March 2008

Rev 3

1/8

AN440

Application note

QII and QIII TRIAC triggering with positive power supply

Introduction

New TRIACs with high commutation and dv/dt performances are now available on the

market.

Generally these TRIACs can be triggered only in the first three quadrants (case of

Snubberless™, logic level and Snubberless high temperature TRIACs) as shown in

Figure 1.

This paper describes a trigger circuit supplying a negative gate current for quadrants II and

III implemented in a system using a positive power supply.

Without a new design, just by adding a capacitor and a diode, new series TRIACs can

replace conventional TRIACs.

Figure 1.

The quadrants of Snubberless, logic-level, and Snubberless high

temperature TRIACs

TM: Snubberless is a trademark of STMicroelectronics

+ +

--

+-

1 st

2 nd

3 rd

4 th

NOT

TRIGGERABLE

IT

IG

+ +

--

+-

1 st

2 nd

3 rd

4 th

NOT

TRIGGERABLE

IT

IG

A1

IG

IT

A2

A1

IG

IT

A2

www.st.com