When using a "regular" (i.e. not medical grade) AC-DC isolated SMPS in the following fashion, a strong mains common-mode voltage is created on the DC side due to - I believe - the stray capacitance of the isolation transformer. When attaching one of the DC rails to the Protective Earth (PE) a typical mains frequency leakage current on the order of ~mA is typically observed to return through the Earth contact.
I was wondering what "medical" grade SMPS do different to achieve a much lower mains leakage to the DC side.
I know that the mains frequency leakage current can be prevented entirely using the scheme below, but I guess that this is not what medical SMPS do, because the below scheme would probably achieve a PE leakage at the output of below µA, whereas medical SMPS are usually rated in the ballpark of ~100 µA.