rackmount DC power inverters?

Sat Jun 25 07:38:49 UTC 2005

At 03:16 AM 6/25/2005, you wrote:
>I have no idea if this is on or off topic (apolgies if the latter).
>
>Right now we're running 48 1u servers in a cabinet off AC.  We're 
>considering switching to DC power supplies with the hope that any cost 
>increase in the power supply and rectifier would be more than offset by 
>the cost savings in electrical and cooling.

Maybe I'm missing something, but I don't expect they will generate any less 
heat nor will your electric bill go down. Modern switching power supplies 
are very efficient. A DC power supply would either use a linear regulation 
circuit which is less efficient or using a DC-DC converter which is simply 
a switching power supply again to convert the -48VDC into the +/-5VDC and 
+/-12VDC needed by your servers internally. I suspect that if anything, the 
additional DC supplies combined with the loss in efficiency of the 
AC-DC-AC-DC conversion vs. AC-DC will produce more heat and use more 
electricity. Setup a DC power supply on the bench and setup an AC supply 
too. Measure the number of watts used in both cases. Make sure the 
computers are processing the normal workload. DC-DC converters rise from 
40-50% efficiency to near 90% when they are at or near full design power 
output. The efficiency of the CPU, HDDs, etc. will remain the same so any 
variation is due to power supply efficiency differences. More power in with 
the same work out = more heat generated! Also factor in the efficiency loss 
of the DC rectifier you want to buy. If I am off-base here, I welcome any 
differing opinions. Matt, please let us know what your experiment tells 
you.

-Robert


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