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: -

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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.

  • \$\begingroup\$ To expand on this - you need to develop a respect for Murphy's Law and margin. Using a 19 amp part for 18 amps is just asking for trouble. If possible, toss in a 50% margin on power and current ratings. If you must cut your margin, you must also start paying very close attention to your system parameters to make sure that what little margin you do have does not get eaten up. \$\endgroup\$ – WhatRoughBeast Apr 22 '16 at 18:19
  • \$\begingroup\$ @WhatRoughBeast plus there's the environment to consider especially high ambient temperatures and even hotter electronics. \$\endgroup\$ – Andy aka Apr 22 '16 at 18:21

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