|align=left| On-state current, non-repetitive peak

|align=right|

| A

| <math>V_\text{t}</math>

|align=left| On-state forward voltage

|align=right|

| V

High commutation (two- and three-quadrant) TRIACs

Three-quadrant TRIACs only operate in quadrants 1 through 3 and cannot be triggered in quadrant 4. These devices are made specifically for improved commutation and can often control reactive loads without the use of a snubber circuit.

The first TRIACs of this type were marketed by Thomson Semiconductors (now ST Microelectronics) under the name "Alternistor". Later versions are sold under the trademark "Snubberless" and "ACS" (AC Switch, though this type also incorporates a gate buffer, which further precludes Quadrant I operation). Littelfuse also uses the name "Alternistor". Philips Semiconductors (now NXP Semiconductors) originated the trademark "Hi-Com" (High Commutation).

Often these TRIACs can operate with smaller gate-current to be directly driven by logic level components.

Operation

Quadrant 2

Critical di/dt

Example data

High commutation (two- and three-quadrant) TRIACs

See also

References

Further reading

External links