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It's not AC because the flow of current doesn't reverse direction. It's pulsed on/off. The last stage isn't rectification, it's an LC circuit to smooth out the resulting voltage. The reason higher frequency switching is for smaller value capacitors and inductors.

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If we're talking off-line power supplies, it most definitely _is_ AC for the simple reason non-ideal transformers will pass no DC. And, further, inductors and capacitors (LC) are all merely losses to DC signals. These components only have a meaningful function for AC waveforms (even though, in the case of a buck converter, a DC signal is superimposed).

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