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Triac triggering: Positive, negative supply basics

Posted: 26 Nov 2014 ?? ?Print Version ?Bookmark and Share

Keywords:power supplies? triggering? AC switch? Triac? ACST?

Non-insulated power supplies are often necessary when a power semiconductor, which has to be controlled using this supply, has its drive reference connected to the mains (line or neutral terminal).

This is the case, for example, for triggering an AC switch such as a Triac, ACST, ACS or SCR (Silicon Controlled Rectifier).These devices are all controlled by a gate current. This gate current has to be applied on the gate pin and circulates between the gate and the AC switch reference terminal, which is the Cathode (K) for SCR, A1 for Triac, or COM for ACST and ACS.

As the AC switch control circuit, and so its power supply, has to be referenced to the device reference terminal (connected back to the line voltage), a non-insulated power supply is required.

There are two ways to connect this drive reference to the non-insulated power supply:

Solution 1: connect the control circuit ground (VSS) to drive reference.

Solution 2: connect the control circuit voltage supply (VDD) to drive reference.

Supply polarity definition

Figure 1: Supply polarity definition.

Solution 1, shown in figure 1a, is the most common solution, as the switch drive reference is also the zero voltage point (VSS). Such a topology is called a positive power supply as the voltage supply (VDD) is indeed above the mains terminal potential (line or neutral), which is connected to the drive reference (VSS).. If the supply is a 5 V power supply, then VDD is 5 V above the mains reference (neutral terminal in Figure 1a example).

This topology can be used directly only with standard Triacs and SCR, not with non-standard Triacs, ACS and ACST, as explained below. But some simple modifications can be implemented to control all these devices from a positive supply as explained at the end of this article.

Solution 2, shown in figure 1b, is called a negative power supply. The voltage supply reference (VSS) is indeed below A1 or COM, which is connected to the mains reference. If the supply is a 5 V power supply, then VSS is 5 V below the line reference, or at C 5V compared to the line.

This topology can be used with all Triacs, ACS and ACST, but not with SCR, as explained below.

Compliance of supply output polarity with AC switch technology
To switch on an AC switch, like any bipolar device, a gate current must be applied between its gate pin (G) and its drive reference terminal (refer also to STMicroelectronics' AN3168 application note).

Then several cases occur.
???For a SCR, this gate current has to be positive (circulating from G to K).
???For a Triac and an ACST, the gate current could be positive or negative (depending on the voltage applied to the device).
???For an ACS, the gate current has to be negative (circulating from COM to G).

An SCR can so be easily driven from a positive supply. If its Cathode is connected to VSS as it is on Figure 1a, a current will be sourced to the SCR gate when the Control Circuit (usually a microcontroller) output pin will be put at high level.

On the other hand, an ACS needs a negative supply to be directly driven as shown in figure 1b. A current will thus be sunk from the SCR gate when the Control Circuit output pin will be put at low level.

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