400 V N-channel MOSFET for offline power switching
The STMicroelectronics STD5NK40ZT4 is a 400 V N-channel MOSFET from the SuperMESH series, built with ST's strip-layout technology to reduce on-resistance per area and gate charge. The DPAK (TO-252) surface-mount package with exposed drain tab suits automated assembly on medium-power PCBs, but the tab's thermal path to the board copper is part of the dissipation budget — 45 W maximum at case temperature is achievable only with adequate copper area and vias.
The 4.5 V threshold maximum at 50 µA means the device is fully enhanced with a standard 10 V gate drive; logic-level drive at 5 V will not guarantee the rated Rds(on) — the 1.8 Ω spec is measured at 10 V only. For designs running from a 5 V bias rail, check the transfer curve in the datasheet to confirm conduction loss at the actual gate voltage.
Temperature range and package — storage and reflow notes
The ±30 V maximum gate-source rating is generous, but a gate-source zener or TVS is cheap insurance against ringing in a half-bridge layout.