AN3401

Thermal runaway

Doc ID 018842 Rev 1

7/18

4

Thermal runaway

If the application is not properly designed in terms of heat dissipation, Schottky diodes can

go into thermal runaway. This phenomenon permanently damages the diode, which works

like short-circuit. As the SPV100x is based on MOSFET technology, it is free from the above

mentioned phenomenon.

Normally, Schottky diodes with lower forward losses, have higher leakage current and so

they are more sensitive to thermal runaway; for this reason the correct design of the

application comes also from a trade-off between forward voltage drop and leakage current.

When the diode is in forward mode the temperature increases due to the high power

dissipation, while, when it goes into reverse polarity it can have a relatively high leakage

current due to the high temperature coming from the previous condition.

If the power losses generated from the leakage current are higher than those in forward

mode, then the diode goes into thermal runaway until permanent damage occurs.

Figure 6.

Thermal runaway positive loop

Therefore, in all the photovoltaic applications, the use of Schottky diodes, as the bypass

device, may be dangerous because of the risk of thermal runaway.

Equation 4

During the fast switching of the diode from forward to reverse mode, the junction

temperature, due to the preceding forward mode, stays continuos and determines the

the new junction temperature trend.

This variation trend, between the initial junction temperature (due to forward mode) and the

new one (due to reverse mode), gives the Tj variation and the rotation sense that can be

seen in Figure 6.

T

j

T

a

R

thJA

V

ak

I

ak

⋅

()

⋅

+

=