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Safe operation area DC in MOSFET

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berger.h

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Any MOSFET in datasheet SOA nod include DC area, but for example 10ms and short area.
My question si if is possible calculate DC area of SOA from max Power disipation?

Her is SOA for any MOSFET with 250W max power disipation and in SOA is only 10ms lide,
I calculate blue line from Pmax and max resistnce It is OK or not?

SOA.jpg
 

Hi,

The answer is given in your question:

You say "a 250W Mosfet" --> DC power dissipation = 250W...as long as your heatsink fulfills the test conditions: tc = 25°C, tj = 150° C.
The datasheet can't know about your heatsink, therefore you find additional informations in the datasheet like r_th to calculate max power dissipation for other heatsink conditions.

Klaus
 

Let us say your heat-sink is 70C in real life, so the die can get to 150C, 80C higher, on the data sheet there will be a de-rating figure so many W/degC, such that at 150 die temp (ambient) the useful power dissipation is zero. Let us say this figure is 4.0 W/degC in this case, the die temp is higher than the heatsink due to the Rj-c (junction-case) ~ .1 say, and the Rc-sink, say 0.1 again.

Let us assume Tsink = 70C, and we want 150W dissipation in the device:

We have 0.2 degC/watt thermal resistance junc to h-sink, 0.2 x 150watts = 30C, thus the die will be at 100C on a 70C h-sink. thus we have only 50C margin ( to 150C)

At 100C, 75C above 25C, the derating factor is (per above) 4 x 75 = 300 W from 500W = 200W allowable dissipation - so we have 50W margin, or we could let the h-sink temp rise above 70C...
 

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