[SI-LIST] Re: HIGH DC Current on GND Plane
- From: steve weir <weirsi@xxxxxxxxxx>
- To: eric@xxxxxxxxxxxxxxx
- Date: Tue, 19 Jun 2007 12:09:23 -0700
Eric, while 3D tools provide the most accurate answer, 2D tools and a
little effort are generally more than adequate for solving power
distribution drops. They have the advantage of being much faster than
3D tools.
Best Regards,
Steve.
Eric Bogatin wrote:
> 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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> ****************************************
> -----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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