[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
www.BeTheSignal.com 
**************************************** 
-----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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