Sandwich　winding　is　usually　used　for　low　conduction　loss　and　low　leakage　inductance　in　insolated　converters.　But　it　also　tends　to　deteriorate　the　common　mode　(CM)　EMI　noise　because　of　the　bigger　parasitic　capacitances　between　primary　and　secondary　windings.　This　paper　analyzes　the　CM　noise　transferring　path　for　typical　isolated　converters.　Influence　of　different　ground　point　location　of　the　shielding　on　CM　noise　is　investigated.　A　novel　CM　noise　reduction　method　by　optimizing　the　spacing　between　shielding　and　windings　is　proposed.　The　advantages　of　the　proposed　concepts　and　strategies　are　verified　and　demonstrated　on　a　Fly-back　converter.　More　than　20dB　CM　noise　attenuation　is　achieved.