Switching-speed and voltage rating for the BOM
The 1N6621 is a fast recovery rectifier diode from Microchip Technology, classified with a recovery time under 500 ns at forward currents above 200 mA — the actual 30 ns trr puts it in the fast-switching class for a 440 V part. Rated for 440 V reverse voltage and 1.2 A average forward current, the 1N6621 handles boost-stage and flyback-clamp duties in offline power supplies where the reverse voltage exceeds 400 V bus levels. Forward voltage is 1.4 V maximum at the full 1.2 A forward current — this conduction loss figure sets the thermal budget for the axial-lead package in free-air or heatsink mounting.
Junction temperature and leakage stability
Reverse leakage is only 500 nA at the full 440 V rating — this low leakage at high reverse bias means the diode does not thermally runaway in high-impedance snubber or clamping circuits at elevated temperature. Junction capacitance measures 10 pF at 10 V reverse bias and 1 MHz — a low enough figure that the diode does not load the switching node in a 100 kHz converter beyond the driver's gate-charge budget.
The through-hole axial package (DO-41 or equivalent A-case) is a standard footprint that places without tombstoning risk in wave-solder or hand-assembly lines — check the MSL rating before the bake step if the reel has been exposed to humid storage.