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Re: rackmount DC power inverters?

  • From: Robert Boyle
  • Date: Sat Jun 25 03:42:05 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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