EMC design concepts

Product Category and EMC Requirements Level

EMC requirements vary depending on the product category, application, and sales region, and are defined by various laws, regulations, and standards.
For example, consumer electronics, industrial equipment, and automotive equipment are subject to different EMC standards, requiring different design considerations for each. Therefore, in EMC design, it is crucial to thoroughly understand the standards and requirements applicable to the target product from the initial design stage before establishing a design policy.

Design process from an EMC perspective

While everyone understands the importance of EMC design in product development, in reality, constraints such as functional realization, ensuring general performance, cost, and prototyping schedules often take priority, and EMC considerations tend to be put on the back burner. As a result, it is not uncommon for EMC problems to only become apparent in the later stages of the development process.

EMC countermeasures may involve fundamental design changes, such as altering the layer configuration of the circuit board or reviewing the enclosure structure. Depending on the nature of these changes, additional prototyping steps may be required, significantly impacting design effort, cost, and schedule.
Therefore, it is no exaggeration to say that how well EMC (Electromagnetic Compatibility) is considered from the initial design stages is a crucial factor in determining the success or failure of product development.

This column explains the fundamental concepts of EMC design that should be understood in the early stages of design.

① Prevent noise generation (source control measures)

In source control, it is crucial to design the circuits and wiring that generate noise in a way that minimizes noise generation.

• In electronic circuits, current always has both a "forward" and a "return" path, so it is especially important to be clearly aware of the "return" path when designing circuits and layouts.
  If this return path is interrupted or separated, the closed-loop area formed by the current's "forward" and "return" flows increases, leading to an increase in electromagnetic noise generation. Therefore, care must be taken to keep the loop area as small as possible from the initial design stages.

Figure 2

In multilayer substrates, by designating the adjacent layer to the signal layer as a GND plane layer, an environment is naturally created where the return current flows directly beneath the signal via the shortest path.
If a GND plane cannot be secured in an adjacent layer due to layer configuration constraints, a similar effect can be achieved by running a return GND trace parallel to the signal line.

• Particular caution is needed in circuits with large current fluctuations, such as SWITCHING POWER SUPPLIES.
  During the component placement stage, consider arranging components so that high-current loops close within the smallest possible area. Many IC manufacturers provide recommended layout patterns, so actively utilizing these is effective.
• When wiring, we aim for a straightforward and continuous routing, avoiding abrupt changes in wire width and unnecessary branching.
  Furthermore, right-angle wiring should be avoided as it can unintentionally cause variations in wiring width. Instead, aim for smoother routing using 45-degree or curved wiring.

Figure 3

These are important design concepts that can be effectively implemented simply by being aware of the basic principles of circuit board structure and wiring. By ensuring these are firmly established during the initial prototyping stage, a significant reduction in the amount of additional work required in later stages can be expected.

② Prevent the generated noise from leaking to the outside (containment measures)

Gaps in the enclosure, CABLES, and I/O CONNECTORS are places where noise is easily leaked to the outside, so it is important to design the system to structurally contain the noise to prevent it from spreading from these areas.

• The metal casing eliminates gaps, including at joints and mounting points, suppressing noise leakage.
• CABLES shields are generally recommended to be connected to the chassis ground of the enclosure to suppress noise radiation.
• Around the I/O CONNECTORS, CAPACITORS or similar devices are used to ensure that noise carried on the internal wiring does not spread to the outside, thus providing a path to GND.

These containment measures are particularly difficult to modify later in development, and depending on the measures taken, they can significantly increase man-hours and costs. Therefore, thoroughly considering them from the early stages of design and ensuring they are reflected in the structure and layout will lead to a reduction in overall development man-hours and costs.

③ Attenuate the generated noise (attenuation measures)

Source control and containment measures alone may not be sufficient to meet the noise level requirements specified by the equipment.

The solution applied in this case is attenuation to reduce residual noise.

• POWER SUPPLIES line: An LC FILTERS or similar device is placed at POWER SUPPLIES input to effectively attenuate internally generated noise.
• Signal lines: For differential signals, common-mode noise is suppressed using a common-mode choke. For single-ended signals, the optimal configuration is considered by combining damping resistors, FERRITE BEADS, and ground-to-ground CAPACITORS while checking the signal quality.

However, since it is often difficult to reduce noise to standard levels through attenuation measures alone, the basic policy is
① Source control measures → ② Containment measures → ③ Attenuation measures
It is appropriate to develop countermeasures in the order listed above, and to position attenuation countermeasures as merely supplementary.