AN443 datasheet(4/14 Pages) STMICROELECTRONICS

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AN443 APPLICATION NOTE

4/14

Figure 5. The variation DIR is the dispersion of IR at max operation junction temperature (DIR1)

plus the variation due to Tj (DIR2)

The junction temperature is given by the thermal resistance junction to ambient Rth (j-a) and the power

dissipation due to the conduction losses (PC) and the switching losses (PS).

PC is linked to the forward voltage (VF) and PS is linked to the reverse recovery charge (QRR).

So the variation of the junction temperature is:

where

∆VF is the dispersion of the forward voltage and QRR the dispersion of the reverse recovery charge.

For series operation, it is recommended to use pieces coming from the same lot, so the dispersion on the

parameters VF, QRR and Rth is minimized;

In most cases the evaluation of

∆Tj is difficult but, from experience, it is generally lower than 10°C.

We propose to take a safety margin and to use:

∆IR = 0.85 IRM

Simplified formula

The relation (2) is often used by using the following approximations:

k = 1: supposing the reverse current IRM constant, whatever the blocking voltage across the diode.

a = 0: Neglecting the effect of the tolerance of resistors. thus:

As for the

∆ IR the worst case is taken into account.

∆ IR = IR with IR = IR max at Tj max specified (100°C)

This formula is "pessimistic" and induces a low resistance and then a high power dissipation.

IR1

IR2

IRM

TJ (øC)

D2,D3,...Dn

∆T

∆Rth PC PS

() Rth

∆V

-----------PC

∆Q

----------------PS





RRM

–

()∆I

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

RRM

–

()I

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

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