The STD3NK80ZT4 is an STMicroelectronics N-channel MOSFET built on the SuperMESH™ technology platform, designed for high-voltage switching applications where the drain-to-source voltage reaches 800 V. The 19 nC typical gate charge keeps the drive losses manageable in a 100 kHz-class converter, and the 70 W power dissipation ceiling at the case tells you the thermal design — heatsink or PCB copper area — needs attention if the duty cycle pushes junction temperature near the 150 °C limit.
The base product number is STD3NK80, and the 'Z' suffix indicates lead-free / RoHS-compliant plating. For dual-sourcing or second-source qualification, the pin-compatible alternatives within the same SuperMESH family share the same DPAK footprint and 800 V rating, though on-resistance and current ratings vary by variant — verify the specific Rds(on) and Id against your load profile before substituting.
In a 100 kHz hard-switched flyback, the average gate-drive power is roughly Qg × Vgs × fsw — about 19 mW at 10 V and 100 kHz — which a standard MOSFET driver can handle without a heatsink. If you are pushing higher frequency, the gate-charge curve in the datasheet tells you the Miller plateau voltage — that is where the switching loss actually lives.