500 V, 3 A, 2.7 Ω — what this SuperMESH™ MOSFET is for
The STP4NK50ZD: The Zener-protected gate structure (the 'ZD' suffix) gives it built-in ESD ruggedness — useful in offline flyback converters, LED lighting ballasts, and auxiliary power supplies where the drain sees high-voltage spikes and the gate may float during assembly. The TO-220-3 through-hole package (supplier device package TO-220) handles 45 W dissipation at the case, so a heatsink is expected for anything above light load.
STMicroelectronics lists the STP4NK50ZD as obsolete.
Gate charge and switching — 12 nC at 10 V
The 4.5 V gate threshold at 50 µA means the device is fully enhanced with a standard 10 V gate drive, but it is not a logic-level part; at 5 V gate drive the Rds(on) will be significantly higher than the 2.7 Ω maximum. Keep the gate drive at 10 V to get the rated on-resistance.
The TO-220 package with a proper heatsink can handle the 45 W dissipation at the case, but derating is needed above 25°C case temperature. For continuous 3 A at 500 V, the switching losses and Rds(on) conduction losses must be calculated against the thermal impedance — the 2.7 Ω on-resistance at 1.5 A and 10 V is the design point for efficiency.