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April 2004

AN444

APPLICATION NOTE

TRANSISTOR PROTECTION BY TRANSIL:

DISSIPATION POWER AND SURGE CURRENT DURATION

Rev. D2A - 3582

INTRODUCTION

In a great number of applications, we find the diagram in Figure 1 where a Transil is used to protect a

switch which controls an inductive load.

The switch can be a bipolar or a MOS transistor.

The purpose of this paper is to calculate the dissipated power in the Transil and the pulse current duration.

Figure 1. Basic Diagram

CIRCUIT MODELISATION

When the switch turns off we use the equivalent circuit represented in Figure 2.

This hypothesis will be used in all formulas.

Figure 2. Equivalent Circuit

rd: apparent resistance.

L, r

OR

L

r

rd