Triode for Alternating Current

Triode for alternating current (TRIAC) is a bidirectional thyristor which is used to control power in ac circuits. While in operation a TRIAC is equivalent to two silicon controlled thyristors (SCRs) connected in anti-parallel. Circuit symbol and static VI characteristics of TRIAC is given in Fig. 1a and Fig. 1b respectively.

• In Fig. 1c cross sectional view of a TRIAC is shown in which Terminal G is connected to N3 and P2, terminal MT1 is connected to P2 and N2, and terminal MT2 is connected to P1 and N4.
• With no signal to gate, the TRIAC will block both half cycles of ac applied voltage in case peak value of this voltage is less than the break-over voltage of VBO1 OR VBO2 of the TRIAC.

Turn-ON process of TRIAC

The turn-ON process of a TRIAC can be explained as under:

1. When MT2 is positive and gate current is also positive with respect to MT1.
• Junction P1N1 and P2N2 are forward biased but junction N1P2 is reverse biased.
• Gate current flows mainly through P2N2 junction.
• When gate current has injected sufficient charge into P2 layer, reverse biased junction N1P2 breaks down.
• As a result, TRIAC starts conducting through P1N1P2N2 layers.
• TRIAC turns on like a conventional thryistor and is more sensitive in this mode.
2. When MT2 is positive but gate current is negative with respect to MT1.
• Gate current flows through P2N3 junction and reverse biased junction N1P2 is forward biased.
• As a result, TRIAC starts conducting through P1N1P2N3 layers initially.
• The device structure P1N1P2N3 may be regarded as the main SCR and the structure P1N1P2N2 as the main SCR.
• The device in this mode is less sensitive and therefore, more gate current is required.
3. When MT2 is negative but gate current is positive with respect to MT1.
• The gate current Ig forward biases P2N2 junction.
• Layer N2 injects electrons into P2 layer.
• As a result, reverse biased junction N1P1 breaks down and the structure P2N1P1N4 is completely ON.
• As the TRIAC is turned ON by remote gate N2, the device is less sensitive in this mode with positive gate current.
4. Both MT2 and gate current are negative.
• N3 acts as a remote gate, the gate current Ig flows from P2 to N3.
• Reverse biased junction N1P1 is broken and finally the structure P2N1P1N4 is turned ON completely.
• The device is more sensitive under this condition.

Note: TRIAC is rarely operated in first quadrant with negative current (as discussed in 2) and in the third quadrant with positive gate current (as discussed in 3).

Reference

1. M. H. Rashid, “Power Electronics: Circuits, Devices and Applications,” Prentice Hall India, Second Edition, 2006.
2. P. S. Bimbhra, "Power Electronics," Khanna Publishers, fifth edition, 2012.