1. Voltage Rating & Technology Platform: The FDP26N40 (400V, UniFET™) is suited for typical single‑phase rectified bus voltage environments. The FCP165N65S3 (650V, SuperFET® III) targets higher‑voltage or more voltage‑stressed applications. Although the latter offers greater voltage margin, it is typically designed for higher‑efficiency, superior‑switching‑performance platforms, with different cost‑versus‑performance tradeoffs. 2. Current & On‑Resistance (Rds(on)): At similar Rds(on) values, the FDP26N40’s rated current (26A) is significantly higher than that of the FCP165N65S3 (19A), and its test‑condition current (13A vs. 9.5A) is also greater. The FDP26N40 likely exhibits lower conduction losses under high current and better thermal performance (supported by its higher dissipation rating of 265W). The substitute part could become a bottleneck for thermal design and current capability in high‑current applications. 3. Dynamic Characteristics & Drive Requirements: The FCP165N65S3 has substantially lower gate charge (Qg: 39nC) and input capacitance (Ciss: 1500pF) compared to the FDP26N40 (Qg: 60nC; Ciss: 3185pF). While the former offers very fast switching and low switching losses, it imposes stricter demands on the drive circuit’s peak current capability and layout noise immunity. Direct substitution may result in insufficient drive, switching oscillations, or EMI issues, preventing its performance advantages from being realized.