300 V, 40 A — a workhorse rectifier for line-frequency jobs
The STTH40P03ST is a standard recovery rectifier from STMicroelectronics, rated for 300 V reverse voltage and 40 A average forward current. The recovery time is over 500 ns — this is a slow diode, intended for 50/60 Hz mains rectification, not for a high-frequency switching supply.
TO-220 package — easy to mount, easy to heatsink
The part comes in a standard TO-220-3 through-hole package. The metal tab is the anode connection; the two outer pins are cathode and anode. The tab can be bolted to a heatsink with an insulating washer — the 40 A rating demands thermal management. The junction temperature ceiling is 175 °C, so the thermal path from tab to ambient needs to keep the junction below that under full load.
STMicroelectronics lists the STTH40P03ST as obsolete.
Leakage and reverse recovery — the fine print
Reverse leakage is 50 µA at 300 V, 25 °C. That leakage doubles with every 10 °C rise in junction temperature, so at 125 °C the leakage current can approach several milliamps — enough to shift the bias in a sensitive DC bus. The slow recovery (>500 ns) also means significant reverse-recovery charge at 40 A turn-off, which generates switching loss in any application that commutates the diode at line frequency or above.