The skin effect does not depend on any mysterious underlying forces. It may be completely predicted based on one simple, lumpedelement equivalent model called the concentricring model.
Table 2.5. Assumptions Made by z BETTER
Conductor Geometry 
L i (0) 

Round wire of any radius not in close proximity to a return path ( k p = 1) 

Wire of any convex cross section with perimeter p and area a 

Rectangular wire of size w x t 

Square wire of size w x w 

Very thin rectangular wire, t << w 
Imagine a round conductor divided lengthwise into concentric tubes, like the growth rings on a tree trunk. In this model the current proceeds absolutely parallel to the wire's central axis. Because current naturally passes straight down the trunk, parallel to the dividing boundaries between the rings, you may insulate the rings from each other without affecting the circuit. You now have a collection of n distinct, isolated conductors, shorted together only at each end of the trunk.
You may now separately consider the inductance of each ring. The inner rings, like long, skinny pipes, have more inductance than the outer rings, which are fatter. You know that at high frequencies, current follows the path of least inductance. Therefore, at high frequencies you should expect more current in the outer tree rings than in the inner. That is exactly what happens. At high frequencies the current crowds into the outermost rings. At low frequencies current partitions itself according to the resistance of each ring, while at high frequencies it partitions itself according of the inductance of each ring.
This simple concentricring idea motivates the idea of a redistribution of current at higher frequencies. What it doesn't do, however, is properly indicate the magnitude of the effect. The skineffect mechanism is far more powerful than just the ratio of individual concentricring inductances. Mutual inductance between the rings actually bunches the current much more tightly onto the outer rings than you might at first imagine. The general setup for constructing a treering model, including the mutual inductance, is discussed in the article "Modeling Skin Effect." The concentricring circuit model, if taken to an extreme (hundreds of thousands of rings), properly predicts both skineffect resistance and skineffect inductance, at low and high frequencies, for a circular conductor.
2.9.1 Modeling Skin Effect
Article first published in EDN Magazine , April 12, 2001Why does highfrequency current flow only on the outer surface of a printedcircuit trace? ”Dipak Patel Magnetic fields cause the behavior you describe. The technical name for this property is the skin effect . It happens in all conductors. If you really like mathematics, the following section will help you to better understand why the skin effect happens. If not, this might be a good time to step out for a cup of tea. I'll start our discussion with a perfect coaxial cable. Figure 2.18 divides the center conductor of this cable into a series of three concentric rings with radii r , r 1 , and r 2 ( meters ). A lumpedelement model of this simple circuit demonstrates that highfrequency signal current flows only on ring number 2. Figure 2.18. The total magnetic flux within the shaded region equals L 1,0 . At high frequencies magnetic interactions between conductors become significant. At DC, the longitudinal voltage drop per meter across each conductor n equals the current i n times its resistance per meter. You can express this relation in matrix terms by defining a square matrix R relating the circuit voltages V to the currents I : Equation 2.60
At high frequencies magnetic interactions between the conductors become significant. Figure 2.18 illustrates the pattern of magnetic fields between the center conductor and shield. The magnetic lines of force ( B field) form concentric circles around the conductive rings. The drawing plots the field intensity, B , versus radial position, r , assuming a positive signal current of 1A flowing in ring 0 with the return current flowing in the shield. The field strength is zero within the interior of ring 0 and zero outside the shield, and varies with 1/ r (Ampere's law) in between. The exact field intensity for a current of 1A on conductor m is , where m is the magnetic permeability of the dielectric material (usually 4 p ·10 “7 H/m). You calculate the mutual inductance per meter between conductors n and m (for n m ) using Faraday's law as the integral of the magneticfield strength, B m , taken over the range from conductor n (at radial position r n ) to the shield (at radial position d /2). Integrating 1/ r yields ln( r ) and the following matrix equations for mutual inductance (values for n < m are found using symmetry: L n,m = L m,n ): Equation 2.61 The system equation for the whole coaxial circuit sums both resistive and inductive terms as V = ( R + j w L ) I . As an example, the following is the inductance matrix for a threering model of an RG58/U coaxial cable: Equation 2.62 Now comes the main point of this article: The terms in the righthand column of L are all the same. Why? Because ring 2 concentrates all its flux into the space between ring 2 and the shield. Therefore, all other rings couple 100% to this flux. The constancy of the righthand column greatly simplifies the solution to the system equation. To solve this equation, you must find a pattern of currents I such that ( R + j w L ) I generates the same longitudinal voltage across every ring. You need the same voltage across every ring because the rings are all connected together at their ends. If you operate at a frequency so high that the R term becomes insignificant compared to j w L , the solution is simple. Just fill in the last element of I , leaving all others zero. This solution peels off only the righthand column of L , properly generating the same voltage for every ring. This is one of the few matrix problems you can solve by inspection. The simple solution says that at high frequencies the signal current flows only on the outer ring as governed by matrix L . At DC, the current distributes itself more evenly, according to matrix R . At middle frequencies, you get a mixture of both effects. That's the nature of the skin effect. Realworld conductors behave in a similar manner, as if they were made from a continuum of infinitely thin concentric rings. At higher and higher frequencies, the current squeezes more and more tightly against the surface of the conductor, progressively decreasing the useful currentcarrying cross section of the conductor and raising its effective resistance. 
2.9.2 Regarding Modeling Skin Effect
Email correspondence received July 17, 2001 SE HO YOU writesYou have shown an inductance matrix and indicated that since all the entries in the right column of the matrix are the same, ring number 2 concentrates all its flux into the space between ring number 2 and the shield. Would you please explain why flux concentrates around ring number 2? The behavior of the flux comes first, I think. Then, the inductance matrix should be just a mathematical expression of how nature works. Could you explain more physically? 
ReplyEddy currents flowing in the outer ring create a magnetic shield through which flux cannot penetrate . The shielding effect of the outermost ring therefore prevents flux from reaching the inner rings. Receiving no electromagnetic impulsion from changing flux, no current flows on the inner rings. At low frequencies the eddy currents in the outer ring are impeded by the resistance of the copper , so the shielding effect is imperfect. With an imperfect shield, some magnetic fields do reach the interior and some current does indeed flow on the inner rings. As you go to higher and higher frequencies, however, the shielding effect becomes more pronounced. The improved shielding effect successively robs the inner rings of more and more flux (and current). 
Fundamentals
Transmission Line Parameters
Performance Regions
FrequencyDomain Modeling
Pcb (printedcircuit board) Traces
Differential Signaling
Generic BuildingCabling Standards
100Ohm Balanced TwistedPair Cabling
150Ohm STPA Cabling
Coaxial Cabling
FiberOptic Cabling
Clock Distribution
TimeDomain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. TwoPort Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )
Notes