# PCB Return-Current Distribution in the Stripline Configurations

Last month’s article, [1], discussed the distribution of a PCB return current in a microstrip configuration. This article discusses the current distribution for the stripline configurations.

Return Current Distribution in a Symmetric Stripline Configuration

Consider a symmetric stripline configuration, shown in Figure 1, where a PCB trace of width w is placed in-between two planes, at the same distance h from each plane; x is the distance from the center of the trace.

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The possible plane combinations are shown in Figure 2.

Figure 3 and Figure 4 show the CST Studio simulations of the E and H fields, respectively [2].

The current distribution on each reference plane is described by its current density [3] J(x):

(1)

Eq. (1) represents the current density in just one of the two reference planes. The total reference plane current density is twice of that in Eq. (1).

Figure 5 shows the Matlab plot of (normalized) current density as a function of x/h, for both the symmetric stripline and a microstrip configuration.

Note that the stripline current does not spread out as far as in the case of a microstrip line. At a distance ±4x/h from the center the current density in a stripline rapidly decays toward zero, while in a microstrip there’s still a noticeable non‑zero current density.

Figure 6 shows the % of the total return current for both configurations, contained in the portion of the plane between ±x/h of the centerline of the trace.

Table 1 shows more detailed results for the stripline configuration [3].

In the stripline configuration, 99% of the current is contained within ±3 x/h. Virtually all current is contained within ±10 x/h.

Return Current Distribution in an Asymmetric Stripline Configuration

Consider an asymmetric stripline configuration, shown in Figure 7, where h1 is the distance between the trace and the closest plane, while where h2 is the distance between the trace and the furthest plane.

Figure 8 shows an 8-layer PCB where the signal V1 is placed between two ground planes, while the signal H2 is routed between a power plane and a ground plane.

Figure 9 shows an asymmetric stripline configuration where two orthogonally routed signal layers are placed between the reference planes.

Figure 10 shows a PCB topology where two high-frequency traces are placed between the reference planes.

Figures 11 and 12 show the CST Studio simulations of the E and H fields, respectively.

The current distribution for the close and far reference plane is described by its current density [3] J(x) as

(2)

(3)

Figure 13 shows the Matlab plot of (normalized) current density as a function of x/h, for both planes.

Note that, directly under the trace, 75% of the current flows on the closest plane and 25% on the far plane. At distance greater than ±3 x/h the currents in both planes are of the same magnitudes.

Finally, Table 2 shows the percentages of the return current in each plane for different h2/h1 ratios [3].

References

1. Bogdan Adamczyk, “PCB Return-Current Distribution in a Microstrip Line,” In Compliance Magazine, November 2020.
2. Scott Piper, CST Microwave Studio Simulations, Gentex Corporation, 2012
3. Henry W. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.

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