900 V N-Channel MOSFET with hermetic package
The IRFAF40 is an N-Channel enhancement-mode MOSFET from Infineon's HEXFET® series, rated for 900 V drain-source voltage and 4.3 A continuous switching current. The through-hole package (likely a TO-254AA hermetic can) suits high-reliability and military/aerospace boards where solder-joint inspection and rework are standard.
Gate drive and switching budget
The gate-source voltage is rated ±20 V, so the drive rail must stay within that window — a 15 V gate drive is safe, but a 24 V industrial supply needs a zener clamp or series resistor to protect the oxide. The maximum gate charge is 120 nC at Vgs=10 V, which means the driver must supply 120 nC per switching cycle; at 100 kHz that is 12 mA average gate current, well within a standard gate-driver IC's capability. The threshold voltage is 4 V maximum at 250 µA drain current — the gate must be driven above 4 V to fully enhance the channel. A 10 V drive is typical to achieve the specified Rds(on) and minimise conduction losses.