620 V N-channel in a full-pack — what the ratings tell you
The STF3LN62K3 is an N-channel MOSFET from ST's SuperMESH3™ series, built for offline switched-mode power supplies and auxiliary converters where the DC bus sits at 400 V or below. The 620 V drain-source breakdown gives you about 220 V of headroom above a typical 400 V bus — enough to handle flyback leakage spikes without avalanche stress eating into the reliability margin. That 3 Ohm figure is the conduction loss floor at rated current — at 1.25 A the dissipation is about 4.7 W, which the TO-220FP full-pack can sink into a heatsink up to its 20 W limit.
Gate drive and switching — the numbers that matter for the driver stage
Total gate charge is 17 nC at 10 V Vgs, which means a typical gate driver delivering 1 A peak can switch this MOSFET in under 20 ns — fast enough for a 100 kHz flyback without excessive cross-conduction. Input capacitance is 386 pF at 50 V drain, a light load on the driver's output stage. Gate threshold voltage is 4.5 V maximum at 50 µA drain current, so a 5 V logic-level gate drive will barely turn it on; the recommended drive voltage for minimum Rds(on) is 10 V. If your controller only puts out 5 V, you'll need a gate-driver IC or a bootstrap charge pump to get the gate above 8 V.