Thermal Design advice needed

[Mosaic]

I have a bank of 4 of these:
http://www.irf.com/product-info/datasheets/data/irfp3206pbf.pdf

Bolted onto a plate of 4" x 6" x 3/8 "aluminum, with a 120mm,12V Fan cooling.

The load thru them takes them to about 85C (case temp), a 50C delta. Peak 800A pulse currents (shared by 4 FETS) at < 50uS, 2.5% DC.

I was just wondering how to calc. the max safe operating 'case' temp for these units given these conditions.

thanks...

 

[Tony Stewart]

What is your calculation? Keep in mind SOA includes max current for avalanche diode appears to be <70mJ @ 85'C Tj , but you measured Tc, so factor Eas*ZthJC to get rise.