The　paper　detail　analyze　the　potential　distribution,　electric　coupling　and　the　principle　of　induced　charge　in　the　primary　and　secondary　windings　of　the　LLC　transformer.　The　analysis　shows　that　copper　shielding　foil　between　the　primary　and　secondary　windings　to　block　the　noise　flow　from　primary　to　secondary　can＇t　eliminate　the　CM　noise　completely　due　to　the　structural　asymmetry.　The　optimize　design　methods　by　adjusting　the　shielding　copper　foil　is　proposed.　The　fact　is　revealed　that　the　CM　noise　by　the　electric　coupling　between　the　primary　and　the　secondary　of　the　transformer　can　be　evaluated　by　the　net　induced　charge　in　the　secondary　windings.　Based　on　the　principle　of　the　minimization　of　the　net　induced　charge　in　the　secondary　windings,　the　optimization　design　methods　of　the　shielding　copper　foil　in　the　LLC　transformer　is　analyzed.　And　the　experimental　results　verify　the　proposed　optimization　design　methods　to　be　correct　and　feasible.　©　2013　IEEE.