Re: [SI-LIST] : 20-H Rule for Power Planes

Larry Smith (ldsmith@lisboa.eng.sun.com)
Wed, 26 May 1999 14:10:25 -0700 (PDT)

Michael - You are correct that there will be fringing fields between
power planes at the edges of a PCB. My major point is that neither the
Vdd plane nor the Gnd plane is preferential. They are both power
planes that have a differential voltage between them. If they are in a
laptop computer, one plane could easily have 1000V and the other plane
1003.3V WRT earth ground, especially if I walk around on a carpeted
floor while holding it.

There is no "spice node 0" even in systems that are tied to earth
ground with a copper telephone pole driven into the earth and connected
to the frame with a 2 inch bus bar. The impedance of the earth ground
connection is still much higher than the impedance between power
planes.

Every voltage measured in a digital system is measured differentially
WRT some other point in the digital system. Single ended oscilloscope
probes measure the difference in the voltage between the measurement
point and the point where you attach the probe ground. (We have to be
a little careful of ground loop currents but they are usually much
lower frequency than the frequencies we care about.)

Even in well grounded systems, I don't think there is any preference
between the Vdd and Gnd plane at high frequencies, so I am not sure
which plane to make 20-H larger than the other plane.

But let's take a closer look at the fields at the plane edges. At
every xy location of the power planes, a differential voltage could be
measured. At low frequency, you get the same measurement at every
location. At higher frequencies where the board size is significant
compared to the wavelength, you begin to get different voltages
depending on the xy location of the measurement.

One particularly interesting frequency is the one where 1/2 wavelength
fits into the length of the board (250 MHz for a 12 inch FR4 board...
hmmm, pretty close to where I live...). This is a cavity resonance,
boundary value problem. We know that the current in the copper planes
is zero at the edge of the board (open circuit), therefor the voltage
fields are at a maximum. Going back to a transmission line analogy for
a 1/2 wavelength resonator, we know that the differential current
(difference between the current in the Vdd plane and Gnd plane) in the
middle of the board is at a maximum and the differential voltage is 0.

This explains why power planes have resonant peaks at frequencies
associated with 1/2, 1, 3/2, 2... wavelengths. The differential
voltage at the power plane edges will be maximum at those frequencies
and the differential current will be 0 (edge of the board). Istvan
Novak has done some interesting work in terminating the edges of these
power planes in order to stop the resonances.

If the edges are not terminated or damped by some other means, we
have potential EMI issues from voltages and currents on the power
planes. Suppose we make one plane larger than the other. The current
stops at the edge of the smaller plane. The larger plane sees a major
impedance discontinuity at the edge of the smaller plane, but some
current keeps going to the edge of the larger plane. It is _not_
differential current. It is a one way current whose return path is
displacement current in the FR4 or air.

A voltage is developed at the edge of the larger plane that is not
balanced by a voltage on a nearby plane. In fact, the nearest
reference is 20H away instead of 1H. We have just changed nice
differential currents and voltages on a set of well behaved power
planes into common mode voltages and currents. Not a good thing to do
if we are trying to prevent EMI.

