[Beowulf] single power supply for multiple nodes

[Jim Lux]

With modern PWM supply design, the max efficiency you can get is fairly
independent of power output (that is, you're going to consume the same watts
whether you have a raft of little supplies or one big one).  Note that this
is NOT true of "designed for low cost" supplies of the type in PCs, where
efficiency isn't a huge driver (other than what's necessary to meet the CE
harmonic content requirements and the EPA Energy Star).

Here are some tradeoffs:
Single supply has fewer total components than multiple supplies, so, the
probability of any one supply failing is lower.  However, the impact of that
supply failing is higher, since it takes down multiple nodes.
Power distribution from a big supply will be a problem.  It's one thing to
run 50A at 3V a few inches from PS in the chassis to the mobo, it's another
to run it 2-3 meters.Don't forget that you need regulated voltages, and a
few 10s of milliohms of resistance will result in the voltage at the load
changing under load. 2-3 meters of AWG 10 (which is fairly large) will be in
the 10-20 mOhm range (especially with connectors). Yes, you can use remote
sense wires back to the PS, but that brings in its own set of problems
(stability, more wires, etc.)

If you're ever thinking of servicing the nodes, you'll probably need to
think about connectors and/or service loops in the power cables.  For
distributed power conversion (as used conventionally), you're distributing
power at a reasonably high voltage, and with exceedingly standardized (and
cheap!) connectors and cables.
The actual power conversion hardware is rougly constant power density.  That
is, converting 1 Watt from AC to DC takes about the same volume and mass,
regardless of scale (within reason).  You'll save on the packaging volume (1
chassis instead of N, with the one big supply) even if the innards scale
normally.

Jim Lux

----- Original Message -----
From: "Frank Joerdens" <frank at joerdens.de>
To: <beowulf at beowulf.org>
Sent: Monday, June 28, 2004 11:33 PM
Subject: Re: [Beowulf] single power supply for multiple nodes


> On Mon, Jun 28, 2004 at 04:04:47PM -0700, Joel Jaeggli wrote:
> []
> > at the point where you get a whole rack of nodes off a single
> > powersupply the size of the conductors for some of the voltages get
quite
> > large (the 3.3volt and 5volt especially if you need a couple hundred
> > amps)...
>
> Having large conducters needn't necessarily be a problem. You might get
> creative and use aluminium tubes that could even double as part of the
> vertical support structure of the rack itself.
>
> If you can power 4 dual opterons off a KW, and you're thinking of
> stacking 20, that'd be 5 KW, a.k.a. a 1,000 amps at 5 Volts and a little
> over 1,500 at 3.3. Sounds a lot, I admit. The power supply itself might
> get just too expensive that way ...
>
> > 1kw wouldn't
> > be that huge anyway that's enough for 4-6 dual opteron nodes.
>
> Sorry, I meant multiple-kilowatt; wasn't expressing myself very well.
>
> I'm not trying to argue at this point that this would make sense,
> economically or otherwise, just sorta thinking about it.
>
> Regards,
>
> Frank
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