Geeetech

Hi, i have experimented problems with the mosfet that control the voltage of the hotbed output, it still working but gets overheat really fast and turn on the hotbed to the maximum temp as soon as i power on the 3d printer , i've changed the burned mosfet with a new one but the hotbed its not working anymore i change it with a FQP50N06 the actually mosfet on board is a STP55N06 the new one works with 52 amp the old one with 55 amp, i wanna know if i have to change something in the firmware for make the new mosfet works, or i can change in the firmware that the hotbed works with the output for the 2th extruder ?. btw i'm new on everything about arduino .

Double check the mosfet specs that it is direct replacement compatible, i beleive the capacitence on the original is around 230uf if the new one is different it might be tricking the board into thinking ghere is still something wrong!
I would also check the rest of the board for damaged components. It would be very unusual that something else didn't get damaged when that burnt out!

Although it's not in focus there looks like there has been some browning of the solder on that chip just in front of the mosfet, that's usually an indication that something has gotten hotter than it should have!

Failing that.... replacing the whole board is the best option without metering out every possibly affected component. Even then If it turns out to be an smd cap or resistor then it's still near impossible to replace and that's assuming there are even any details on it!
Most unbranded Chinese smd components don't even have details printed on them so without having a diagram of the pcb which states whats what it's a shot in the dark!

Edit:
Ps. When you get that fixed you now know that your first print should be a custom duct to run from that fan directly to those mosfet's!
And double check that fan should be running when you power up the printer, if it doesn't then you have that fan plugged into the extruder fan socket!