Why Your Triac Won't Turn Off
Let’s set the scene: you apply a signal to your triac’s gate, and the triac toggles into its conducting state. Suddenly, when you try to toggle the device off, the device will not switch states, and it remains in the conducting state with power applied. What can be done to fully modulate the device and switch it off?
The internal structure of triacs creates a certain defect where remaining minority charge carriers can persist in the internal layers of the device, and this can cause the triac to latch into the ON state. However, with the proper switching approach between the in/out terminals and the gate terminal, it’s possible to ensure the triac does not latch and can recover into an OFF state.
Why a Triac Conducts
Triacs are a simple type of electrically self-modulating switch with a latching function. The latching function arises once the triac is switched on, and the latching function may not switch off even if the gate terminal is reverted to zero current. As long as the current flowing across the terminals is larger than a particular threshold, known as the latching current, then the device will continue to conduct even if the gate terminal is set to zero input or set to float.
In short, this means we can see a few instances where the triac fails to totally turn off, even if the gate terminal bias has been removed:
- The triac continues to have DC bias after the gate terminal bias is removed
- There is some phase difference between current and voltage (reactive load)
- The edge rate of the switched AC signal is too fast
The typical structure of a triac is shown in the layer diagram below. In this diagram, stacked n-type and p-type layers are stacked in reverse order (4 layers in total) as you would see in a pair of thyristors. The gate terminal is used to drive the triac into its conduction mode, and the main terminals (MT1 and MT2) are the input/output terminals for the switched signal.
Triac circuit symbol and structure.
It is sometimes said that a triac cannot work with DC current, but this is not completely true. If the gate bias is used to drive the triac into conduction, then the triac will pass a DC current. However, if the gate bias is removed, the triac will continue to pass the same DC current as long as the DC current is greater than the triac’s latching current. The triac will continue conducting until the DC current is turned off, at which time the triac will return to its insulating state.
AC with DC Offset
If the triac remains conducting with this type of signal, this is essentially the same thing that can happen in the DC bias case. If the DC bias is larger than the AC amplitude, and both are larger than the triac’s DC latching current, then the triac will remain conducting even if the AC component of the signal is eliminated. This is not a typical case that would be experienced in a typical triac application, where line voltages are involved and there is no offset. However, in specialty applications where an arbitrary waveform is conducted through a triac, the DC bias needs to be considered.
AC With Reactive Load
In the case of a reactive load, or technically any driven circuit with reactive input impedance, the current and voltage across the triac can be out of phase. It is possible that the driving voltage across the triac terminals (MT1 and MT2) is too fast for the triac to fully modulate off, even if the gate bias is removed.
Suppose we have a reactive load being driven through a triac with an AC signal. If the gate is not fed then the triac will attempt to turn off as the current drops to zero. However, the voltage signal will still be oscillating, and the reactive load can still discharge or induce a current into the triac. If the voltage drop rating across the triac changes too quickly (called the commutating voltage, or dV/dt), then the device will not turn off. This is related to reverse recovery in a diode, where a sufficiently fast signal can keep the rectifier forward biased even if the instantaneous voltage momentarily drives the diode into reverse bias.
Other Electrically Controlled Switching Elements
Triacs are very common as electrically modulated switching elements, and they are commonly seen in motor control and appliances operating with input line voltages. Their small size and simple switching rules with AC circuits make them very useful compared to electromechanical switching elements. However, there are other options for electrical, mechanical, or electromechanical switching in electronics systems, including:
- Silicon controlled rectifier (SCR)
- Analog switches/multiplexers
When you’re ready to build switching circuits with components like triacs, you can design and simulate your circuits with the simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.
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