AN444 Datasheet, PDF (1/5 Pages) STMicroelectronics – DISSIPATION POWER AND SURGE CURRENT DURATION

AN444
APPLICATION NOTE
TRANSISTOR PROTECTION BY TRANSIL:
DISSIPATION POWER AND SURGE CURRENT DURATION
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
VCC
L, r
OR
CIRCUIT MODELISATION
When the switch turns off we use the equivalent circuit represented in Figure 2.
The worst case is to consider V_CL = V_BR min.
This hypothesis will be used in all formulas.
Figure 2. Equivalent Circuit
L
r
V_CC
Note: V_CL: clamping voltage;
V_BR: breakdown voltage;
rd: apparent resistance.
V_CL = V_BRmin
rd
V_CL
V_BR
April 2004
Rev. D2A - 3582
1/5