AN2689 datasheet(25/42 Pages) STMICROELECTRONICS

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AN2689

Supply rail protection

Doc ID 14310 Rev 3

25/42

where Rs is the serial resistor of the generator. In this case Rs = 2 

So the surge generator capacitor is C = 2.7 µF. Therefore, when considering the use of an

SM15T33AY, the current duration at Ip/2 is:

(tp is the current duration through the SM15T33AY measured between Ip and Ip/2)

It is now possible to compare this current duration with the curve given in Figure 31.

For a current duration of 4 µs, the peak pulse power the SM15T33AY can dissipate is

approximately 20 kW.

Figure 31.

Peak pulse power versus exponential pulse duration

In the same way as for the previous examples, a derating factor needs to be considered

regarding the ambient temperature. For 85 °C ambient temperature the derating is close to

70% (see Figure 32). So the maximum peak power the SM15T33AY can withstand for a 4

µs is instead 14 kW.

Figure 32.

Peak power dissipation versus initial junction temperature

()

µs

0.1

1.0

10.0

100.0

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

(kW)

t (ms)

500

1000

1500

2000

100

125

150

175

P (W)

T (°C)

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