DEMUX, thanks for the prompt and exhaustive answer.
As for the second part, that is exactly the information I was looking for. I will go looking at lostcircuits.
As for the first part, I am still not sure I am getting your point (but then, I am not an EE major). While there might a theoretical advantage to have the intermediate conversion voltage at the square root of the two, I am pretty confident that this effect will be swamped by the actual implementation (topology, controller and quality of switches and chokes). Again, just pointing out circumstantial evidence: why would Intel for this particular motherboard as well as Dell, IBM (now Lenovo), HP and most Japanese laptop makers choose an external power supply voltage of about 20 V when they are based in and have their main markets in countries with 100 to 120 V mains? According to your formula, the optimum conversion point between 180 V rectified and filtered mains and something like 1.3 V operating voltage would be closer to 12 V.
So what we should be doing is exactly what has been done for AMD FM1 and Bobcat/Zacate boards:
- choose those that have both good idle power as well as full load power to identify those with efficient on board power conversion
- among those, chose those with the BIOS most amenable to undervolting CPU, IGP/NB and residual chipset
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