How can I calculate allowable DC current that can be processed at all times when the MOSFET is attached to the heat sink and enough heat is released?
I heard In real life, safe operation area (SOA) parameter is most important. Can mosfet handle more DC current than SOA with enough heatsink?

For example, in IRF1404
ID = 202A (data sheet title, package limit is shown as 75A)
ID= 40A (SOA, VDS=10v )

 

You cannot move 202 A through a TO-220 pin. The metal leg will blow like a fuse. If the drain current is DC (not pulsing), what is Vds in your application?

What is your application?

ak

 

SOA is an energy limit for Short tern overloads, not steady state DC.

"Enough heatsink" begs several questions. Do you mean some theoretical infinite sink? What temperature is it at?

Still you get about 1 degree C per watt rise junction to sink best case. Max junction temp of 175 puts you safe up to 100 degree ambient for your 75 amp current.

Junction temp = ( Therm resistance * power ) + heatsink temp

Wikipedia has a good article on thermal resistance, do read it.