March 2008

Rev 3

1/16

AN439

Application note

Snubberless™ and logic level TRIAC behavior at turn-off

Introduction

The use of TRIACs is limited by their switching behavior. Indeed, there is a risk of spurious

triggering after conduction if the slope of the decreasing current is too high, and/or if the

slope of the reapplied voltage is too high. The designer must then take some precautions:

device over-rating, switching aid network (snubber), and junction temperature margin, and

so on. This generally involves additional costs.

After a brief discussion of commutation when a TRIAC is turned off, this article will describe

the behavior of the logic level and Snubberless TRIACs, which present high commutation

capabilities.

Contents

1

TRIAC turn-off description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1

Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2

(dI/dt)c versus (dV/dt)c characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.3

Application requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.1

TRIAC with resistive load

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3.2

TRIAC with inductive load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.4

TRIAC without snubber network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2

Logic level and Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.1

Operation in Q1-Q2-Q3 quadrants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2

Performances and specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1

Logic level TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.2

Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3

Typical applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3.1

Logic level TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3.2

Snubberless TRIACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

www.st.com