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Bring the Noise — And Then Filter It: Practical EMC Design for Modern Engineers
Electromagnetic interference, or EMI, should not be seen as something mysterious or feared, for many design engineers it remains a recurring obstacle that leads to repeated board revisions, rushed fixes, and filters that are added more in hope than certainty.
I
n reality EMI is not magic; it is physics, and with the right knowledge, the correct simulation tools, and a sound grasp of the relevant
standards, engineers can deal with noise in a controlled and predictable way. This article looks at EMI and electromagnetic
compatibility from a practical standpoint. It covers how to choose the correct filter topology, what other factors affect behaviour, and how nonlinear effects can undermine even the best theoretical design and how working with companies that specialise in EMC filtering can help create products that are robust and compliant.
EMI and EMC in Context EMI is unwanted electrical noise that disrupts the operation of electronic systems. EMC is the ability of a product to work correctly without generating interference or being affected by it. Achieving EMC compliance means addressing both emissions and immunity. Filters are usually the first line of defence, but their effective use requires understanding how interference moves through a system and how it can be blocked.
Practical Considerations The environment often decides filter success. High IP-rated filters are essential outdoors or in transport where dust and vibration are constant threats. For high-power systems such as EV chargers or industrial drives, water-cooled filters keep performance high while saving space. Long cable runs can act as antennas, so specialised filters are needed to reduce both common-mode and differential noise.
Wide Bandgap Devices Gallium Nitride and Silicon Carbide semiconductors enable faster switching and higher efficiency, but they also create sharp transitions that raise EMI risk. Designers must use advanced filters, optimised layouts, and improved shielding. Pre-compliance testing and careful simulation are essential to avoid redesigns.
Figure 2 – High IP-rated EMC filter
Differential and Common Mode Most EMI problems involve both differential-mode noise, which flows between conductors, and common-mode noise, which appears on both conductors relative to ground. Both must be addressed. Common-mode chokes combined with capacitors are effective against common-mode emissions, while series inductors or resistors work best for differential suppression.
Understanding the Standards Standards such as EN 61000-6-4 for emissions, EN 61000-6-2 for immunity, and the EN 50121 series for railway equipment define acceptable limits and test methods. Designing filters is essential for meeting these requirements, particularly in industries where compliance is closely enforced.
Figure 1 – REO Choke Coil
Choosing the Right Filter Filters are built from passive components such as inductors and capacitors, arranged to suppress unwanted signals while passing useful ones. LC filters are well-suited for high-frequency suppression, RC filters often appear in low-power applications, and Pi or T networks provide higher levels of attenuation. Common-mode chokes target common-mode noise but also give some differential suppression. Choosing correctly depends on the expected noise, system impedance, and layout. A poor choice can make noise worse instead of better.
The Real World In practice capacitors change behaviour with temperature and frequency, magnetic cores can saturate, and parasitic elements create resonances. What looks good on paper may fail in reality. This is why simulation and testing at the earliest stage are essential.
Simulate First, Test Once SPICE-based simulation tools allow engineers to model filters before building hardware. They give insight into attenuation, impedance, and resonance behaviour. Simulations do not replace laboratory testing but they reduce prototypes and improve the chance of first-time compliance.
16 ELECTRICAL ENGINEERING • SEPTEMBER 2025
Working with Specialists While many applications can be addressed in-house, partnering with an EMC filter specialist is often the best option. Specialist manufacturers provide pre-certified filters, custom designs, accurate data, and consultation on placement and grounding. This support is especially valuable in regulated sectors such as rail, medical and energy.
Figure 3 – REO DC Power Supplies
Conclusion EMC filtering is no longer an afterthought but a vital part of delivering products that are reliable and compliant. However, with intelligent design, simulation tools, and attention to installation factors, engineers can control noise instead of fearing it.
electricalengineeringmagazine.co.uk
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