In　order　to　construct　a　low-impedance　loop　for　common-mode　electromagnetic　interference　(EMI)　signals,　traditional　method　is　to　use　Y-capacitors　as　filtering　components.　However,　in　the　commonly　used　isolated　ac-dc　switching-mode　power　supplies　(SMPS),　the　Y-capacitors　branch　also　behaves　as　a　terrible　leakage　current　loop.　For　the　safety　of　human　beings,　this　leakage　current　is　not　allowed　in　commercial　used　equipment,　such　as　chargers,　medical　instruments,　etc.　Therefore,　certain　works　should　be　done　to　both　acquire　good　EMI　performance　and　satisfy　the　strict　leakage　current　limitation.　The　goal　of　this　paper　is　try　to　meet　these　two　demands　at　the　same　time.　In　this　paper,　a　novel　non-Y-capacitor　EMI　design　concept　for　SMPS　is　proposed　for　the　first　time.　By　getting　rid　of　traditional　EMI　filtering　component-the　Y-capacitors,　the　leakage　current　can　be　eliminated　entirely.　Meanwhile,　to　face　with　EMI　design　challenge,　optimized　transformer　architecture　is　presented.　Analysis　of　the　transformer　architecture　as　well　as　the　auxiliary　winding　has　been　carried　out.　Then,　a　novel　topology　suitable　for　non-Y-capacitors　converter　is　proposed　and　the　design　procedure　of　the　proposed　topology　is　discussed　in　detail.　The　proposed　concept　is　applied　to　several　popular　converter　topologies.　The　experiment　results　demonstrated　the　effectiveness　and　feasibility　of　the　proposed　non-Y-capacitor　design　schemes.