The 20 nC typical gate charge keeps switching losses manageable in hard-switched topologies like flyback and forward converters, PFC stages, and auxiliary power supplies in industrial and consumer off-line equipment.
STMicroelectronics lists the STF9NM50N as Obsolete.
The 500 V Vdss rating gives enough headroom for 240 VAC offline rectified rails (≈340 VDC) plus switching overshoot, but the 5 A continuous current is derated by the 25 W power dissipation ceiling at Tc=25°C. In a real enclosure with a heatsink at 70°C ambient, the usable current drops further — thermal design must be checked against the SOA curve. The 560 mOhm Rds(on) at 10 V drive means conduction loss at 3.7 A is about 7.7 W, which consumes nearly a third of the package's dissipation budget. The 20 nC gate charge is modest; a standard gate driver with 1 A peak source can switch it in the 50–100 kHz range without excessive drive loss.