AN2689 datasheet(27/42 Pages) STMICROELECTRONICS

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AN2689

Supply rail protection

Doc ID 14310 Rev 3

27/42

Where tp is the current duration at Ip/2 and T the minimum surge period.

Then the average power is:

The junction temperature can be calculated as:

with Rthj-a = 75 °C/W.

The junction temperature will rise by less than 1 °C over the maximum ambient temperature.

So the maximum junction temperature will not be exceeded. Therefore the SM15T33AY will

be suitable for the application.

In the meantime, comparing the 274 W power induced by the surge with the SM15T33AY

admissible power, the SM15T33AY is extremely safe regarding the ISO 7637-2 pulse 2

surge, even over-specified.

For this reason it would be more effective to choose a smaller Transil, like, for instance, an

SM4T35AY (Vbrmin = 33.3 V). The same calculations show the power dissipation is quite

similar to the SM15T33AY results. Figure 35 and Figure 36 show that the maximum power

this SM4T35AY can dissipate for 4 µs exponential pulse duration is still higher than that the

surge imposes. The SM4T35AY is able to withstand more than 3 kW for 4 µs surge duration.

Figure 35.

Peak pulse power versus exponential time duration of an SM4T Transil

Figure 36.

Peak power dissipation versus initial junction temperature of SM4T

Transil

Vcl

7.52

⇒

AV Rth

Tamb

0.1

1.0

10.0

1.0E-03

1.0E-02

1.0E-01

1.0E+00

1.0E+01

PP(kW)

j initial = 25 °C

tP(ms)

500

1000

1500

2000

100

125

150

175

P (W)

T (°C)

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