Planar　PCB　high-frequency　transformer　is　the　key　component　that　affects　the　characteristics　of　conducted　common　mode　(CM)　electromagnetic　interference　(EMI)　of　switching　power　supply.　In　this　paper,　the　calculation　method　of　the　effective　capacitance　of　the　CM　port　of　the　planar　transformer　is　explained,　and　the　effective　capacitance　of　the　CM　port　of　the　planar　transformer　is　further　determined　by　the　simulation　method.　Then,　the　method　of　CM　EMI　characteristics　evaluation　of　planar　transformer　insertion　loss　(IL)　is　explained　and　expounded,　and　its　simulation　and　measurement　methods　are　introduced.　Finally,　based　on　the　analysis　and　research　of　the　defects　of　the　original　transformer　IL　model,　the　high-frequency　model　of　transformer　IL　is　proposed,　and　the　advantages　and　effectiveness　of　the　proposed　high-frequency　model　in　the　conduction　frequency　band　are　verified　by　the　actual　measurement.　©　2023　IEEE.