400 V N-channel MOSFET for offline power conversion
STMicroelectronics has marked the STP5NK40Z obsolete. Buyers with an existing BOM line should qualify a replacement for new builds and plan for last-time-buy coverage on legacy service runs.
The 400 V Vdss gives the designer 50–80 V of derating headroom for a 240 VAC offline rail after rectification and ripple, assuming a snubber-clamped flyback topology. The 3 A continuous current rating at 25°C case temperature must be derated per the thermal curve above 100°C case — at 100°C case the practical continuous current drops to roughly 2 A. The 1.8 Ω Rds(on) at 10 V gate drive means conduction losses of about 1.8 W at 1 A; the TO-220 package in still air with a small heatsink handles that, but at 3 A the loss hits 16 W, requiring a substantial heatsink or forced air. The 17 nC gate charge keeps the switching transition fast enough for 50–100 kHz operation; beyond that the Miller plateau stretches and switching losses climb.