Column: EMC


Considerations for EMI filter design By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK D


esigning for electromagnetic interference (EMI) may seem like black magic to some, but it is a highly necessary skill that requires


finesse. Engineers can perform multiple EMI mitigation procedures, only to find a product still fails the electromagnetic compatibility (EMC) test. Others might install an EMI filter without understanding the specific problem. In general, filters are used to protect


devices and circuits from lightning and electromagnetic interference. To design an effective filter that allows a product to pass EMC standards, time must be spent understanding or learning the fundamentals of the circuit. However, the physics and mathematical equations are complex and can be daunting, and what’s been learnt at university often doesn’t solve the practical engineering problems. That’s why it is imperative to explore the underlying principles of EMI.


Issues around frequency Filters only work within a certain frequency range, so any discussion about them is meaningless without a


Initial design considerations Filters are a network of passive components that attenuate signals within a certain bandwidth. Choosing the best filter configuration for the design at hand can be difficult with so many different filter types to choose from. The so-called “passive” filters are made up of passive components like resistors, capacitors and inductors, which have no amplifying capability and no signal gain. Some filters are designed for specific


challenges, such as power surges and voltage spikes; an example is the transient voltage suppressors (TVS) that reacts to sudden overvoltage conditions. Nonlinearity in a circuit is sometimes


defined frequency spectrum. In the world of EMC we use terms such as “low/high frequency” and “narrow-/ wide-band signals”. The definition of low or high frequency is relative and depends on the working frequency of the circuit in question. In a grid-tied motor drive, for instance, the total harmonics distortion (THD) of the unit must meet regulation standards. “Low frequency” applies to mains-


Choosing the best filter configuration for a design can be difficult, especially with so many different filter types to choose from


related harmonics. A modern motor drive will have a switching frequency of around tens of kHz, and the EMI impact can extend to a few hundred MHz, even GHz. Tens of kHz is the low frequency range, whereas anything above 1MHz is high frequency. Generally speaking, the distinction


between low and high frequency is important when passive components affect circuit operation and can’t be ignored. The frequency range from a few kHz to a few hundred MHz is critical to a wide range of electrical and electronic products, so it must be considered when designing and implementing an EMI filter.


08 September 2022 www.electronicsworld.com


overlooked by engineers, particularly when frequency increases. In capacitors, for instance, the behaviour of dielectrics, which are inherently poor insulators, is nonlinear at high frequencies. With magnetic materials in the


circuit, the relationship between magnetic flux density (B) and magnetising force (H), as measured with the B-H curve, is also nonlinear. With these materials, the permeability, or the magnetisation obtained in response to an applied magnetic field, falls off with frequency. As a result, the demand for magnetising current increases significantly as the magnetic core approaches saturation. Engineers must consider and address these nonlinearities during the design phase. Finally, the difficulties of filter perfor-


mance can be avoided by modelling the processes beforehand with simulation tools. A Simulation Program with Integrated Circuit Emphasis (SPICE) based simulation is quick and easy to use. With sufficient allowance for the parasitic parameters, SPICE-based simulations yield accurate filter performance models that match real-world test results.


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