Hasn't that problem mostly been solved already with switch mode power supplies that are pretty small.
With most personal electronics being battery powered these days I'm curious where you see a real need for further reduction in the size of power converters. In other words what technologies or devices are currently limited by the size of their power converters ?
I waste a ton of board space with power supplies. Not everything is personal electronics with custom ICs* in the volume of an iPhone. So this means on my current design there's power supplies for 1v, 1v8, 2v5, 3v3, ~4v, 5v. Each of these the biggest component is the inductor.
I've sometimes joked, I make power supplies with a computer attached.
Edit: since the volume is low we use off the shelf parts which means we tend to end up with a variety of voltage requirements. And yes...reduction of rails is something that's part of components selection, alas....
Perhaps you are thinking of "wall wart" power supplies that used line-frequency transformers in their rectifier design. These have been replaced and miniaturized by using offline switched-mode power supplies and point-of-load converters, but the magnetic components remain the largest components in these designs.
> With most personal electronics being battery powered these days I'm curious where you see a real need for further reduction in the size of power converters.
It's now common for modern SoC devices to require multiple voltage rails. It may require 3.3 V, 1.35 V, 1.8 V on separate pins and each requires a point-of-load converter for efficiency, and each of these take up board space.
It would still be useful to make them even smaller. For example, it’s often not possible to use all sockets in a power strip because of the size of power converters. Also, such smaller converters typically are a bit of a hassle to toss into a bag because of their somewhat awkward shape (especially if all other things in the bag are pencil-shoes or flat slates (laptop, tablet, notebook)
A transformer that fits inside an AC power plug would solve both problems.
I think that, _if_ we could build that, most people would start calling the current tiny converters bulky.