ST AN869 APPLICATION NOTE
AN869
APPLICATION NOTE
Tj MAX LIMIT OF SCHOTTKY DIODES
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
V
SCHOTTKY FAMILY
STPSxxL10 10 100
STPSxxL25 25 125
STPSxx45 45 150
STPSxx100 100 125
RRM
(V)
Tj Max
(°C)
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 RUNAWAY 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 resistance Rth(j-c) ...) and application parameters (reapplied voltage across the diodes: V
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 different limits for Tj max.
First example:
Application parameters:
(1-δ) = 0.5
V
= 15V
R
Rth(c-a) = 8.5°C/W
Diode parameters: (STPS10L25D):
I
max (15V, 125°C) = 210mA
R
Rth(j-c)=1.6°C/W
, duty cycle of the
R
REV. 2
1/4May 2004
AN869 APPLICATION NOTE
The limit of the reverse current at VR and Tj max before reaching thermal runaway is given by:
IR15VTjmax,()
Where c is a thermal coefficient c ≈ 0.055
Rth(j-a) = Rth(j-c) + Rth(c-a)
----------------------------------------------------=
V
R
1
1 δ–()cRth j a–()
We have: I
Tj max is given by:
Second example:
Application parameters:
Diode parameters (STPS10L25D):
We have I
and Tj max = 144°C
(15V, Tj max) = 242mA
R
Tjmax 125=
Tj max = 127°C
(1-δ) = 0.5
V
= 5V
R
Rth(c-a) = 18.5°C/W
max (5V, 125°C) = 125mA
I
R
Rth(c-a=) = 1.6°C/W
(5V, Tj max) = 363mA
R
+
1
--- -
In
C
I
15VTjmax,()
R
--------------------------------------------------
I
max 15V 125 °C,()
R
CONCLUSIONS
This application note shows that the maximum limit of Tj of the SCHOTTKY diodes given in the datasheet
is mainly due to the thermal runaway phenomenon. This limit doesn’t only depend on diode parameters
but also application parameters. In the first example corresponding to a typical application we will find Tj
max given in the datasheet. The second example shows that in some applications SCHOTTKY diodes can
be used with a junction temperature higher than the Tj max given in the datasheet.
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