Column: EMC


The importance of inductors By Dr Min Zhang, EMC consultant at Mach One Design, and design engineers at REO UK W


e all learnt the concept of inductance in our high-school physics class, where we saw


how a solenoid develops voltage when the current going through it changes. Te basic function of an inductor for a filter is to provide an in-line high-impedance path at certain frequencies.


An essential component Inductors are essential passive components in electronics applications, from starting engines to delivering power to devices. Inductors are mostly used with capacitors and resistors to create filters for analogue circuits and signal processing. On their own, inductors function as low- pass filters, since the impedance of an inductor increases as the frequency of its signal increases. A basic inductor can be modelled as


a circuit using the Simulation Program with Integrated Circuit Emphasis (SPICE) tool. We call the whole circuit an inductor, rather than just the inductor symbol. This is important,


powdered iron. Te core can be toroidal, E-shaped or many other shapes. Te winding can be a single-strand conductor, or multi-strand (rope-type) winding, or even Litz wire. Engineers should select the right choice


of material for their inductor for a specific application. For example, nanocrystalline materials have become popular for inductors in common-mode choke applications due to their performance in the broadband spectrum. But, for motor- drive or high-power SMPS applications, where EMI issues oſten start in the few kHz range, the manganese-zinc core is a better choice. Increasing the number of turns of an


since a schematic symbol provides little information on the physical structure of a component, which often limits our understanding of what is actually happening in an electromagnetic field. An inductor is always associated


Generally speaking, an inductor is used for differential- mode noise


suppression, whilst a common-mode choke handles common-mode noise suppression


10 October 2022 www.electronicsworld.com


with a parasitic capacitance due to its turn-to-turn winding structure. It is only inductive until its self-resonance point, aſter which it behaves more like a capacitor. As the frequency increases, not only does the parasitic capacitance become dominant, there are also skin and proximity effects associated with eddy currents. Most of the time, from an EMC design perspective, both skin and proximity effects increase the lossy component in the inductor and reduce the leakage inductance, so the filtering impact is affected.


Inductor structures Te basic structure of an inductor is simple: winding an enamelled wire around a magnetic core material is an inductor. But there are many different types of core materials, such as ferrite or


inductor’s winding, you would expect the inductance value to increase. In reality, however, the inductance of an inductor does not have the proportional squared relationship with the number of turns. As the number of turns increases, so does the turn-to-turn capacitance of the winding. Tis will shiſt the resonant frequency lower; meaning, the capacitative part of an inductor starts to dominate. In fact, this is a primary reason why engineers sometimes find that an inductor has little impact on a design. Since inductors store a magnetic field


in space, rather than in the winding or core, any nearby components can potentially couple with the field in the same space. A shielded or semi- shielded inductor is designed to contain its magnetic field, thus reducing potential coupling. In other words, a shielded inductor has much less leakage inductance compared with an otherwise identical non-shielded inductor.


General layout rules for inductors Te number one rule of placing an inductor is to make sure the its magnetic flux is kept to a minimum to avoid


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