In　isolated　power　converters,　the　transformer　is　a　key　part　of　voltage　transformation　and　isolation.　Since　common-mode　(CM)　noise　is　rather　difficult　to　suppress　compared　with　different-mode　(DM)　noise,　more　and　more　scholars　are　paying　attention　to　the　characteristics　of　CM　noise,　especially　in　high-frequency　CM　noise　behaviors.　CM　noise　can　be　further　divided　into　conducted　CM　noise　and　radiated　CM　noise,　and　the　main　focus　of　this　paper　is　on　conducted　CM　noise.　The　CM　coupling　capacitance　of　the　transformer　is　one　of　the　main　contributors　of　CM　noise,　which　has　been　verified　in　many　previous　studies.　Hence,　eliminating　the　CM　noise　in　a　transformer　coupling　path　can　significantly　lower　the　whole　CM　noise　level　of　the　converter.　Professional　conducted　electromagnetic　interference　(EMI)　testing　instruments　are　quite　expensive.　In　this　paper,　a　general　transformer　evaluation　technique　for　CM　noise　behavior　is　proposed.　Only　a　signal　generator　and　oscilloscope　can　achieve　transformer　CM　noise　behavior　evaluation.　PCB　planar　flyback　transformers　are　designed,　and　a　series　of　noise　spectrums　and　voltage　waveforms　can　verify　the　effectiveness　of　the　proposed　transformer　evaluation　method.　The　flyback　adapter　porotype　can　pass　the　EMI　standard　limited　line　EN55022　class　B　by　the　proposed　evaluation　method.