Abstract: Power devices are the core components for energy conversion in switched-mode power supplies, yet they represent the most vulnerable elements prone to failure. Investigating their failure mechanisms and establishing appropriate models are essential for improving system reliability. This paper first analyzes the failure process and degradation mechanisms of power MOSFETs, identifying the on-state resistance as a key sensitive parameter. A parameter acquisition circuit is then designed, and to address the significant random noise in sampled signals, a stochastic-effect nonlinear Wiener process considering measurement noise is adopted for modeling. Finally, the effectiveness of the proposed failure model is experimentally validated.Keywords: switched-mode power supply; power device; on-state resistance; failure model