For the 18v25 driver, the datasheet for the IRLR8743PbF MOSFETs lists a “safe operating area curve”, and I’m having a bit of trouble deciphering it, especially since there is no continuous line.
But, it seems to say that at 18 volts and 3 amps, it’s only rated for 10ms pulses. Is this correct?
Basically, if I’m running my motor at 18 volts, what’s the max continuous current I can push through it with this MOSFET without risking breakdown, assuming proper cooling?
The 18V rating you are looking at is if that whole 18V is applied across the MOSFET. However, if you run the motor driver at 18V, most of that goes to the motor. If the MOSFET has 3 milliohms of on-resistance, at 25A, that is only 0.075V across the MOSFET.
The driver should be good for 25A continuous. At that current, the power dissipation per MOSFET would be 0.075V * 25A = almost 2W. At 50 degrees per watt of thermal resistance, those 2 watts will cause about 100 degrees of temperature rise. As we say on the product page, if you actively cool the MOSFETs, you can get to about 35A continuous.