AN4242 datasheet(10/26 Pages) STMICROELECTRONICS

10 / 26 page

Forward thermal runaway

AN4242

10/26

DocID024196 Rev 1

Equation 19

Due to the exponential function in the expression of Tj(t), if B · Rth - 1 > 0 then the limit

leads to the diode destruction if the current I0 is not interrupted.

Thus, the stability condition is given by:

Equation 20

Equation 21

The detailed expression gives:

Equation 22

Note that the limit of thermal runaway can be directly found by:

Equation 23

Numerical application:

An application with an average current I0 = 6 A using the STPSC6H065 is considered here.

The typical forward voltage curve versus forward current of the STPS6H065 datasheet is

calculated between 3 A and 9 A and between 25 °C and 150 °C:

Vt0150°C = 0.85 V, Rd150°C = 0.175 , Vt0 = -800 µV/°C, Rd = 600 µ/°C.

So, the “B coefficient” is equal at 0.017 W/°C, and the critical Rth, beyond which the thermal

runaway is reached, is Rth > 59.5 °C/W.

Considering the Rth(j-a) of the TO-220 package in air to be around 60 °C/W, the stability

condition will not be respected since B · Rth(j-a) - 1 > 0 and thus the diode cannot be used

without a heatsink for this value of current.

The diode must be mounted on its own heatsink, in choosing the Rth value checking

B · Rth(j-a) - 1 < 0 and respecting Tj < Tjmax. In this case of a stable condition,

if Tj targeted is 125 °C with Tamb = 40 °C, the heatsink should be chosen for an Rth(j-a) value

equal to 7.45 °C/W.

T (t) =

· (A + T

· B)

amb

B · R

- 1

· e

B · R

- 1

C· R

· t

+A· R

amb

B · R

- 1

lim

® ¥

Tj(t)

® ¥

B · R

- 1 < 0

Rth <

· I +

· I

dP (T )

dTj

Rth(j-a)

lim

® ¥

Tj(t)

® - T

+A· R

amb

B · R

- 1

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