I'm a member of a Formula Student Team of VIT University, India. We currently are building a power control board where our Engine Control Unit switches the relays which drive the Fuel Pump which consumes 20A approx and Fan which consumes 8A peak. Our Engine Control unit gives grounding signal to switch on the relays and I'm planning to replace the Relays with MOSFETs. I've bought IRF9540 MOSFETs and tried to switch the devices. Everything seems fine except for the MOSFET driving the Fuel pump, Which gets very hot even if a heatsink is installed. Is there any way to reduce the temperature of the MOSFET as there are temperature sensitive devices near the MOSFET.
The on-resistance of the Vishay IRF9540 is 0.2 ohms: -
This means that when 20 A are being passed the power loss is \$20^2\$ x 0.2 = 80 watts. Choose a MOSFET with a much much lower on resistance.
Apart from that it's only rated at 19A (the Vishay part) so you are definitely asking for trouble.