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

Larry Smith (ldsmith@lisboa.eng.sun.com)
Tue, 25 May 1999 14:44:35 -0700 (PDT)

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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