This would mean, that ripple current in winding ws is equal to zero. Our next move is to make the ripple current of Lext bypass the load. If C is an ideal one with infinite capacitance, its voltage is always equal to output voltage Vo, so nothing changes in the filter behavior after modification, except ripple current no more flows through the load. 3c-d shows other equivalent modifications of the same coupled filter. The main advantage of the above simple analysis is that it can easily be corrected for real elements, containing parasitic parameters.
More general analysis including some parasitic parameters is to be obserwed below.
While learning more about coupled filters, I tried to apply this principle to any basic topology from Fig. As a result, it appeared, that every basic topology has at least two modifications with either input or output zero-ripple property. The only exclusion is Buck I, which I could not find in available literature. So I claim no authorship for these topologies except collecting them together and proposing equivalent schemes, which can be assumed as simplified versions of the originals. As can be seen from the above pictures, Flyback converter has four zero-ripple modifications, while Buck and Boost have only two each.