Package and mounting
The IRF630NPBF is an N-channel enhancement-mode MOSFET from the HEXFET series, built on a planar stripe DMOS process that gives it a 200 V drain-source breakdown and a continuous drain current rating of 9.3 A at 25°C case temperature. The 300 mOhm maximum on-resistance at Vgs=10 V and 5.4 A sets the conduction loss floor — at 5 A the dissipation runs about 7.5 W, which the TO-220AB package can handle with a modest heatsink before the junction hits 175°C. Gate charge totals 35 nC at 10 V, so a gate driver sourcing 1 A can switch the FET in about 35 ns — fast enough for a 100 kHz SMPS primary, but the 575 pF input capacitance at 25 V means the driver sees a capacitive load that needs a low-impedance loop back to the source pin.
Thermal and switching — the numbers that decide the heatsink and the driver
The 82 W maximum power dissipation at Tc=25°C is a case-temperature-limited figure — real-world dissipation at a 100°C case drops to about 35 W, which is where the TO-220 tab bolted to a finned heatsink earns its keep. Threshold voltage is 4 V maximum at 250 µA drain current, so a 5 V logic gate drive will barely turn it on — the datasheet specifies 10 V drive for the rated Rds(on), and anything below 8 V risks operating in the linear region with high dissipation. The ±20 V maximum gate-source rating gives headroom for gate-drive overshoot, but a 10 V drive is the sweet spot — driving to 15 V gains no Rds(on) improvement and eats into the margin on the gate oxide.
Where the scorch mark tells you it failed — repair bench reality
On a failed offline flyback supply, the IRF630NPBF typically dies shorted drain-to-source when the primary-side snubber or the reflected voltage exceeds 200 V — the scorch mark on the TO-220 tab is usually at the drain pin exit.