AC transformers have fixed thermal power density, but load power scales with mass to the 4/3rd power (i.e., scales with linear size by the 4th power, while volu

AC transformers have fixed thermal power density, but load power scales with mass to the 4/3rd power (i.e., scales with linear size by the 4th power, while volume and mass and loss only scale to the 3rd power). And no, 99% isn't hard for a resonant switched capacitor converter. They just happen to be restricted to integer voltage ratios. (With sometimes a few percent regulation around this ratio without substantial efficiency loss.)