A buck converter in front of the bridge shapes the bus. It is simple to reason about, and most of the wall runs on it. The buck can be a proper converter or, in QCW 1's phrase, “a plain switch.” The idea of a buck in front of the bridge is older than the ramp. Jan Wagner's 2002 page on the self-resonant SSTC already says that fast power modulation has to be done in a “buck step-down” pre-regulator, for audio in his case.
Sources
This is the card from the driver tree, written from what the builder published. Send corrections through the forum.