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May 2004

AN869

APPLICATION NOTE

Tj MAX LIMIT OF SCHOTTKY DIODES

REV. 2

INTRODUCTION

This application note is about the limit of Tj max given in the datasheet of SCHOTTKY rectifiers.

It explains the real meaning of this parameter and why in some applications, the component can operate

with a junction temperature higher than Tj max.

Table 1 shows the Tj max specified in the datasheet for the different families of SCHOTTKY diodes.

Table 1. Tj max versus SCHOTTKY family

For a classical ultra fast rectifier, this limit is equal to 150°C. For a SCHOTTKY diode this limit is lower (for

instance 125°C for a STPSxxL25). This lower limit of Tj max is only due to thermal runaway phenomenon

linked to the leakage current. This phenomenon is explained in the application note: "THERMAL RUN-

AWAY IN RECTIFIER".

This application note describes the rules for the calculation of the limit before thermal instability is reached.

This limit depends on the characteristics of the diodes (leakage current: IR, junction to case thermal resis-

blocking of the diode: (1 -

δ), thermal resistance of the heatsink: Rth(c-a) ...).

The following two examples show that for the same diode (STPS10L25D) different conditions lead to dif-

ferent limits for Tj max.

First example:

Application parameters:

(1-

δ) = 0.5

Rth(c-a) = 8.5°C/W

Diode parameters: (STPS10L25D):

Rth(j-c)=1.6°C/W

SCHOTTKY FAMILY

(V)

Tj Max

(°C)

STPSxxL10

10

100

STPSxxL25

25

125

STPSxx45

45

150

STPSxx100

100

125