ADVERTISEMENT FEATURE COVER STORY PRACTICAL EMC DESIGN
This article from REO (UK) examines the practical aspects of EMI and EMC, from selecting the appropriate filter topology to understanding how environmental conditions, cable runs, and
real-world nonlinearity impact performance. It also highlights how companies with specialist expertise in EMC filtering can support robust, standards-compliant product development
E
lectromagnetic interference (EMI) doesn’t have to be feared but needs to be understood. It’s a recurring roadblock
for many design engineers: countless PCB revisions, last-minute fixes, and filters added more in hope than confidence. But EMI isn’t magic – it’s physics. With the right approach, simulation tools, and a solid grasp of the standards, engineers can bring the noise – and then filter it – with precision, efficiency, and compliance in mind.
FROM INTERFERENCE TO COMPLIANCE: UNDERSTANDING THE EMI CHALLENGE EMI is unwanted electrical noise that disrupts the intended operation of devices, systems, and communications. In contrast, electromagnetic compatibility (EMC) is the ability of a product to function properly without generating or being affected by that interference. Achieving EMC compliance means addressing
emissions and immunity; filters are often the frontline solution. But filtering isn’t just about adding a capacitor here or an inductor there – it’s about understanding how interference propagates and how to block it effectively.
FILTERING 101: TOPOLOGIES THAT TAME THE NOISE Filters are built from passive components – inductors, capacitors, and resistors – arranged to attenuate unwanted signals while preserving the useful ones. Standard filter configurations include: • LC filters: Ideal for suppressing high- frequency noise.
• RC filters: Useful in low-power or signal-line applications.
• Pi (C-L-C) and T (L-C-L) filters: Provide increased attenuation and flexibility.
• Common-mode chokes: Target common-mode interference but offer differential-mode attenuation due to leakage inductance. Designers must consider the filter type and
how it integrates with the system, including impedance, expected frequencies, and physical layout. Using the wrong filter or applying it incorrectly can do more harm than good, amplifying rather than attenuating noise.
12 DESIGN SOLUTIONS APRIL 2025
STANDARDS IN THE SIGNAL CHAIN: WHAT ENGINEERS NEED TO KNOW A wide range of EN standards apply depending on the product category and operating environment to ensure EMC performance is not just good practice but legally mandated. For example: • EN 61000-6-4 – Generic emission standards for industrial environments
DIFFERENTIAL VS COMMON MODE: DUAL FRONTS IN EMI DEFENCE Most real-world EMI problems involve both differential-mode and common-mode noise. Differential-mode noise flows between conductors in opposite directions, typically generated by switching components. On the other hand, common-mode noise appears identically on both conductors relative to a reference, such as ground or chassis. An effective EMI strategy filters both. Common-mode chokes, for example, are invaluable in addressing common-mode emissions, especially when paired with capacitors to ground. Meanwhile, series inductors or resistors may be better suited for differential suppression.
Figure 1 – REO Choke Coil
• EN 61000-6-2 – Generic immunity standards for industrial use
• EN 50121 series – EMC standards for railway equipment. These standards define acceptable emissions
levels, and the tests required to demonstrate compliance. Filter design plays a crucial role in meeting the conducted and radiated emissions criteria, particularly in sectors where regulatory scrutiny is stringent.
THE REAL WORLD ISN’T IDEAL: NONLINEARITY AND ENVIRONMENTAL EFFECTS Theory can only go so far. In real-world designs, nonlinear effects begin to show, particularly as switching frequencies increase and power densities rise: • The dielectric behaviour of capacitors changes with temperature and frequency.
• Magnetic cores can saturate or lose permeability under high currents.
• Leakage inductance and parasitic capacitance can introduce unexpected resonances. What works on a datasheet might not work in a
test chamber. That’s why experienced engineers now turn to simulation and practical testing earlier in development.
SIMULATE FIRST, FILTER ONCE Modern simulation tools – especially those based on SPICE – enable engineers to model filter behaviour before committing to hardware. With parasitics and nonlinearities factored in, SPICE simulations offer a reliable window into: • Frequency response and attenuation • Impedance matching • Susceptibility to resonance • EMI mitigation across a broad frequency range. Simulations won’t replace EMC testing, but
they can significantly reduce the number of physical iterations needed – and improve the likelihood of a first-time test pass.
FILTERING BEYOND THE SCHEMATIC: WHEN INSTALLATION GETS TOUGH Specific design environments make filtering incredibly challenging. That’s where practical considerations come into play – ones that extend beyond the circuit diagram. For example:
BRING THE NOISE —
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