Anti-phase　winding　is　an　effective　method　to　suppress　the　CM　EMI　conducted　noise　in　Boost　converter.　However　the　parasitic　parameter　of　the　anti-phase　winding　has　critical　effect　on　high　frequency　ranges.　By　the　detail　analysis　of　the　CM　EMI　equivalent　model　of　Boost　converter　considering　the　parasitic　parameter　of　the　anti-phase　winding,　the　key　factor　is　confirmed　which　is　due　to　the　serial　resonant　between　the　leakage　inductance　and　stray　capacitance　of　secondary　winding　plus　the　compensated　balance　capacitance.　Optimizing　the　parasitic　parameters　will　bring　the　benefit　to　increase　the　resonant　frequency　and　expend　the　effective　bandwidth　in　CM　noise　suppression.　Based　on　the　theoretical　analysis,　a　novel　method　is　also　proposed　which　use　an　anti-phase　1:1　transformer,　instead　of　anti-phase　winding.　With　the　new　method　the　leakage　inductance　and　the　stray　capacitance　can　be　controlled　to　decrease　greatly,　resulting　in　great　increase　of　effective　bandwidth.　The　result　of　experiment　shows　that　a　much　higher　effective　bandwidth　and　then　the　much　great　CM　noise　suppression　in　higher　frequency　ranges　can　be　obtained　using　the　anti-phase　transformer　than　that　using　the　anti-phase　winding.　©　2013　Chinese　Society　for　Electrical　Engineering.