Yuriy,
Nice simulation! Thanks for sharing it! Note that even if you had terminated
the loop on the top plane (forcing all the signal current to flow in the
trace), there would still be a current in the plane under the trace. At kHz
frequencies, the magnetic field induces an eddy current that attempts to cancel
the magnetic flux passing through the plane. The current flowing in the trace
loop would be approximately the signal voltage divided by the load impedance.
At kHz frequencies, the current flowing in the disconnected plane would be
approximately equal to that. Now if we terminate the source and load in the
plane (as in your simulation), the signal current flowing directly between the
source and load is canceled in the plane by the eddy current and we get the
current distribution you are observing (i.e. the signal current returns in the
plane under the trace).
Of course, the rule that's often quoted is:
"At kHz frequencies and below, currents take the path(s) of least resistance.
At MHz frequencies and above, currents take the path of least inductance."
This is generally correct for circuit board and system geometries. For a
straight trace over half-ounce copper, I believe the currents in the plane will
spread out significantly at 1 kHz and be basically the same as the DC
distribution at 100 Hz.
Thanks again for sharing this! Your simulations have really helped to visualize
fields and currents in a variety of important situations.
Best regards,
Todd
---------------------
Todd Hubing
LearnEMC, LLC
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
Yuriy Shlepnev
Sent: Thursday, May 7, 2020 10:47 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AC current return path
Hello Everyone,
I did a quick investigation (numerical experiment) of AC current return path on
PCB - it is #2020_23 at https://www.simberian.com/ScreenCasts.php ;(see "How
interconnects work"
section, YouTube version is at https://youtu.be/qeJ2EqaiBdc).
It is about a curved trace over large plane and attempt to force current to
"return" directly between input and output. At DC current follows the path of
least resistance (no inductance effect) - about straight line between input and
output or direct path. At high frequencies (or AC) current follows the path of
least impedance - mostly below the trace, as you can see. The resistance of
such path is larger, but inductance is much smaller comparing to the loop
inductance. When frequency goes down, the total impedance of the straight line
path becomes smaller and the return current flows directly.
The question is at which frequency AC current switches to the direct path?
It is relatively easy to estimate the crossover frequency, but not so easy to
simulate and visualize for PCB geometries. My conclusion is that it happens at
Hz frequencies (depends on stackup), rather than at KHz or MHz frequencies -
can it be true? As usual, I will appreciate any feedback.
Best regards,
Yuriy
Yuriy Shlepnev, Ph.D.
President, Simberian Inc.
www.simberian.com
Simbeor THz - Accurate, Productive and Cost-Effective Electromagnetic Signal
Integrity Software to Design Predictable Interconnects!
Simbeor SDK - The Industry-First Signal Integrity Analysis Automation Kit!
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