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Technically all DC-DC converters at a minimum have an AC current in their input and output capacitances. Since we see a small ripple voltage across the capacitors, it must be true that the direction of the current flowing in the capacitors is alternating.

It is true that in most cases the inductor current isn't changing direction though.

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My point was the core of a DC-DC converter like the simple boost converter I mention does not rely on AC to work. It relies on building up a magnetic field in the inductor and then redirecting that stored energy somewhere.

Any AC inside the circuit is due to imperfections, like the reverse recovery time of the diodes or similar.

Sure, direction of current in the magnetics does capture the intended distinction better. Ripple currents are usually non-ideal behavior, and cases where it isn't (discontinuous conduction mode in the boost converter) are probably arguably but needlessly pedantic.

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