> The interesting question is, is it more efficient to always let 12V->5V regulators run for 5VSB or to use an extremely small separate regulator? To complicat

> The interesting question is, is it more efficient to always let 12V->5V regulators run for 5VSB or to use an extremely small separate regulator? To complicate this further, full system power-off is not the only time you want to do wake-on-lan... if you want to wake from system sleep then you have to be running the 12V rail anyway, because that 12V rail drives the VRMs which drive Vcore and Vmem. No 12V rail means you lose your memory and CPU state... As such I don't think you can really frame this as "having a separate 5V regulator with the 12v rail turned off", or at least that use-case must exist in addition to the deep-sleep use-case, it cannot supplant it entirely. Anyway though, power-supply 5V rails are usually much much larger than they need to be, and that has higher loss inside a transformer. Even if you make that a solid-state thing inside the transformer, moving power at 5V is less efficient than moving it at 12V and stepping it down at the point-of-consumption. So actually the answer is probably that it's more efficient to always run a step-down regulator.