40 V, 1.85 mOhm — the conduction-loss floor
The Infineon IRF40DM229 is an N-Channel StrongIRFET™ MOSFET rated at 40 V drain-source and 159 A continuous drain current. The headline figure is the 1.85 mOhm maximum on-resistance at 97 A, 10 V gate drive — this sets the conduction-loss floor for a high-current switching design. At 100 A, that is about 18.5 W of I²R loss before accounting for temperature derating.
Total gate charge is 161 nC at Vgs = 10 V. For a 1 A gate driver, that translates to roughly 160 ns to charge the gate to the Miller plateau.
Thermal and package reality
The DIRECTFET package is a surface-mount can with a large exposed drain pad on the bottom — the thermal path runs through the PCB copper, so the board layout and via count directly set the effective RthJA. 83 W maximum power dissipation assumes an infinite heatsink; real-world derating depends on the board's ability to sink heat.