Common-mode　(CM)　conduction　electromagnetic　interference　(EMI)　is　a　challenge　for　switched-mode　power　supply　(SMPS)　designers.　A　flyback　converter　is　a　traditional　topology　used　in　low-power　isolated　applications.　The　size　and　cost　of　EMI　filters　are　important　considerations　for　higher　power　density　and　lower　cost　of　SMPS　design.　In　most　cases,　the　CM　current　is　generated　by　voltage　pulsation　(dv/dt)　assigned　on　the　primary　winding　and　the　secondary　winding.　The　CM　current　conduction　paths　can　be　mainly　categorized　into　two　categories.　One　is　the　interwinding　parasitic　capacitance　of　the　transformer,　and　the　other　is　the　parasitic　capacitance　between　the　semiconductor　switches　and　the　protective　ground.　This　paper　proposed　an　evaluation　method　to　measure　the　equivalent　CM　noise　capacitance　of　the　two　main　CM　current　conduction　paths,　respectively.　Then,　this　paper　proposed　a　CM　noise　cancellation　scheme　based　on　the　transformer　winding　design　combining　the　measurement　technique　and　the　FEM　simulation　tool.　Planar　transformers　with　PCB　windings　also　have　advantages　in　low　profile,　good　heat　dissipation,　and　good　stray　parameter　consistency.　Based　on　these　considerations,　an　18-W　adapter　is　designed　and　tested　to　verify　the　effectiveness　of　the　proposed　evaluation　method　and　the　design　scheme.