Hermetic HEXFET for high-reliability power stages
The IRFAF52 is an N-Channel hermetic MOS HEXFET rated for 900 V drain-source and 5.7 A continuous switching current, dissipating 150 W in a through-hole package. The hermetic seal — a ceramic or metal-can construction rather than epoxy overmold — qualifies this part for military, avionics, and downhole environments where moisture ingress or outgassing in a sealed enclosure would kill a plastic FET within months.
900 V, 5.7 A — reading the safe operating area
The headline 150 W dissipation is the thermal budget at the case, not a free lunch. At 900 V the current must be derated well below 5.7 A — the switching SOA at that voltage is limited by the linear-mode region and the RDS(on) temperature multiplier. This is a part for a 400–600 V DC bus where the switching edges are fast and the duty cycle keeps the average current under 2–3 A in a typical flyback or two-switch forward converter.
Active lifecycle — no end-of-life pressure
Infineon lists the IRFAF52 as Active. There is no official pin-compatible successor — the hermetic HEXFET line is a niche, not a commodity family, so a replacement would require a package and pinout change.