Which UPS?

[James Knott]

Scott Allen wrote:
> On 12 June 2012 11:59, James Knott<james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org>  wrote:
>> Whether AC or DC supply to the power supply, there are two stages involved,
>> conversion of AC to DC and then converting that DC to the required voltages
>> through a switching converter/regulator.  I'd have to question why having
>> both in the same package results in higher efficiencies of the order you
>> claim.
> I showed some math backing my claim, but I had to estimate the
> efficiency of converting 120VAC to 48VDC in the UPS (and I think I was
> quite liberal at 85%). Show me the math that would have you question
> my claim.

An AC supply has two stages, rectifying and converting/regulating.  A DC 
supply would have only the one stage, as rectifying wouldn't be 
necessary.  How can two stages be more efficient than one, particularly 
if the converting/regulating stage is essentially identical either way?
>>   Many "server farms" run on about 200V DC or so and get better
>> efficiencies than AC powered systems.  The rectified DC inside a typical
>> power supply would be in the range approaching 170V.  The conversion of the
>> high voltage DC to the voltages required by the computer should have about
>> the same efficiency, no matter what the source, all else being equal.
> How is that 200VDC produced? Is it regulated? Just rectifying AC is
> quite efficient. It's chopping and changing to different voltages and
> regulating that costs efficiency. Also, multiple conversions cost
> efficiency.
>
There would be large rectifiers and battery banks for the DC.  And yes, 
multiple conversions cost efficiency.  In a data center, UPS is a must.  
With AC supply, you rectify and then convert back to AC in the UPS, 
then, in the computer rectify again, covert back to AC for the converter 
and then rectify again.  Having a DC supply eliminates the first DC to 
AC and second AC to DC stages.

BTW, I read about this in an article about air conditioning savings in 
server sites that have high voltage DC supplies, rather than AC.  Those 
sites may consume 50 KW or more for running the computers and generate a 
lot of heat in the process.

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