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

Well put.


> [Original Message]
> From: Eric Bogatin <eric@xxxxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Cc: <eric@xxxxxxxxxxxxxxx>
> Date: 6/19/2007 10:32:11 AM
> Subject: [SI-LIST] Re: HIGH DC Current on GND Plane
>
> Guys-
>
> Just to throw in some numbers to this discussion, keep in mind that the
> sheet resistance of 1/2 oz copper is about 1 mOhm/square. Of course, 1 oz
> copper is 0.5 mOhm/square.
>
> In the simplest case, if all the current is injected in a bar from one
edge
> of the power plane, and it goes to the other end, a distance equal to its
> width, the resistance is 1 square. 
>
> For 1/2 oz copper, at 100 A, the voltage drop is 100 A x 1 mOhm = 100 mV
per
> plane. You can scale these numbers roughly depending on the aspect ratio
of
> the board, the number of power layers and the copper thickness of each
> layer.
>
> This is as much as you can do with a simple estimate. It represents a best
> case estimate. The complication arises when the current is not injected
in a
> bar from one end of the board to the other, but as via contact pads from a
> connector to the via contacts of a BGA. Then, you get constriction
> resistance that can increase the series resistance by a factor of 2-5
> depending on the geometry.
>
> To minimize the increase in constricting resistance, you want the current
to
> be able to spread out as much as possible and have the lowest possible
> current density throughout the entire path. This means, large via holes
for
> the power connector, multiple via contacts spread out along the board
edges
> from the power connector, multiple via contacts to the BGA, spread around
> the periphery of the package footprint.
>
> The constricting resistance can easily dominate the series resistance in
the
> planes. Since it's hard to estimate the impact of the constricting
> resistance with simple approximations, especially for complex via contact
> patterns, it's important to use 3D planar or full 3D tools, such as
Ansoft's
> Q3D and SI wave, Optimal, Agilent's Momentum, CST and others.
>
> Of course, one way of getting around some of the low frequency (DC) IR
> voltage drop problems is to use sense lines located on the BGA package and
> regulate the DC output voltage based on the local voltage at the load.
This
> works up to the feedback loop bandwidth of the supply, typically in the
> audio range. Then it's about the PDS design.
>
> --eric
>
>
> **************************************
> Dr. Eric Bogatin, President
> Bogatin Enterprises, LLC
> Setting the Standard for Signal Integrity Training
> 26235 w 110th terr
> Olathe, KS 66061
> v: 913-393-1305
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> e:eric@xxxxxxxxxxxxxxx
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> **************************************** 
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On
> Behalf Of Lee Ritchey
> Sent: Tuesday, June 19, 2007 11:02 AM
> To: Eddy; si-list@xxxxxxxxxxxxx
> Subject: [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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> > Browse Top Cars by "Green Rating" at Yahoo! Autos' Green Center.
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