Datasheet avalanche ratings

AN2344

12/27

have to be taken into consideration. These are mainly systemic in nature because the

datasheet thermal impedance is the response of the userfs system to a rectangular power

pulse, while maintaining the package case temperature at 25°C. This response can be

expressed as follows:

Equation 1

where,

t= Triangular and Rectangular response time, and

P= Power dissipated

The application of Equation 1, without any modification cannot give precise information. In

fact, a comparison between a rectangular and a triangular power pulse (both with the same

peak), indicates that the peak temperature between them is quite different (see Figure 7.).

To use the Zth, the triangular pulse can be approximated as rectangular pulse, with scaled

amplitude and width.

Another important concern about the use of thermal impedance is that usually it is

experimentally and theoretically calculated according to the ON state of the device, so the

normal power distribution within the device may be different from the one that occurs during

the avalanche state.

ST's approach to an EAS statement starts from a theoretical thermal model of the die with

Equation 2

where:

This solution is derived from the general heat transmission equation (Fourier equation) for

the special case where an infinite media (mono-dimensional case) is subordinated to a

short rectangular power source pulse and uniformly distributed over a die area.

Equation 2, when it is applied to a triangular power pulse, is a simple and good

approximation to use for the temperature increase calculation within the device during the

avalanche operation, if the pulse is of short duration.

∆T

jc

Zth

()P

=

T

j

P

0

AK

-------- t

av

T

starting

+

=