1. Gate Charge (Qg): The AUIRFR8403 (99 nC) is 2.15 times that of the FDD9409‑F085 (46 nC). Implication: In switching applications such as DC‑DC converters or motor drives, this means driving the AUIRFR8403 requires more than twice the gate drive current or results in longer switching times. If the original gate driver circuit lacks sufficient margin, a direct replacement would cause a sharp increase in switching losses, excessive MOSFET temperature rise, and potential overloading of the driver IC. This is the primary obstacle to substitution. 2. Maximum Power Dissipation (Pd): The FDD9409‑F085 (150 W) is 52% higher than that of the AUIRFR8403 (99 W). Implication: This reflects differences in package thermal resistance (RthJC) and die thermal performance. Under identical power losses, the junction temperature of the AUIRFR8403 will rise more rapidly, significantly reducing thermal headroom. To achieve the same operating current, the system may require improved heat‑sinking; otherwise, the device must be derated. 3. Relationship Between On‑Resistance (Rds(on)) and Continuous Current (Id): The FDD9409 is specified with a lower Rds(on) at a higher current (3.2 mΩ @ 80 A vs. 3.1 mΩ @ 76 A), while its rated continuous current is slightly lower (90 A vs. 100 A). Implication: This highlights differences in manufacturer testing and definition standards. Although the AUIRFR8403 appears to have a higher current rating, its lower maximum power dissipation becomes the practical bottleneck for continuous current capability. This discrepancy reminds engineers that current ratings should not be judged solely on the Id spec; thermal parameters must be considered in the assessment.