Application constraints

AN3432

14/24

Doc ID 019041 Rev 1

3.1.2

Thermal runaway risk

The thermal runaway results in the loss of temperature control, due to the inability to

exhaust the power losses generated by the diode operation.

In the bypass diode application, the most critical case is when the diode conducts and

runaway risk:

Equation 14

The junction temperature depends on the junction box thermal resistance between ambient

The power losses and these parameters are linked by the thermal law.

Equation 15

and

The conduction losses are given by:

Equation 16

A graphical interpretation is presented in Figure 10 with STPS20L45C diode, the current is 8

the heat power that the junction can dissipate. The blue curve is the conduction losses

generated by the diode. The curves cross point gives the junction temperature at the

Figure 10.

)

T

(

P

j@to

cond

j@to

rev

)

T

(

> P

j

Thermal

) =

T

(

P

(Tj -Ta)

Rth(j-c) + Rth(c-a)

)

T

(

P

j@to

cond

j@to

Thermal

)

T

(

= P

sc

j

sc

F

j

cond

I

)·

T

,

I

(

V

)

T

(

P

=