AN3168

Triggering circuits

Doc ID 17193 Rev 1

8/20

Figure 7.

Gate current amplification with positive supply topology

With the bipolar solution, to keep the current sourced to the gate, the only way is then to use

a PNP transistor. A PNP transistor has to be used to set its drive reference to a stable bias,

This is another drawback of the positive power supply topology. To amplify the control circuit

output current, a PNP transistor has to be used instead of an NPN transistor. And a PNP

transistor has a lower current gain and a higher price than an NPN one.

2.4

Triac and ACS / ACST triggering circuit with negative power

supply

Such a topology is the preferred one. The gate current is sunk from the gate by the control

circuit. The device then works in Q2 and Q3 quadrants. Such a topology is adapted for all

devices: standard, snubberless or logic level Triacs, and ACS or ACST.

For SCRs, the gate circuit has to be modified to source the current to the gate, as explained

in Section 2.5.

It should be noted that another advantage with this topology is that the control circuit output

current can be easily amplified if needed (see Section 2.3) with an NPN transistor as shown

in Figure 8.

Figure 8.

Gate current amplification with negative supply topology

CONTROL

CIRCUIT

+V

DD

V

SS

Vac

LOAD

T

CONTROL

CIRCUIT

+V

DD

I

G

V

SS

Vac

LOAD

T

Vac

I

G

LOAD

CONTROL

CIRCUIT

+V

DD

V

SS

LOAD

CONTROL

CIRCUIT