1. Current Rating & Conduction Characteristics: The STTH302 has a lower rated current (3A) than the MUR420 (4A). Its forward voltage drop (Vf) is already higher (950mV vs. 890mV) at a lower test current (3A vs. 4A). At the same operating current, the STTH302 will exhibit higher on-state losses and junction temperature, resulting in reduced thermal margin. This is the primary rationale for the derated substitution. 2. Dynamic & Reverse Characteristics: Both diodes share the same reverse recovery time (trr) of 35ns, placing them in a similar performance class. The STTH302's reverse leakage current (3μA) is slightly better than that of the MUR420 (5μA), though this difference is typically negligible in practical circuits. 3. Cost & Manufacturer: The STTH302 (ST) is priced significantly lower than the MUR420 (Vishay), which is the main driver for substitution. However, this must be weighed against the performance trade-off. Differences in manufacturing process, quality control, and long-term reliability data between the two suppliers should also be considered. Core Recommendation: Prior to substitution, a thorough re-evaluation of the maximum operating current, transient peak current, and thermal conditions in the actual application is mandatory. This ensures the STTH302's junction temperature operates with adequate margin. For high-frequency or hard-switching applications, it is strongly advised to measure the switching losses and temperature rise after the replacement.