MOSFET power calculation

[r4r]

Hello everyone!

I need to limit current with MOSFET IRF4905PBF to -20A. Datasheet says max current -70A and Rds(on) is 0.02Ohm.
Not really sure how to calculate Rds and power dissipation for -20A and, let's say, -18V? Vgs = -4.7V.

Rds = 18/20 = 0.9 Ohm
P = 20^20*0.9 = 360W?

Is it correct?

[Ice-Tea]

Your calculation is right but you took a bit of a detour. Just U*I works just fine. 

Also: forget about dissipating 360W in that FET. It has a Junction-to-Case thermal resistance of 0.75°C/W. So, your die will sit at 270°C above ambient even if you cool the case with a generous helping of pixie dust and hail marys.

[r4r]

Thank you, Ice-Tea! =)

Maybe I should use two IRFs in parallel?
Don't have any other idea...   

[Ice-Tea]

I suggest you look into a topology that doesn't have you dissipate 300W. Perhaps you should start by telling us exactly what you are trying to achieve..

[T3sl4co1l]

A switching current limit will dissipate a whole hell of a lot less power... but isn't much of a beginner project.

Does it have to be continuous, or can it be intermittent?  Controllers like TPS2491 can be used to handle this, with a constant-power characteristic that optimizes on-time within thermal limits.  You still only get 10s of ms though.

Tim