SuperFungus

There's quite a few practical details that weren't addressed. Whats the self discharge of these caps? Sure, maybe they can hold a ton of energy in a small volume but who cares if it bleeds off in 2 hours? What's the performance over temperature? Does the capacity drop off at temperature? Does the available power drop off? A lot of conventional supercaps can even permanently age quite quickly at moderately high temperature.

Thats the discussion. In reality, you care about both. Power density to move the energy into and out of the cell quickly to shorten charge times and supply thirsty loads. Energy density to run lower loads for longer.

You don't need superconductors to get around the skin effect. It's actually interesting that you mention superconductors, as the skin depth in a perfect conductor is actually zero. (Anyone know what happens at DC in theory in a perfect conductor? I originally thought it was zero at DC as well, but to me skin depth seems like it becomes undefined.)

Even without superconductors, at 100 MHz the skin depth is still around 2 thousandths of an inch. Litz helps too. High power RF transistors exist. All the things you mention are limiting factors, but I still say the lack of good magnetic materials is the dominant barrier to higher frequency converters.

"Further increases are not recommended due to increased hysteresis loss."

This is why I am REALLY hoping the materials revolution everyone keeps talking about takes off and we get some decent high frequency, high permeability, low loss, high flux density ferrites out of it (not too much to ask huh?). You can do lots of cool stuff with magnetics, even beyond just power conversion but the materials to do it really well just dont exist.

I can see the advantage, but I still would argue that just having some DC power standard like USB will always be more economical and practical than trying to build "do it all" supplies to work with all gear despite the absence of a standard.

Hey, not to shoot you down but there is definitely a transformer in the iphone supply. Second, building switching supplies that can supply an arbitrary output voltage/current isn't practical or economical. Third, don't you want a power supply for every piece of equipment? You don't really want to have to unplug one thing to plug another thing in do you?