Forward drop and the conduction loss trade-off
The STPS5H100AF is a 100 V, 5 A Schottky rectifier in the SOD-128 flat package. At the rated 5 A forward current the maximum forward drop is 760 mV — that is the conduction loss floor at full load, and it sets the heat that must be sunk through the exposed pad to the board copper. Reverse leakage at 100 V is 3.5 µA typical. In a lightly loaded rail or a standby supply that leakage current adds to the quiescent draw; at elevated junction temperature it rises exponentially, so the 175 °C max junction rating is the thermal ceiling, not a continuous operating target.
The SOD-128 flat package (SOD128Flat per the supplier device code) is a surface-mount footprint with an exposed cathode pad. The thermal resistance to ambient depends on the copper area under that pad — a 1-inch² pour on a 2-layer board is the typical starting point for the datasheet's RthJA figure.