1. Switching Performance & Drive Requirements: This is the most critical distinction. The FCP380N60 exhibits significantly higher gate charge (Qg) and input capacitance (Ciss) than the STP18N65M2 (Qg: 40nC vs. 20nC; Ciss: 1665pF vs. 770pF). At the same switching frequency, the FCP380N60 demands higher drive current, resulting in increased switching and gate drive losses. The existing driver circuit may lack sufficient peak current capability, leading to slower switching transitions and elevated temperature rise. 2. Rds(on) Test Conditions: The specified maximum Rds(on) values are similar (380mΩ vs. 330mΩ), but the test currents differ (5A vs. 6A). This implies their conduction characteristic curves likely differ, necessitating re-evaluation of conduction loss under actual operating current based on specific application conditions. The ST part maintains a lower resistance at a higher test current, suggesting potentially better performance in its linear region. 3. Voltage Ratings & Thresholds: The FCP380N60 has a 50V lower Vdss rating (600V vs. 650V), requiring the original design to have sufficient voltage derating margin (e.g., for 310V bus or PFC applications below 400V). Its Vgs(th) is slightly lower (3.5V vs. 4V), offering a marginally lower noise immunity threshold, while its Vgs(max) is also lower (±20V vs. ±25V), reducing the design margin for the gate drive voltage. 4. Technology Platform: The STP18N65M2 utilizes ST's MDmesh™ M2 technology, focusing on a balance of low Qg and low Rds(on). The FCP380N60 employs onsemi's SuperFET® II technology, emphasizing low conduction loss. This difference in technological approach directly manifests in the aforementioned trade-off between switching and conduction characteristics. Summary: In hard-switching topologies (e.g., flyback, PFC) with high switching frequencies, substitution may lead to reduced efficiency and thermal management challenges. For low-frequency switching or linear mode applications with adequate voltage margin, substitution feasibility is higher. It is imperative to re-validate driver capability, switching waveforms, and temperature rise.