AN3168

Triggering circuits

Doc ID 17193 Rev 1

7/20

In the following, we assume that the device gate is directly connected to a microcontroller

This means that the minimum supply voltage and the maximum voltage drop of the gate

The actual resistance value also depends on its tolerance. Typically, 1% precision resistors

in the current limitation.

Equation 1

50

Ω (ex: 1.5 V for 30 mA for an 85 °C junction temperature).

Equation 2

The normalized value closest to 166

Ω is 165 Ω (1% precision resistor).

2.3

SCR and Triac triggering circuit with a positive power supply

With positive power supplies, the gate current can be only sourced from the control circuit to

the gate. Such a topology is adapted for SCRs control. For Triacs, the devices are then

triggered in quadrants Q1 and Q4. Such an operation is not advised for Triacs as the gate

current level is the highest for Q4 (see Section 1). Also Triac resistance to dI/dt at turn on is

lower for Q4.

As a control circuit designed with a positive power supply can be used only with standard

Triacs, the whole design has to be changed if the designer wants to switch from this

standard Triac to a snubberless or logic level Triac, or to an ACS or ACST. Indeed these

latter devices can not be triggered in Q4 (refer to Section 1).

When the gate current required to trigger the device is higher than the control-circuit output

current capability, the control-circuit output current has to be amplified. For example, today a

lot of MCUs feature output pins with a current capability around 30 mA. They can switch

by such an MCU, the two solutions are then:

●

Use several MCU output pins in parallel (the best is to use a separate gate resistor

between each output pin and the Triac gate to ensure a good current repartition

between each pin).

●

Use a bipolar transistor as shown in Figure 7.

()

C

0

I

01

,

1

R

R

V

V

gt

G

max

DSon

max

GT

min

DD

°

>

⋅

+

−

Ω

=

−

⋅

−

<

166

50

01

,

1

0135

,

0

55

,

1

5

,

4