AN3432 datasheet(14/24 Pages) STMICROELECTRONICS

14 / 24 page

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

suddenly turns off. At this specific time t0 the following rule is applied to eliminate thermal

runaway risk:

Equation 14

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

temperature (Ta) and diode case temperature (Tc) and the thermal resistance between the

diode junction and the diode case, given in the diode datasheet as the Rth(j-c) parameter.

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

A, the ambient temperature is 60 °C, Rth(c-a) = 3 °C/W, Rth(j-c) = 1.3 °C/W. The green curve is

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

specific time t0. In this condition, the junction temperature Tj@t0 = 70 °C is shown in

Figure 10.

)

(

j@to

cond

j@to

rev

)

(

> P

Thermal

) =

(

(Tj -Ta)

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

)

(

j@to

cond

j@to

Thermal

)

(

= P

cond

)·

(

)

(

AN3432 Datasheet(PDF) 14 Page - STMicroelectronics

AN3432 Datasheet(HTML) 14 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL