ST AN1542 APPLICATION NOTE

AN1542

®

THE THERMAL RUNAWAY LAW IN SCHOTTKY

INTRODUCTION

Nowadays,somecriticalapplicationsrequirevery
high available power supplies. Typically, these
applications are servers or telecommunication
base stations.

In such systems, the power supplies are built with
several power supplies connected in parallel in
order to be fault tolerant. Thanks to redundancy,
the total failure rate stays very low and the avail­ability can exceed 99.99%.

The connection of several power supplies needs
the OR function, commonly built with diodes, to
tolerate faults in the SMPS.

Fig. 1: Supplies connected in parallel

APPLICATION NOTE

USED IN OR-ing APPLICATION

by Y.LAUSENAZ

2. TYPICAL PREFERRED DEVICE

In normal operation the diode is conducting in
forward mode. So, the first requirement of the
component,irrespectiveof the maximum repeti­tive reverse voltage (V
ing (I

), is the forward voltage drop (VF).

F(AV)

The lower the forward voltage drop, the lower the
forward losses in the diode, and the better the
SMPS efficiency.

For this reason, Power Schottky diodes are com­monly used in OR-ing application. The L series
(for example STPS60L30CW) are optimized to
provide very low forward voltage drop:

= 0.33V (30A @125°C per diode).

V

F typ

The following graph presents the typical Schottky
used in OR-ing application on common voltage
outputs:

) and the current rat-

RRM

Or function

SMPS 2

SMPS 1

Load Vout

1. OR-ing FUNCTION PRESENTATION

TheOR-ing function iscommonlybuilt with diodes.
The diodehasto let the current pass through when

the associated SMPS is working in normal opera­tion. When a SMPS fails in short circuit, the diode
has to block reverse voltage in order to maintain
output voltage on the load.

The purpose of the OR function is to prevent fault
propagation between supplies connected in parallel.

Fig. 2: Typical Schottky used as OR-ing function
on common voltage outputs

Output
voltage

48V
24V
12V

5V

3.3V

L15 L30L25 L45 L60 H100

Schottky
voltage

Using Schottky diodes provides very low forward
losses.But the mainimportanttechnology trade off
for Schottky is between forward voltage drop and
leakage current:

The optimization of forward voltage drop is inevita­bly made to the detriment of leakage current.

High leakage current gives rise to the thermal
runaway problem.

May 2002

1/6

APPLICATION NOTE

3. THERMAL RUNAWAY RISK

The risk of thermal runaway comes from the fact
that leakage current increases quickly with the
junction temperature.

3.1. Problems

Using a Schottky as OR-ing function provides a
very low forward voltage drop. But when the diode
is blocking because its associated supply has a
fault in short circuit mode, the diode has to operate
in reverse mode with high junction temperature
(due toprecedingforwardlosses) and so with rela­tively high reverse current.

This high reverse current can generate high re­verse losses, and so increase junction tempera­ture, and so reverse current as well… This is the
thermal runaway phenomenon.

Fig. 3: Thermal runaway diagram

Reverse

current

Reverse

Junction

losses

temperature

The problem is to quantify the risk of thermal
runaway in order to prevent it.

3.2. Result in classical cases

In the classical simple case where both the following
assumptions are made:

Constant thermal resistance system

■

OR-ing diode on its own heatsink

■

The reverse losses in the Schottky diode, due to
associated SMPS short circuit failure, is a mo­notonous function of the time. Consequently the
thermal runaway diagram of fig. 3 is covered in
only one rotation- sense.

To determine if the Power Schottky will goes into
thermal runaway mode consists of finding the ele­ments that will determine the rotation sense of
fig. 3.

During the forward mode, the forward current (I
defines the junction temperature (T
forward voltage (V

) and ambient temperature (T

R

th(j-a)

TT R IxV

=+

jambthjaFFI

), device thermal resistance

F

()

−() @

) (linked to

j

):

amb

fwd

F

During the fast mode change of the diode (from
the forward mode to the reverse one, the change
is fast in comparison to device thermal constant),
the junction temperatureduetothepreceding for­wardmode stay continuous (c.f.fig.5) and willde­termine the leakage current (I
reverse voltage V

ITV I CV e

;;=°×

()( )

rev j rev rev rev

rev

):

100

) (linked to the

rev

−°

100

cT C

(

j

)

c ≈ 0.055°C-1(thermal constant)

This reverse current will determine the new junc­tion temperature trend (linked to reverse voltage
and device thermal resistance). This variation
trend between the initial junction temperature (due
to forward mode) and the new one (due to reverse
mode)givesthe T

variationandthe rotation-sense

j

in fig. 3.
In a constant thermal resistance system, the ther-

mal stability can be determined by comparing for­ward losses (P
before the SMPS failure (t
losses (P

) occurring just after (t0+δt) the even-

rev

) in the power Schottky just

fwd

-δt) and the reverse

0

tual SMPS short-circuited fault.

)

2/6

The stability can be guaranteed if P

fwd>Prev@t0