Forward thermal runaway

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DocID024196 Rev 1

2

Forward thermal runaway

In some particular application conditions a thermal runaway loop can be triggered (see

Figure 4) and the thermal system of the diode may become unstable.

Figure 4. Thermal runaway loop

Two kinds of application conditions can be linked to the thermal runaway risk:



the stationary regime during the regular working mode



the critical transient phases.

2.1

Thermal runaway risk in regular working mode

During the regular operating mode, the average current in the diode can modeled with a

constant current generator as shown in Figure 5.

Figure 5. Simple electrical model

The electrical model given by Equation 10, simulates the variation of the forward voltage

Equation 10

Equation 11

Equation 12

Losses P(Tj)

Positive loop

0

Io

Rd(Tj)

Vt0 (Tj)

V (I ,T ) = V

+

(T - 150) + [R +

· (T - 150)] · I

F0

j

t0

V

j

d

R

j

0

150°

t0

d

aa

V (T ) = V

+

(T - 150)

t0

j

t0

V

j

150°

t0

a

R (T ) = R

+

· (T - 150)

dj

d

R

j

150°

d

a