What this Schottky pair does on the board
The Infineon BAS40-04B5000 is a dual series-connected Schottky diode in a SOT-23-3 package, qualified to AEC-Q101 for automotive and high-reliability applications. Each diode handles 120 mA average rectified current with a 40 V reverse voltage rating, and the Schottky construction gives a forward voltage of 1 V at 40 mA — fast switching with low forward drop for signal-level rectification, clamping, and OR-ing in 12 V and 24 V automotive buses.
40 V reverse voltage and 120 mA — where the margin lands
The 40 V reverse voltage gives comfortable headroom on a 12 V nominal rail (derated 50% for load-dump transients) and still holds margin on a 24 V system. The 120 mA per-diode average current covers signal-level loads like relay coil flyback clamping, logic-level translation, and small-signal rectification. Reverse leakage is 1 µA at 30 V, which keeps quiescent draw low in battery-connected modules.
The AEC-Q101 qualification means this part has passed the automotive stress tests: temperature cycling, high-temperature reverse bias, and humidity. It is rated for a junction temperature of 150°C, so it can sit near hot components in an under-hood ECU or a transmission control module without derating the life target.
Package and footprint — SOT-23-3
The SOT-23-3 (TO-236-3 / SC-59) footprint is a standard three-lead small-signal package. The supplier device package is PG-SOT23. It is a surface-mount part, so it reflows with the rest of the board. No special thermal pad or via pattern needed at 120 mA — the junction stays well within the 150°C limit with standard copper traces.