1. Rated Current and Forward Voltage Drop: The MUR420 is rated for 4A, higher than the STTH302's 3A rating. Additionally, its forward voltage drop (890mV @ 4A) is slightly lower than that of the STTH302 (950mV @ 3A) under equivalent test conditions. At the same operating current, the MUR420 typically exhibits lower conduction losses and temperature rise, providing greater thermal reliability margin. 2. Current Handling and Thermal Design: The core difference lies in the current ratings of 3A versus 4A. If a direct substitution is made, it is essential to ensure the circuit's maximum steady-state current has sufficient derating (e.g., not exceeding 70-80% of the 3A device's rating, approximately 2.4A). Otherwise, the STTH302 risks overheating, compromising long-term reliability. 3. Reverse Leakage Current: The STTH302 offers superior reverse leakage characteristics (3µA @ 200V) compared to the MUR420 (5µA @ 200V). In high-temperature, high-reverse-voltage applications, the STTH302 contributes slightly lower static power dissipation, though this difference is generally not critical for most applications. 4. Operating Temperature Range: The MUR420 specifies a full junction temperature range (-65°C to 175°C), whereas the STTH302 datasheet only indicates a maximum junction temperature (175°C). For extreme low-temperature startup or storage conditions, the suitability of the STTH302 must be verified.