[Beowulf] Interesting google server design

Lux, James P james.p.lux at jpl.nasa.gov
Thu Apr 2 15:01:49 EDT 2009


> However, I would imagine this is a trivial amount at 12 VDC 
> vs 5 VDC and the incredibly short distances the electricity 
> travels on a PCB trace.

Actually, it's a non-trivial amount. Consider that a processor that draws 100W at 3.3V is drawing 30Amps. 

If you're using 2oz copper (typical for a high power application), and you allow a 10C rise, the trace, if inside the board, needs to be 1.7  inches  wide.  A 3 inch long trace will have a loss of 0.4W (which is small).

If you can put the trace on the outside of the board, it can be about half the width, but you'll lose a watt in a 3" long trace.

Drop the current to 10A, and you're down to a .4" trace inside the board, and .15" wide outside.

You could let it get hotter, but that gets you into junction temperature problems and/or thermal distortion issues.

Play around here
http://www.circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
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