I'm looking for some circuit designs that would work to drive a 12 volt 20 amp dc pump. Do i just need the right mosfet and diode or do i need a more complex circuit? I've done a fair amount of reading on pwm and dc motors but most of the circuits I have found have been for loads under 10 amps. I also need a fanless design because the circuit will be used in a dusty environment.
You'll need a MOSFET with Rds(on) less than 0.005 ohm and a heatsink really... perhaps two at 0.01 ohm is more realistic. It would be wise to use a driver chip like the MIC4422 for three reasons:
It allows faster switching PWM due to much lower switching losses.
It protects against capacitive coupling back from drain to gate.
It allows a non-logic level MOSFET to be used - much greater choice.
Use a MOSFET rated for at least 2 times the supply voltage. Find a good schottky diode pair (TO220) for the flyback diode (and that needs good heatsinking). Again get suitable high voltage rating.
Diecast ally boxes make good integral heatsinks BTW
I second the recommendation of using a purpose designed mosfet driver chip for high current applications. Those higher power mosfets that have such great max current and minimum Ron specs come with very high gate capacitance values that an arduino output pin can just not provide the high current drive required to insure the mosfet switches as fast as possible between transitions, that can be near instant death to high power mosfets, they are very unforgiving in that way.
Thanks for the info on the mosfet driver chip I hadn't read anything on that. So I take it that I want no more than about 2 watts of waste heat per mosfet with a good heat sink? Do you have a certain diode in mind I've read a lot on the mosfet but not too much on the diodes.
Without a cooling fan a few watts is quite a lot. Standard TO220 schottky common-cathode rectifier diodes - there's tons available, 2x15A will be enough if in parallel. I've some MBR30100CT's for instance.
Thanks for the example diode just want to make sure we were on the same page so to speak. What amount of waste heat is reasonable for a fanless design in a metal enclosure?
You might want to consider a motor controller like below.
co2shaun: Thanks for the example diode just want to make sure we were on the same page so to speak. What amount of waste heat is reasonable for a fanless design in a metal enclosure?
Depends totally on the surface area of the enclosure through which heat can leave...
What amount of waste heat is reasonable for a fanless design in a metal enclosure?
Without any sort of ventilation very little, maybe 0.25 W. Put a few air holes in and that will increase it considerably. Better yet attach the heat sink to the metal case.
Can't you use the metal case itself as a heat sink? Run a heat pipe to it, and let it freely radiate. As long as you don't get above 40C or so in total, things on the inside should be OK (but check specs to make sure -- especially batteries!) Some fins on the case will further increase surface area, but may be a problem (read: clog magnets) in dusty environments.