How Shielding Materials Affect Electromagnetic Radiation and the Effectiveness of a Power Inductor

Throughout this blog series on the electromagnetic (EM) radiation of power inductors of DC-DC converters, we’ve explained how EM radiation works and factors that influence it.

In this third and final post, we’ll discuss how shielding materials affect electromagnetic radiation and, by implication, the effectiveness of your power inductor

Shielding

Every core material has inherent advantages and disadvantages that are particularly suited to a specific application. In these circumstances where a core material cannot be substituted, external solutions may be necessary to mitigate emissions.

For example, inductors made of iron powder/metal alloy core material have excellent saturation characteristics and can be made in extremely small sizes, but they have limited shielding characteristics at frequencies above 1 MHz. So to shield the emissions from the ringing frequency caused by a fast switching transition time, external shielding may be required to ensure electromagnetic compatibility.

Metal and magnetic shielding solutions can be optimized based on the application. Metal shielding materials are made of copper, aluminum, metal alloys, and composite mixtures. Metal shielding usually consists of an enclosure that is attached over the source to reflect or absorb the noise. The thickness and type of material can be chosen based upon shielding effectiveness, frequency, and cost.

Interestingly, some iron powder inductor manufacturers are integrating a metal bridge on the top of the inductor to improve shielding performance. However, this approach is less advantageous than it may first appear. These inductors are not as flexible to design and emission requirements, as they have a limited effect at a constrained range of switching frequencies and fixed source properties.

Alternatively, magnetic shielding can be achieved using magnetic materials or µ-metals and their effectiveness are dependent on material permeability, impedance, and thickness.

Effect of Shielding in the Near-Field

The switch node ringing frequency on one demo-board is ≈130 MHz, and on the other demo-board it is ≈ 180 MHz. Since the advantages of iron powder and metal alloy inductors cannot be compromised most of the time, Würth Elektronik offers a huge variety of metal and µ-metal shielding materials, such as copper tape, various composite metal shielding cabinets with and without vents, NiZn and ferrite plates, and more.

These products offer flexible and adaptable solutions that are suitable for specific design restraints and can be selected for the particular range of frequencies where attenuation is required (Figure 15). For example, the effectiveness of metal shielding of a Würth Elektronik WE-LHMI iron powder inductor is demonstrated by a 10 dB reduction in E-field emissions (Figure 16).

Effect of Shielding in the Far-Field

The effectiveness of shielding is not only limited to near-field radiation. Notable reductions in far-field emissions can also be achieved using metal and ferrite solutions.

The same demo-board was tested in the EMI chamber for far-field radiation. The value of enclosing an iron powder core inductor with a 1.5 mm thick aluminum shield is demonstrated in Figure 17 and Figure 18, where the ringing frequency has been substantially reduced. In addition, attenuation was also notable over the whole frequency range, including harmonics. Similarly, the addition of adding a 3 mm-thick ferrite plate has a similar effect when placed on the iron powder inductor.

Summary of EM Radiation of Power Inductors in Power Magnetics

EM radiation is an incredibly vast and intricate topic, as seemingly small variations in any one parameter can influence emission sources and therefore the near-field and far-field characteristics. The characterization of the near-field can be a complicated and lengthy process because of the necessity of many experiments and observations to fully comprehend and resolve EMI issues.

Plus, the move toward high switching frequencies for higher power densities and better efficiency — as the availability of new technologies in Mosfets becomes readily available (GaN, SiC) — further necessitates the consideration and control of emissions. When switching at higher frequencies, the usual approach for power magnetics is no longer valid.

Würth Elektronik completely understands these concerns with electromagnetic radiation. Our team is fully prepared to support with issues that arise due to changing technologies, designs, and parameters.

Designing one specific inductor that’s valid for only few conditions is not the WE way. A small change in the switching device can affect the source characteristics of the inductor dramatically. Specific components for a specific design for a specific application is required.

For this reason, WE offers a wide variety of products together with unparalleled levels of support and technical assistance to ensure your device meets EMI standards. View our selection of power inductors here!