[SI-LIST] Re: HIGH DC Current on GND Plane

In the last seven years, I have done the SI work on 3 half rack router
style products, all of which used more than 5 KW at 48 volts.  The maths
says that is around 100 amps.  Every plane, both ground and power, are 1/2
ounce.  There are certainly voltage drops across the planes, but their
magnitude is much smaller than people guess. The worst case is on the order
of 15 mV.

There is good science out there to allow any engineer to calcualte the
voltage drops in planes.  It's been around a very long time.  Using it
would ease some of the fears that seem to pop up on this site.  It would
also enlighten engineers as to how good copper is as a conductor- much
better than most expect.

Lee Ritchey


> [Original Message]
> From: Eddy <eddyvk@xxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 6/18/2007 8:36:59 PM
> Subject: [SI-LIST] Re: HIGH DC Current on GND Plane
>
> "Saturation at DC is spelled meltdown"  ROTFLMAO
>
> 60 Amps is an awful lot. That's like the current
> required to start a good size car! My first thought
> was "that has got be noisy as hell!". Theoretically
> the DC current should not cross-talk anything into
> your signal lines but I have a very hard time
> believing that 60 Amps of current can be quiet enough.
> However, I have seen Amps being confused with
> milli-Amps before....  60 Amps is unrealistic to be
> flowing through a PCB. The copper would be curling off
> the FR4. There has to be a typo here somewhere....
>
> Eddy
>
> --- "Townsend, Fred" <ftownsend@xxxxxxxxxxxx> wrote:
> >
> > I don't know the parties involved here but I think I
> > would start
> > analysis with psychoanalysis. It would take a lot of
> > convincing for me
> > to buy into this methodology. You don't say if the
> > 60 Amps is static but
> > if it changes there will certainly by cross talk!
> > Can you say ground
> > bounce? Normally one would want such current on the
> > outside of the board
> > where I would be looking for 4oz copper if your
> > vendor can supply it.
> > Saturation at DC is spelled meltdown. Have you done
> > temperature rise
> > analysis?=20
> > 
> > The only good news about this design I can see is
> > iron magnetically
> > saturates. Air and epoxy doesn't.
> > 
> > Fred Townsend
> > 
> > 
> > -----Original Message-----
> > From: si-list-bounce@xxxxxxxxxxxxx
> > [mailto:si-list-bounce@xxxxxxxxxxxxx]
> > On Behalf Of Joe Paul M
> > Sent: Saturday, June 16, 2007 3:07 AM
> > To: si-list
> > Subject: [SI-LIST] HIGH DC Current on GND Plane
> > 
> > 
> > 
> > I have a midplane carrying 60Amps of current and
> > Same GND plane is used=20
> > as reference for multiple of 6.6 Ghz serdes lines ,
> > PCI-e lines and GE
> > lines
> > Is there anything similar to 'saturation' in
> > Inductors applicable to GND
> > 
> > plane?
> > ( Due to saturation of flux in the flux carrying
> > medium , beyond a=20
> > certain amount of DC current inductors saturate and
> > wont act as=20
> > inductors for any AC superimposed on the DC. )
> > Will cross talk increase due to the absence of
> > mutual inductance between
> > 
> > signals and gnd plane ?
> > Is there anything else that can go wrong due to this
> > DC , AC current=20
> > mixing in planes ??
>
>
>
>  
>
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