Xiaofan Chen wrote:
> Say if you have a single-stage buck converter, the efficiency from
> 24V->5V will be better than 48V->5V. 12V-->5V is even better.

Not necessarily.  There is no theoretical basis for your statement, and this
is highly dependent on the circuit.  There are various factors that get both
better and worse as voltage goes up.

For example, one product I designed works from a wall wart that can provide
as low as 20VDC, or POE which is nominally 48VDC.  The circuit itself was
designed to work up to 60V for some margin.  The first stage power supply
takes this input and bucks it down to about 5.5V.  I have measured the power
supply, and efficiency is pretty constant accross the 20V - 60V range.


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