If not with FET, how else?
Now i putted another FET (IRL2505) in the same circuit and after more than hour it's almost completely cold. Without heatsink on it, everything is in completely sealed plastic box.
I saw that maximal Gate-to-Source voltage for IRL2505 is 16V while for IRL540 is 10V. Since i am powering the whole circuit with a little bit more than 12V, this could be problem?