12 A, 600 V, 21 ns — what the numbers mean in a real PFC or boost stage
The STTH12S06FP: At 12 A average rectified current and a 21 ns reverse recovery time, it is sized for the output rectifier of a continuous-conduction-mode PFC stage or the boost diode in a 1–2 kW SMPS. The 3.4 V forward drop at 12 A is the conduction loss number that drives the heatsink size; at 12 A that is roughly 40 W, so plan for a decent thermal path.
How it sits against the STTH15R06D — the higher-current sibling
The closest peer in the same STTH family is the STTH15R06D: same 600 V blocking, same standard diode type and through-hole mounting, but rated 15 A average with a 50 ns trr — more than twice the recovery time. The STTH15R06D also comes in a TO-220AC (non-isolated) package, so it needs an insulator if the heatsink is grounded. If your design already has an isolated heatsink or you need the faster 21 ns recovery for higher switching frequencies, the STTH12S06FP is the better fit despite the lower current headroom.