The STMicroelectronics STTH506D is a 600 V, 5 A fast recovery epitaxial diode in a TO-220AC through-hole package. Its 50 ns reverse recovery time (trr) qualifies it for hard-switched power conversion stages — boost PFC front-ends, secondary-side output rectification in flyback and forward converters, and freewheeling diodes in motor-drive snubbers.
No official replacement part number is listed by ST; a functionally equivalent fast-recovery diode in the same voltage and current class should be qualified on the board before substituting.
The 50 ns reverse recovery time is the key switching parameter. In a continuous-conduction-mode boost PFC operating at 60–100 kHz, a 50 ns trr keeps the reverse-recovery charge low enough that the MOSFET turn-on loss stays manageable without resorting to silicon-carbide diodes. The 5 µA reverse leakage at the 600 V rated blocking voltage is a room-temperature typical; expect the leakage to rise with junction temperature, so the off-state dissipation should be checked at the 175°C limit if the diode runs hot.