regards,
Larry Smith
Sun Microsystems

> From: Michael E Vrbanac <mvrbanac@cisco.com>
> Subject: Re: [SI-LIST] : 20-H Rule for Power Planes
> To: si-list@silab.eng.sun.com
> Date: Tue, 25 May 1999 18:50:40 -0700 (PDT)
> Cc: mvrbanac@cisco.com (Michael E Vrbanac)
> MIME-Version: 1.0
> Content-Transfer-Encoding: 7bit
>
> Larry,
>
> Interesting proposition....
>
> The fields do exist and have been shown to be troublesome whether you
> believe in the rule or not.
>
> Judging from the assumptions that you have presented, you certainly
> could have a bigger problem than fringing field radiation/coupling. You
> probably won't need to worry about the 20H rule. That indeed won't be
> the bigger issue in that case.
>
> Which brings me back to the point I mentioned earlier... it depends
> on "the design criteria"... "how much", how you implement your system
> design, etc.
>
> BTW, the plane that is larger in the 20H rule is the one connected to
> system reference. For folks who do not reference planes to the system
> reference, then the rule is not usable. The fringing field will
> remain uncontrolled (the 20H rule applied in the PCB would be pointless
> as the whole board assembly now becomes the energized element in the
> "planar structure" generating the fringing field) and other
> methods/intervention must be used to alleviate any problems arising
> from the situation.
>
> re: differential circuits, common mode current, et al
> I think one would be hard pressed to define the typical digital signal
> as purely differential unless the driver/receiver and transmission line
> constructs are specifically built that way. The last time I looked,
> most of that is "single ended or unbalanced" and not "differential
> or balanced".
>
> re: common mode radiation
> The basic model for common mode radiation is an "energized element"
> against a ground reference which typically happens when cable shields
> aren't grounded to chassis and signal grounds do not have low
> impedance attachments to the system reference (i.e. chassis)... hence
> my comment above that the situation for a system like this does not
> need to worry about the 20H rule in a PCB as a primary problem... it will
> likely have more serious electromagnetically related issues elsewhere.
> I will concede that controlling those issues in such a case is
> possible but certainly more difficult and oftentimes more expensive.
>
> Regards,
>
> Michael E. Vrbanac
>
>
>
> >
> > I don't believe in the "20-H Rule". Suppose the power plane was at
> > 3.3V and the ground plane was at 0V. It would be easy to reconfigure
> > the system so that the "power" plane is at 0 volts and the"ground"
> > plane is at -3.3V. Does this mean that the power plane should now be
> > bigger than the ground plane?
> >
> > The only difference between the power and ground plane is that one is a
> > 0V and the other 3.3V WRT (...thats with respect to, lest I start
> > another discussion...) earth ground. But even this is not true in a
> > battery operated system. In any modern digital system, the impedance
> > between the power and ground plane is much less than 1 ohm well into
> > the EMI frequencies.
> >
> > The ground plane probably has a path out to frame ground and eventually
> > earth ground somewhere. But if that path is more than an inch long, it
> > is going to be well over 10 nH. Ten nH is 1 Ohm at 15 MHz (Z=jwL) and
> > higher impedance at higher frequencies. So, above 15 MHz, the voltage
> > between the power and ground planes is insignificant compared to the
> > voltage across the earth ground connection.
> >
> > The power and ground planes should be exactly the same size. To make
> > one larger than the other will simply have the effect of turning nice
> > diffential currents into common mode current and common mode
> > radiation.
> >
> > regards,
> > Larry Smith
> > Sun Microsystems
> >
> >
> > > From: Mark Freeman <msf@stratos.com>
> > > To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
> > > Subject: [SI-LIST] : 20-H Rule for Power Planes
> > > Date: Tue, 25 May 1999 10:09:37 -0700
> > > MIME-Version: 1.0
> > > Content-Transfer-Encoding: quoted-printable
> > >
> > > Now and again I come across references to the "20-H Rule" for reducing
radiation
from
> > power planes. This rule states that the power plane should be smaller than
the
ground
> > plane; The power plane edges should be back from the power plane a distance
of
20-times
> > the plane spacing. This reduces fringing fields from the power plane and
reduces
> > coupling to adjacent planes and free space.
> > >
> > > Best I can tell, this rule originated with Mike King. The earliest
reference I
found
> > is Mark Montrose's "Printed Circuit Board Design Techniques for EMC
Compliance,"
pg.
> > 26. I have not found any numbers - analytical, simulation or measurement -
which
> > indicate the effectiveness of this technique over frequency. Intuition (a
dangerous
> > thing for this digital designer to rely upon) tells me that the dimensions
of the
> > fringing fields are small, thus only affecting GHz-range signals. Is this
technique
> > currently only of interest to cell 'phone designers, or do we need to begin
applying
> > this technique to digital PBW design?
> > >
> > >
> > > Mark Freeman
> > > msf@stratos.com
> > > Stratos Product Development, LLC
> > >
> > >
> > >
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