2.5 mOhm Rds(on) — the conduction-loss floor for a 40 V switch
The Infineon IRFB7440PBF is an N-channel MOSFET from the HEXFET® and StrongIRFET™ series, built for high-current switching where low on-resistance is the dominant selection criterion.
120 A continuous drain — sizing the output stage
Gate charge and drive voltage
Total gate charge is 135 nC at 10 V, with the Rds(on) specified at both 6 V and 10 V drive levels. A 10 V gate drive delivers the full 2.5 mOhm on-resistance; a 6 V rail is usable but the on-resistance will be higher.