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May 2004

AN575

APPLICATION NOTE

CALCULATION OF TRANSIL

APPARENT DYNAMIC RESISTANCE

REV. 2

INTRODUCTION

namic resistance of the device, rd.

This value depends on:

– The thermal impedance and therefore the package

– The pulse current duration tp (standard exponential pulse).

The purpose of this note is to explain the means of calculating rd.

EXPRESSION OF THE DYNAMIC RESISTANCE RD.

rd is defined by the formula:

low level of current (1mA)

There are two distinct cases:

– tp lower than 1 ms

– tp higher than 1 ms

a) rd (tp) with tp < 1 ms

We can thus estimate rd20µs and rd1ms with the following formula:

(1)

(2)

For tp between 20 ms and 1ms we can calculate rd (tp) as:

(3)

with rd in ohms and tp in µs.

rd

20

µs

V

() V

–

I

()

---------------------------------------------------------------

=

rd

1ms

V

() V

–

I

()

-------------------------------------------------------------

=

rd tp

()

rd

–

20

980

--------------------------------- tp

20

–

[] rd

20

+

=