How do medical SMPS achieve lower Earth leakage compared to "regular" AC-DC SMPS?

Question

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.

^{simulate this circuit – Schematic created using CircuitLab}

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.

^{simulate this circuit}

Answer 1

I've seen a medical power supply design (of the switching type) use a shield between primary and secondary. The shield was connected to rectified/smoothed AC and so the switching noise would be shielded from getting onto the secondaries and thus EMC compliance would be achieved at some small loss of power efficiency and at a small extra BoM cost.

Compare this with a conventional switching power supply. It will use a 1 nF or 2.2 nF 'Y' capacitor from the low voltage secondary back to the rectified/smoothed AC in order to satisfy compliance with EMC regulations. The 'Y' capacitor will pass a little AC frequency through to the secondary and that is what might make it unsafe in a medical application.

The upshot of this is that a direct shielding approach is better for safety in that a 'Y' capacitor is avoided (I can't say for sure in all cases of course) thus, there is very little coupled AC to the secondary circuits. The shield will be a capacitive coupler but, it is likely to have a smaller capacitance between it and the secondary (given the extra transformer winding insulation needed in medical power supplies).

To obtain much less capacitive coupling between windings a UI transformer can be used where primary and secondary are wound on different limbs of the magnetic core. Of course, if a protective earth is available it makes life easier because you can just earth the shield mentioned earlier.

Answer 2

The primary factor is lower line-to-output capacitance. It’s not stray capacitance but rather a deliberately inserted physical Y1 or Y2 safety-rated capacitor or capacitors, used to return AC currents from the switching.

The challenge is to reduce that capacitance and still meet EMC standards.