APPLICATION NOTE

2/6

The risk of thermal runaway comes from the fact

that leakage current increases quickly with the

junction temperature.

3. THERMAL RUNAWAY RISK

Reverse

current

Reverse

losses

Junction

temperature

Fig. 3: Thermal runaway diagram

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 to preceding forward losses) and so with rela-

tively high reverse current.

This high reverse current can generate high re-

verse losses, and so increase junction tempera-

ture, and so reverse current as well… This is the

thermal runaway phenomenon.

3.1. Problems

In the classical simple case where both the following

assumptions are made:

s

Constant thermal resistance system

s

OR-ing diode on its own heatsink

The reverse losses in the Schottky diode, due to

associated SMPS short circuit failure, is a mo-

notonous 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 ele-

ments that will determine the rotation sense of

fig. 3.

TT

R

I xV

jamb

th j a

F

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 temperature due to the preceding for-

ward mode stay continuous (c.f. fig. 5) and will de-

()

(

)

(

)

IT V

I

C V

e

rev

j

rev

rev

rev

cT

C

j

;;

=°

×

−°

100

100

c

This reverse current will determine the new junc-

tion 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

in fig. 3.

In a constant thermal resistance system, the ther-

mal stability can be determined by comparing for-

δt) and the reverse

δt) the even-

tual SMPS short-circuited fault.

3.2. Result in classical cases

The problem is to quantify the risk of thermal

runaway in order to prevent it.