[SI-LIST] Re: Interference from planar magnetics


Looking at equation 6-11b and table 6-2 in Henry Ott's text 
"Noise Reduction Techniques in electronic Systems" he gives the
equation for skin depth as:

        depth(in inches) = 2.6/sqrt(f*ur*or)
        
Where the depth is defined as "the distance required for the
wave to be attenuated to 1/e or 37% of the original value"
        
For a few common materials, or and ur:
        
Material        Relative Conductivity (or)      Relative Permeability (ur)
--------        --------------------            ----------------------
Cu                      1.00                    1
Al                      0.61                    1
Steel (SAE1045)         0.10                    1000

        
For copper ur (relative permeability) and or (relative conductivity)
both equal 1 so the equation reduces to

        depth = 2.6/sqrt(f)
        
Using this equation the skin depth for copper is .0026 inches at 1 MHz

table 6-2 from Ott's book summarizes the results as:

Freq            Cu              Al              Steel
------          -----           ------          --------
60 Hz           0.335           0.429           0.034
100 Hz          0.260           0.333           0.026
1 kHz           0.082           0.105           0.008
10 kHz          0.026           0.033           0.003
100 kHz         0.008           0.011           0.0008
1 MHz           0.003           0.003           0.0003
10 MHz          0.0008          0.001           0.00008


Looks like there is a discrepancy between sqrt(F) and F in Ott's
formula and the one Steve presented. Who's correct ?


-Ray Anderson
Sun Microsystems Inc.
        


>
>
>
>Larry, it works out to be just about 1MHz in 1oz Cu.  The formula is depth 
>in inches = sqrt( 2.6/F )  At 1MHz it comes out 1.6mils which is close 
>enough for government work to 1.4mils.
>
>Regards,
>
>
>Steve.
>At 06:50 PM 1/27/2003 -0800, Larry Smith wrote:
>
>>Actually, the copper planes are a good magnetic shield at high frequency but
>>poor at low frequency.  The transition region occurs when skin depth is
>>equal to the copper plane thickness.  The skin depth is the depth to which
>>magnetic fields penetrate the copper (or at least diminish to 1/e of their
>>value on the surface).  At high frequency where skin depth is much less than
>>copper thickness, very little magnetic field penetrates the copper.
>>
>>As I recall, the skin depth of copper is about the same as the plane
>>thickness at 1 MHz (but I cannot remember if this is for 1 oz or half
>>oz copper).  The copper planes will not shield the fundamental switching
>>frequency but will shield the higher harmonics from reaching buried
>>transmission lines.
>>
>>regards,
>>Larry Smith
>>Sun Microsystems
>>
>>Dennis Schmitz wrote:
>> >
>> > "I could, of course put a large copper shape (with plenty of vias tied to
>> > ground planes below) beneath the VRM to block the electric fields from
>> > penetrating into the lower wiring planes.   While I believe that this would
>> > affect the magnetic field (since it affects the electric field), it 
>> will not
>> > block it!."
>> >
>> > I think you'll find that copper is a very effective magnetic shield at 
>> those
>> > frequencies. The principal is that the magnetic fields induce currents that
>> > cancel out the field on the far side of the copper plane. The frequency
>> > components where the magnetic fields are no longer effectively cancelled by
>> > copper are so large that your diffpairs won't even even notice it's there.
>> > You'll get some coupling on the single-ended stuff, though.
>> >
>> > The rule then is that the copper plane needs to be as large as you can make
>> > it, preferably the entire top surface of the board (and be a ground plane).
>> > The higher the conductivity the better, so use 1 oz (or even 2 oz copper if
>> > you can get your board shop to do it). The GHz stuff should be ok too.
>> >
>> > If you have sensitive analog signals, you'll have trouble, though.
>> >
>> > Dennis
>> >
>> > -----Original Message-----
>> > From: si-list-bounce@xxxxxxxxxxxxx
>> > [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Bob Welte
>> > Sent: Friday, January 24, 2003 4:31 PM
>> > To: si-list@xxxxxxxxxxxxx
>> > Subject: [SI-LIST] Interference from planar magnetics
>> >
>> > Hello (once again)
>> > I would like to get some insight, (or references) on the effects of having
>> > planar magnetics (from DC/DC converters) on a digital board in close
>> > proximity to the digital traces.
>> > Specifically, I need to determine the effects that the fields from the
>> > planar magnetics of the DC/DC VRM might have on various types of digital
>> > signals.
>> > The digital signals fall into two classes:  High frequency (1.25Ghz) which
>> > are differential, and low voltage (1V amplitude), and low frequency (<200
>> > Mhz), unbalanced, but with various voltages (some are 3.3V, 2.5, and 1.8V
>> > signals).
>> > The VRMs are relatively low voltage/ high current (2.5 and 1.8V at 10-25
>> > Amps).  The VRMs typically have switching frequencies not much greater than
>> > a couple Mhz.  The magnetics of the VRM are not "part of the board" (ie
>> > board traces do not make up any portion of the inductor/transformer
>> > windings).
>> > Ideally, I would like to be able to route the digital signals near (and
>> > even under) the VRM.   I could, of course put a large copper shape (with
>> > plenty of vias tied to ground planes below) beneath the VRM to block the
>> > electric fields from penetrating into the lower wiring planes.   While I
>> > believe that this would affect the magnetic field (since it affects the
>> > electric field), it will not block it!.
>> > As voltages get lower, and VRMs need to be closer to the ASICs, where board
>> > real estate is at a premium, I thought this topic would have wide interest.
>> > Any thoughts?
>> >
>> > Bob Welte
>> > IBM Microelectronics
>
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