Finding the Right Ferrite: Comparing Core Materials

Only a measurement in the EMC laboratory can confirm the definitively correct ferrite. It is generally not possible to calculate the dynamic impedance (transfer impedance) of an electronic circuit in the highest frequency range. This depends on many factors that are difficult to describe mathematically, especially in the HF range. However, an initial selection can be made using comparison measurements and based on experience.

How Do You Find the Right Ferrite?

Supply voltages or ground leads in good designs are in the range of 1 to 10Ω; useful signal lines, depending on application, are between 50 to 100Ω and greater (e.g., bus systems such as CAN, SCSI etc.).

Beginning with this transfer impedance (ZA or ZB) and the desired interference suppression, you can find the desired ferrite impedance (ZF) based on the nomogram. Knowing the impedance of the ferrite, you look for the ferrite that matches the application, e.g., a ferrite sleeve or an SMD ferrite.

Little tip: Wurth Electronics offers a Component Selector, a free software program for quickly and easily selecting SMD ferrites and other components, as well as for simulating some parts of them.

Comparison of Core Materials

It only makes sense to use the core materials for creating inductors within a limited frequency range due to the frequency-dependent loss components. Core losses increase considerably above a typical frequency limit. Here the core material can still be used as a filter element.

Conclusions:

  • Iron powder materials (Fe): can be used as a pure inductor up to approx. 400kHz; after that point, the loss component dominates in R, which is present to approx. 10MHz (more, depending on the core material). The core is no longer effective in the frequency range beginning at approx. 20MHz and up.
  • Manganese-zinc cores (MnZn) are inductive up to frequencies of 20–30MHz. Increasing losses must be anticipated in the frequency range beginning at 10MHz. In the frequency range beginning at approx. 80MHz, the core material is no longer effective.
  • Nickel-zinc cores (NiZn) are inductive up to frequencies of 60MHz; above that, the core material is lossy up to frequencies of 1GHz and more.

This qualitative comparison makes it clear why nickel-zinc ferrites, in particular, have established themselves in the EMC range. It is the core material that functions effectively as a filter across the largest and most relevant frequency range.

Lately, however, MnZn ferrites have also been used to suppress cable interference in the range of 150kHz to 30MHz. Wurth Electronics offers two series here, the STAR-FIX LFS snap ferrites and the WE-AFB LFS ferrite sleeves.

To learn more about how to select the right ferrite for effective EMC protection, read our previous blog posts or see our many ferrite products.

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