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Also, suppose the Power supply which supplies the SMPS is a mains transformer with diode rectifier and smoothing cap.....most of the time, the diodes arent even conducting...so its just going to look like its big output capacitor.....Ie , no ideal voltage source at all......it will be virtually impossible in this case, for the SMPS's input filter to have an output impedance which is more than ten times less than the SMPS Zin at all frequencies from 0 to crossover.

...as the attached shows