Power supplies (addendum)

Bari Ari bari at onelabs.com
Wed May 23 10:40:52 PDT 2001

Robert G. Brown wrote:

> Dear List,
> Mark Hahn and I have been continuing the PS discussion offline, as I was
> a decade or so off in assuming that PC power supplies have big
> transformers.  The incoming voltage is converted to high frequency and
> stepped down with relatively small transformers before being cleaned up
> and regulated and delivered.  My bad.  A useful link on power supplies:
>   http://www.howstuffworks.com/power-supply.htm
> One thing that came up in our discussion that I hadn't really thought
> about but that is obvious in retrospect is that a "300 Watt" ATX supply
> doesn't deliver 300 Watts to any particular component in a computer --
> this is the (approximate) aggregate of the power delivered on the
> various voltages provided by the supply, with the bulk of the power
> delivered to the +5V and +12V lines.  A typical 250 W supply might
> deliver about 120 W to each of these and a handful to the other lines.
> Thus the motherboard and electronics in a computer with even a 300 W
> power supply might only be able to draw 150 W total, leaving 140 or so
> to run the peripherals on the 12V line.
If you use only 2.5" hard drives you can then eliminate the +12VDC 
supply altogether. The -5 and -12  aren't handy for much in a cluster 
either unless you're building a cluster for audio or multimedia :-) and 
even then you can generate these onboard from the +5. This leaves you 
with just +3VDC and +5VDC as a requirement.

> In some sense this is way more than most beowulf node designs "need" for
> the peripheral supply -- a diskless design or design with only one disk
> might need only 30-50W to run the disk(s) and cooling fans (a useful
> table of typical power requirements is on the article linked above).
> However, the requirements of the motherboard are not so flexible.
> Memory, CPU, the motherboard itself, all of these eat energy to run and
> 150W could very easily be needed to support a full memory configuration.
> You therefore might want to look at the details of how any given supply
> distributes and delivers power -- this is probably what gets a power
> supply "certified" by a CPU vendor more than the absolute wattage.  The
> site above reiterates my earlier post that suggested that it is a bad
> idea in general to operate a power supply at 100% of capacity.  Power
> supplies do indeed get hot in operation and contain large heat sinks
> buffering electronics and they get unhappy if overheated.  A large
> supply has more thermal capacity than a smaller one.
>    rgb

If you wish to use one large supply to power multiple boards the issues 
you run into are isolating the overcurrent fault protection, supply 
distribution and power management. You don't want one board causing a 
current overload to bring down the whole cluster. You also need to 
account for voltage drop and regulation between the supply and each 
node. Most cluster users probably won't care much about power down or 
sleep modes but if you do then you have to have isolated supply 
switching between nodes and 5VSB and 3VSB for stand-by.


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