What this Schottky does in the circuit
The STPS3150 is a 150 V, 3 A Schottky rectifier in a DO-201AD axial-lead package. Its forward voltage drop is 820 mV at the full 3 A rated current, which keeps conduction losses manageable in a power supply output or OR-ing diode position. The fast recovery characteristic — ≤ 500 ns at currents above 200 mA — matters when the diode is switching at medium frequency, though a Schottky's majority-carrier action already gives it a speed advantage over a standard recovery diode.
150 V, 3 A — the numbers that define the replacement
The reverse voltage rating of 150 V gives headroom on a 48 V or 72 V bus, but not on a 120 V rectified line. The 3 A average forward current is the continuous rating; peak repetitive currents will be higher but limited by the junction's thermal capacity. Reverse leakage at 150 V is specified at 2 µA, which is typical for a Schottky of this voltage class. When searching for a functional replacement, these three numbers — 150 V Vrrm, 3 A If(AV), and the 820 mV Vf at 3 A — are the starting point.
STMicroelectronics lists the STPS3150 as obsolete.