What this Schottky pair does in the circuit
The STPS40120CR is a dual common-cathode Schottky rectifier from STMicroelectronics, rated for 120 V reverse voltage and 20 A average forward current per diode. It comes in a through-hole I2PAK (TO-262) package, with a fast recovery time ≤500 ns above 200 mA. The 900 mV forward drop at 20 A is typical for a 120 V Schottky, so conduction losses are predictable in a power supply output or PFC boost stage. The common-cathode configuration directly suits center-tapped transformer rectifiers and half-bridge topologies where two diodes share a single cathode node.
ST lists the STPS40120CR as obsolete. For design-in or volume production, the buyer should verify fit with a cross-reference search against the 120 V, 20 A per-diode, common-cathode I2PAK spec.
The 120 V reverse voltage gives headroom for 85–100 V DC rails in power-factor-correction stages or 48 V telecom rectifiers. The 20 A per-diode average current, combined with the 900 mV forward drop, means about 18 W conduction loss per diode at full load – the I2PAK package's thermal pad must be well heatsunk or the junction temperature will hit the 175°C maximum quickly. The fast recovery ≤500 ns matters for switching frequencies above 50 kHz where reverse-recovery losses in a standard diode would dominate; Schottky recovery is inherently fast, but this spec confirms it's rated for hard-switched converters.