AN3168 datasheet(6/20 Pages) STMICROELECTRONICS

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AN3168

Triggering circuits

Doc ID 17193 Rev 1

6/20

Triggering circuits

2.1

Two kind of power supply bias

To trigger a Triac, ACST, ACS or SCR, a gate current has to be applied on the gate pin and

circulates between gate and cathode (K) for SCR, or between gate and terminal A1 for Triac,

or between gate and terminal COM for ACST and ACS.

For non-insulated control circuits, this means that the reference of the control circuit has to

be related to K, A1 or COM. Then there are two ways to connect this drive reference.

●

Solution 1: connect the control circuit ground (VSS) to K or A1

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Solution 2: connect the control circuit voltage supply (VDD) to A1 or COM

Solution 1 is called a positive power supply. The voltage supply VDD is indeed above the

drive reference (VSS) which is connected to the mains terminal (line or neutral) as shown in

Figure 5. If the supply is a 5 V power supply, then VDD is 5 V above the mains reference.

Figure 5.

SCR / Triac control with positive power supply

Solution 2 is called a negative power supply. The voltage supply reference (VSS) is indeed

below A1 or COM, which is connected to the mains reference (line or neutral) as shown in

Figure 6. If the supply is a 5 V power supply, then VSS is 5 V below the line reference.

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

Figure 6.

Triac and ACS / ACST control with negative power supply

2.2

Gate resistor value definition

The minimum gate current (IGT) required to trigger a Triac, SCR or ACS / ACST increases

as the junction temperature (Tj) decreases (see Figure 4). The worst case appears when Tj

equals the minimum ambient temperature. For appliance systems, the minimum ambient

temperature is generally 0 °C.

For example, the ACS108-6Tx IGT level is given as lower than 10 mA with Tj equals 25 °C.

When Tj equals 0 °C, IGT becomes 15 mA.

Vac

CONTROL

CIRCUIT

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Vac

CONTROL

CIRCUIT

LOAD

CONTROL

CIRCUIT

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CONTROL

CIRCUIT

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CONTROL

CIRCUIT

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CONTROL

CIRCUIT

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Vac

CONTROL

CIRCUIT

LOAD

A1 or COM

CONTROL

CIRCUIT

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