AN2344 datasheet(11/27 Pages) STMICROELECTRONICS

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AN2344

Datasheet avalanche ratings

11/27

Figure 6.

EAS vs temperature STP9NK80Z

3.3

Avalanche rating example

In this example, a user wants to use an STP9NK80Z as a DC/DC converter main switch.

The datasheet ratings of this particular device are:

●

EAS = 350mJ,

●

IAR = 7.5A, and

●

TJ(max) = 150°C.

According to the board analysis, the device used for such power supply condition can

experience a single pulse avalanche operation. The measurements have shown that the

average junction temperature is 100°C, the peak drain current switched during the

avalanche is 4A, and the energy that is dissipated during that single avalanche operation is

0.24mJ.

To understand if the device is working within the datasheet ratings, the switched current

needs to be checked by comparing it to the IAR; because the ID peak value is 4A and below

the IAR, this rating is satisfied.

To confirm that TJ is below TJ(max), it is assumed that TJ before the avalanche is average

(100°C). Figure 6. shows that the energy it takes to bring TJ to the maximum rating (starting

from 100°C) is approximately 50mJ. Since the energy measured is below 50mJ, the TJ

reached during the avalanche will be less than TJ(max). Considering that both of the bonds,

the current below IAR and the TJ value below the TJ(max) are well satisfied, one can safely

state that the device (under that single avalanche event) is working within the datasheet

ratings.

Each MOSFET manufacturer (including ST) presents their own approaches to measuring

EAS values, as well as their findings. These values are not simple to state because it is very

difficult to look at TJ during an avalanche operation. In order to provide as clear an

explanation as possible, some manufacturers set this value by using the thermal impedance

stated in the datasheet. This could be an interesting approach, but some concerns would

Starting TJ=100°C

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