AN575 Datasheet, PDF (1/5 Pages) STMicroelectronics – CALCULATION OF TRANSIL APPARENT DYNAMIC RESISTANCE

AN575
APPLICATION NOTE
CALCULATION OF TRANSIL
APPARENT DYNAMIC RESISTANCE
INTRODUCTION
To estimate the clamping voltage V_CL and the dissipated power in a TRANSIL we need the apparent dy-
namic resistance of the device, rd.
This value depends on:
– The thermal impedance and therefore the package
– The breakdown voltage V_BR
– 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:
rd = (V_CL-V_BR)/I_pp,
Where V_CL is the peak voltage at Ipp and V_BR is the breakdown voltage of the TRANSIL measured at a
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
In the data sheet V_CL max is specified at tp = 20 µs and 1 ms.
We can thus estimate rd20µs and rd1ms with the following formula:
(1)
rd_20µs = ^V----^C----^L---^m----^a----^x-_I--⁽_p--2-_p--0₍---2-^µ--0-^s--_µ-⁾---–_s---₎^V----^B---^R---ⁿ---^o----^m---
(2)
rd_1ms
=
-^V----^C---^L---^m-----^a---^x---⁽--1----^m----^s---⁾---–---^V----^B---^R---ⁿ---^o----^m---
I_pp(1ms)
For tp between 20 ms and 1ms we can calculate rd (tp) as:
(3)
rd(tp)
=
r---d---_1---m----s--–_{---r---d---2--0---µ---s [ t p}
980
–
20 ]
+
rd_20µs
with rd in ohms and tp in µs.
May 2004
REV. 2
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