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Φ Abstract --This article describes observations about power MOSFET failures and degradation experienced during accelerated testing involving high temperature and high humidity stresses. The examined power MOSFETs were operated in commercial variable speed drives in flyback transformer topology as power switches. Known power MOSFET failure mechanisms are summarized and electrical stresses typical for the flyback topology are reviewed. In addition, effects of electrical stresses due to power interruptions were studied. The power MOSFET failure analysis results are presented. The visual appearance of the samples with catastrophic damage was examined with such analysis methods as X-ray, acoustic microscopy (SAM) and optical microscopy. The samples with no obvious failures were also analyzed in research for electrically measurable failure precursor parameters to characterize the physical degradation of the devices. Under these test circumstances, the power MOSFET channel off-resistance R DS-off was discovered to have degraded. This resistive leakage phenomenon was also visualized with backside OBIRCH technique.