[Beowulf] Interesting google server design

Donald Becker becker at scyld.com
Thu Apr 2 11:35:30 PDT 2009

On Thu, 2 Apr 2009, Lux, James P wrote:
> > On Thu, Apr 02, 2009 at 10:11:07AM -0400, Prentice Bisbal wrote:
> > > Or it could be because they make motherboards that convert 
> > 12 VDC to 5 
> > > VDC on the motherboard.
> > Greg Lindahl wrote: 
> > All Itanium and some other x86 boxes take a single 48V input 
> > to the mobo. I talked to a mobo designer once and he claimed 
> > that there was no power savings to be had doing this. Beats me, mon.
> If I were king of the world and could dictate designs, here's what I would do:
> A big bulk 3 phase rectifier to make the 400V bus, which would be fairly
> The batteries hang off this bus at that voltage. (this is standard stuff
> these days, with grid-tie inverters, for instance).

That leads to short battery life.  If you take this approach you really 
want the intermediate voltage to be optimized to the battery float 
voltage.  This is a century-old design used with 48V telephone circuits 
and giant lead-acid cells in the basement, but using it with higher 
voltages and better batteries doesn't work well.
> You distribute the 400VDC (or 350, or whatever it works out to be
> convenient) to all the chassis.

Here is where you run into problems.  If you use telephony-standard 48V, 
you need really hefty power cables.  If you use high voltage, you 
need better insulation and special precautions.  If you run 400VDC to 
the server boards, you need special precautions for the power connections 
and everything connected to the board.  Anywhere it's possible for 
the 400VDC to get to needs to be shielded from human contact and isolated 
from the rest of the system.  A FET shorting 400VDC through a heatsink or 
to a ground plane can fry a whole board, and then the human that goes to 
replace it.

Suddenly running 120V or 240V to isolated metal-boxed power supplies looks 
like a great engineering solution.

> Because you're feeding from an essentially regulated DC bus, the "per
> PC" power supply can be designed with higher efficiency over a narrower
> input voltage range (2-3% should be easy to achieve, with fairly

DC-to-DC converters can be 98% efficient (although 94% is typical), but 
they lose efficency when they have to convert a high ratio.  To drop to 
48V to 3.3V you'll want at least one intermediate stage.. say 12V.  And  
if we are close, we might as well regulate it well and use it for the disk 
drives.  Hmmm, aren't we back at pretty much the standard design?

Donald Becker				becker at scyld.com
Penguin Computing / Scyld Software
www.penguincomputing.com		www.scyld.com
Annapolis MD and San Francisco CA

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