In almost all DC-DC converter books, they introduce the small-ripple approximation to make the derivation of the the Vout-Vin relationship easy.
Usually, they assume that the output of the converter is a constant DC with no higher order components.
Then, they derive the inductor current ripple and capacitor voltage ripple with the assumption that the output is constant DC.
That seems like a contradiction - they use the small-ripple approximation on the output voltage (to say it is fixed DC) but then they use this constant output voltage in the derivations to calculate inductor current ripple and capacitor voltage ripple? It is almost like they are only half applying the small-ripple approximation, they only apply to the output voltage, not to the cap and inductor waveforms!