The　flyback　topology　is　being　widely　used　in　power　adapters.　The　coupling　capacitance　between　primary　and　secondary　windings　of　a　flyback　transformer　is　the　main　path　for　common-mode　(CM)　noise　conduction.　A　Y-cap　is　usually　used　to　effectively　suppress　EMI　noise.　However,　this　results　in　problems　in　space,　cost,　and　the　danger　of　safety　leakage　current.　In　this　paper,　the　CM　noise　behaviors　due　to　the　electric　field　coupling　of　the　transformer　windings　in　a　flyback　adapter　with　synchronous　rectification　are　analyzed.　Then　a　scheme　with　balance　winding　is　proposed　to　reduce　the　CM　noise　with　a　transformer　winding　design　that　eliminates　the　Y-cap.　The　planar　transformer　has　advantages　in　terms　of　its　low　profile,　good　heat　dissipation　and　good　stray　parameter　consistency.　Based　on　the　proposed　scheme,　with　the　help　of　a　full-wave　simulation　tool,　the　key　parameter　influences　of　the　transformer　PCB　winding　design　on　CM　noise　are　further　analyzed.　Finally,　a　PCB　transformer　for　an　18W　adapter　is　designed　and　tested　to　verify　the　effectiveness　of　the　balance　winding　scheme.