One of the most important concepts in designing printed circuit boards (PCBs) for low electromagnetic interference (EMI) is the concept of fields. Specifically, as engineers, we need to focus on containing these fields.
EMI, or electromagnetic interference, is fundamentally about field containment and controlling where the fields need to be strong and where they need to be weakened.
When we talk about fields, we are referring to both electric and magnetic fields. Therefore, if our goal is to design PCBs with low EMI, we must understand how these fields behave and how to control them.
By doing so, we can not only achieve a high-performing circuit but also ensure that the design passes electromagnetic compatibility (EMC) tests, which are essential before the product can be sold in the marketplace.
One of the things I found most useful for understanding electromagnetic (EM) fields is to actually visualize them. While these phenomena can be very complex, having a simplified visual representation can significantly help us grasp the fundamentals. This understanding is essential when designing or debugging products for low EMI.
The first image we need to have in mind when discussing fields is how these fields surround a conductor.
Figure 1- Electric Field of Conductor.
In this scenario, the circle in the center represents the conductor, viewed in cross-section. Surrounding this conductor are the magnetic and electric fields, which together form the electromagnetic field.
The magnetic field is quantified by the number of lines of magnetic force, denoted by the symbol Φ (PHI). These magnetic field lines encircle the conductor, forming closed loops that represent the magnetic flux generated by the current flowing through the conductor.
For the electric field, these are represented by dashed lines in the picture.
These lines originate from the conductor and are measured by the number of lines of electric force, denoted by the symbol Ψ (PSI). In the case of a single conductor, the electric field lines radiate outward as straight lines from the conductor.
However, when a return conductor is introduced, the behavior of the fields changes, as illustrated in Figure 2. In this scenario, the magnetic field lines become concentrated between the two conductors, forming stronger, more focused loops in the space between them. Outside of this space, the magnetic field lines spread out more broadly.
For the electric fields, when we have the return conductor, the lines of force change their behavior as well. Instead of radiating outward from a single conductor, the electric field lines now start from one conductor and terminate at the other.
Figure 2 - Electric Field of Circuit.
The key takeaway from these visualizations is that magnetic field lines always form closed loops, while electric field lines terminate at conductors.
This distinction leads us to describe the magnetic field as a "magnetic circuit," where the field lines loop continuously. In contrast, the electric field can be described as an "electric circuit," where the field lines originate and terminate at specific points.
Another way to represent electromagnetic fields is through the concept of polarities:
Figure 3 - Electric bodies with equal but opposite charges;
In this case, the electric field lines from one polarity terminate on the opposite polarity. This shows how fields interact between positive and negative conductor, with lines of force originating from the positive conductor and ending at the negative conductor.
Conversely, if two conductors or bodies have the same polarity, the fields will repel each other:
Figure 4 - Electric bodies with equal charges;
In this scenario, the electric field lines push away from each other, illustrating how like charges repel and how this affects the distribution of the electric field.
These principles can be directly applied to printed circuit board (PCB) design, as shown in Figure 5.
Figure 5- Electric and Magnetic Field in a PCB.
In this representation, we have a printed circuit board (PCB) formed by a two-layer stackup. The stackup consists of a signal layer, shown in red, which contains the signal traces, and a return reference plane, depicted in blue.
The term "stackup" refers to the specific arrangement of the PCB layers. This includes the conductive copper layers where signal traces and reference planes are located, as well as the insulating layers and prepreg or core materials that separate and support these conductive layers.
Figure 6 - Electric and Magnetic Field in a PCB, cross-section view.
When we observe the fields in this case, we find that the behavior is similar to the scenario with two conductors shown in Figure 2. However, in this instance, the two conductors are oriented at a 90-degree angle.
Figure 7- Electric and Magnetic Field in a PCB in compared to two wires topology.
This picture lays the foundation for understanding how to design printed circuit boards (PCBs) with low electromagnetic interference (EMI). Our objective will be to design the PCB to contain and redirect the fields exactly where we want them by carefully shaping the geometries of the circuit board to achieve this control.
In practical terms, this means that the fields of the signal trace in a PCB are influenced by the return reference plane. The fields not only terminate at the surface of the return plane but also tend to bend towards it from the opposite side. This bending helps contain the fields and prevents them from spreading outwards as they would in the absence of the return plane.
The return reference plane acts as a boundary that contains the fields, preventing them from dispersing freely into the surrounding space. Without this plane, the fields would continue spreading outward from the signal trace, as illustrated in Figure 1.
Understanding how the fields are contained by the return reference plane is essential for designing PCBs. The choice of stackup—how the layers of the board are arranged—will be based on this principle.
The key takeaway here is that removing the return reference plane allows the fields to expand uncontrollably. This is a major reason why PCBs often fail EMC tests and generate EMI issues. Signal traces without an adjacent return reference plane lack the necessary containment for the fields, leading to excessive radiation.
When we measure the fields during EMC testing, we often find that the levels exceed the requirements. This is because, without a return reference plane, the PCB effectively becomes a radiator, emitting unwanted electromagnetic energy.
In practical scenarios, particularly with common electronic consumer products, if a PCB lacks a return reference plane adjacent to the signal traces, the earth ground will act as the return path. This causes the fields to continue expanding until they eventually reach the earth ground or other nearby conductive structures.
This expansion can lead to the formation of common-mode currents, which are highly effective at radiating electromagnetic energy. These currents can easily cause a product to fail EMC tests if the radiated fields exceed acceptable limits.
To mitigate this issue, the first and most important rule of thumb in PCB design is:
For each signal layer, there must be a return reference plane adjacent to it. This return plane should be free of discontinuities, such as large holes, gaps, cuts, or any other malformations that could compromise its effectiveness.
Figure 9 - Electric and Magnetic fields in two layers PCB.
The return reference plane should be as integral and as close as possible to the signal trace. This approach is key to successfully passing EMC tests on the first attempt. Proper design with an adjacent, continuous return reference plane ensures that fields are effectively contained, reducing the chances of EMI issues.
Figure 10 - Electric and Magnetic fields in a two layers PCB.
This image is one of the most critical concepts to keep in mind when designing printed circuit boards. It serves as a visual reminder of the importance of field containment through proper layer stackup.
I hope this helps,
Dario
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