October 2007

Rev 2

1/18

AN437

Application note

RC snubber circuit design for TRIACs

Introduction

When a TRIAC controls inductive loads, the mains voltage and the load current are not in

phase. To limit the slope of the reapplied voltage and ensure right TRIAC turn-off, designer

usually used a snubber circuit connected in parallel with the TRIAC. This circuit can also be

used to improve TRIAC immunity to fast transient voltages.

The subject of this paper is, first of all, to analyze the snubber circuit functions and to

propose a method for snubber circuit design in order to improve turn-off commutation.

Contents

1

Snubber circuit functions and drawback . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1

Turn-off improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.1

TRIAC turn-off reminder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.1.2

Snubber circuit benefit at TRIAC turn-off . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2

Overvoltage limitation at turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.3

Immunity to fast voltage transient improvement . . . . . . . . . . . . . . . . . . . . . 5

1.4

Turn-on stress due to snubber circuit discharge . . . . . . . . . . . . . . . . . . . . . 5

2

How to design snubber circuit for turn-off improvement . . . . . . . . . . . 7

2.1

Step response of an RLC series circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2

RC snubber circuit design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.1

Is the snubber circuit required? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2.2

Resistor and capacitor snubber circuit design . . . . . . . . . . . . . . . . . . . . 10

3

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

Appendix A

RLC series circuit step response explanation . . . . . . . . . . . . . . . . 15

Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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