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.

The possible plane combinations are shown in Figure 2.

Figure 3 and Figure 4 show the CST Studio simulations of the ** E** and

**fields, respectively [2].**

*H*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 ±4*x*/*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 *h*_{1} is the distance between the trace and the closest plane, while where *h*_{2} is the distance between the trace and the furthest plane.

Figure 8 shows an 8-layer PCB where the signal *V*_{1} is placed between two ground planes, while the signal *H*_{2} 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

**fields, respectively.**

*H*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 *h*_{2}/*h*_{1} ratios [3].

**References**

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