> The power transformer that crosses the boundary should also have a split bobbin, so the AC line side coil and the output side coil are separated by a barrier,

> The power transformer that crosses the boundary should also have a split bobbin, so the AC line side coil and the output side coil are separated by a barrier, not wound on the same bobbin. Separate bobbin is not required for robust isolation. For example, just right now on my bench there is a medical separation transformer. No separate bobbin (hint: it is toroidal).