What this Schottky pair is and where it goes
The STPS40L45CG is a dual Schottky rectifier from STMicroelectronics, configured as a common-cathode pair in a single D2PAK surface-mount package. The low forward drop of 530 mV at 20 A keeps conduction losses down in high-current output rectification stages — think switch-mode power supply secondaries, OR-ing diodes in redundant power feeds, and freewheeling clamps in DC-DC converters. The common-cathode layout pairs naturally with centre-tapped transformer windings or as two independent diodes sharing the same output rail.
ST lists the STPS40L45CG as obsolete. If this part is on your BOM, you are looking at last-time-buy inventory that is already in the field or surplus stock held by independent distributors. For a production line that still needs this footprint and current rating, the only path is sourcing from the open market — and that means verifying date codes, storage condition, and authenticity before the reels hit the pick-and-place.
The D2PAK (TO-263) footprint with the tab soldered to the PCB copper plane is the intended thermal path — the tab is the cathode common node, so it carries both current and heat.