What this 50 ns fast recovery diode is for
The UF5402-E3/73: The UF5402-/73: The Vishay UF5402-/73 is a standard fast recovery rectifier rated 200 V reverse and 3 A average forward current, with a 50 ns reverse recovery time. That trr puts it in the class of diodes used for the output rectification in flyback converters, the boost diode in PFC stages, and the freewheeling path in low-to-mid-frequency SMPS where you need to cut switching losses without stepping up to an ultrafast part. The DO-201AD axial package is a through-hole form factor that handles the 3 A continuous load with a reasonable junction-to-ambient thermal path when the leads are soldered into a board or a terminal block.
The 50 ns reverse recovery time is the spec that separates this from a standard 1N400x-type rectifier (which can take several microseconds to recover). At a 50 kHz switching frequency, that 50 ns recovery window means the diode stops conducting reverse current fast enough that the switching transistor doesn't see a shoot-through spike on every cycle. The 45 pF junction capacitance at 4 V reverse bias is low enough that it won't ring badly with the transformer leakage inductance in a typical flyback layout — but you still want a snubber across the secondary winding if the trace lengths exceed 50 mm.
DO-201AD axial package, through-hole mount.