High current Switch for starter motor

[Rodney]

I need to fabricate a high current switch for a large starter motor (drawing about 35 to 40 amps). Power is a 12 volt Pb lawn mower battery. Since I have a number if the IRFZ44Mosfets (rated for greater than 30 amps) I am thinking the attached circuit might work. I have little or no experience in high current FET's so will appreciate any comments pro or con. If I need to parallel more than one FET, I plan to attach the added on ones at point X<Y and Z as indicated. Please comment, thanks Rodney

[JohnMuchow]

Looks pretty good. I always like to place the "kickback" diodes right across the MOSFET's drain-source pins to protect from voltage spikes created from any wiring inductance. It's the FET you're protecting so I like to place the protection there.

Looking at the datasheet for that MOSFET, I'd probaby start with 3 in parallel. Each one is rated to carry only 34A when hot and that's with a 10V gate voltage. Your gate voltage is lower than that (which doesn't turn it on as hard) and you'll be drawing more than 34A (MUCH more when the motor starts). A lot depends on how long the current is flowing and whether they're heat-sinked or not.

[mjsas]

I would eliminate the 4.7K and 22 ohm resistors, they don't really do anything. Also the 10K to ground should be a lower value to turn off the FET faster, something like 1K or less. You might think about putting a filter between the battery and input to the switch to eliminate any large voltage spikes. Also a large diode for kickback.
The sparking at the brushes will put out some nasty voltage spikes, high enough to blow the FETs. You will need some way to calm things down, not sure if caps will do it.
Even a small Pb battery can put out very large currents, I would design for 500 amp turn on spikes.
When you say large starter motor I think in terms of something like a starter for a 500 CID racing engine that draws 150 amps no load and much more under load. I can't think of any starter that draws 35 to 40 amps under load, maybe a small lawn mower.

[JohnMuchow]

Good points.
But, I think that having the turn-off even slower than he has now would be a good thing here.

If I understood him correctly, he's only using it as a on/off switch (not PWM control) so minimizing on/off times really isn't needed to keep the FET cooler. A slower off time will minimize the spike he's going to get though.

[ghoti]

The 22 ohm resister should not be left out. It suppresses high frequency oscillation. Bill