45 ns reverse recovery — the switching-loss ceiling
The STTH20R04D is a 400 V, 20 A fast-recovery epitaxial diode from STMicroelectronics, in a TO-220AC through-hole package. Its 45 ns reverse recovery time (trr) is the number that decides whether it works in a high-frequency PFC boost stage or a line rectifier.
20 A continuous, 1.7 V forward drop — thermal budget
400 V blocking, 20 µA leakage — rail margin
The 400 V reverse voltage rating covers a 230 VAC rectified rail (≈325 VDC) with 75 V headroom. Reverse leakage is 20 µA at 400 V, 25 °C — negligible for standby power, but it rises with temperature; at 125 °C junction the leakage can increase by an order of magnitude, so factor that into the off-state loss estimate for high-temperature operation.