AN4242 datasheet(9/26 Pages) STMICROELECTRONICS

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DocID024196 Rev 1

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AN4242

Forward thermal runaway

Tj. Vt0 and Rd are thermal coefficients. They represent the junction temperature impact

on Vt0 and Rd.

Using the electrical model previously defined, the conduction power losses P(Tj) can be

estimated. Using the analogy between thermal and electrical units, a simple electro-thermal

model is described in Figure 6. The thermal model is defined by the thermal resistance

Rth(j-a) and the thermal capacitance Cth(j-a) junction to ambient.

Figure 6. Simple electro-thermal model

The resolution of the above electro-thermal system gives the Tj(t) expression used to find

the thermal runaway limit and to highlight the stability condition of the diode. The equation

giving the conduction power losses versus Tj is the following:

Equation 13

with

Equation 14

and

Equation 15

A simplified version is:

Equation 16

The global system equation is defined by:

Equation 17

Or again

Equation 18

Solving the differential equation gives the stability condition on the junction temperature:

Tamb

Cth(j-a)

Rth(j-a)

P(Tj)=I0·VF(Tj,I0)

P(T ) = [V

(T - 150)] · I + [R

· (T - 150)] · I

150°

A = I · (V

- 150 ·

) + I

· (R

- 150 ·

)

0t0

150°

I ·

· T + I

· T

(I ·

+ I

) · T

B T

Rj =

Rj =·

P(T ) = A + B · T

T= T

+ R

amb

P(T ) - C

jth

dTj

T · (1 - B · R ) + R

· C

jth

dTj

= T

+A· R

amb

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