Using complex permeability to design with Fair-rite suppression products

Fair-rite allocates some of its closed loop ferrite products to two different categories:

  • inductive; and
  • suppression.

Sometimes the same dimensioned cores are available in both categories with different part numbers and possibly different prices, implying some real difference in behavior, eg 5943003801 and 2643803802 are both FT240-43 sized cores.

Material datasheets often contain a note like this from the #43 datasheet:

Characteristic curves are measured on standard Toroids (18/10/6 mm) at 25°C and 10 kHz unless otherwise indicated. Impedance characteristics are measured on standard shield beads (3.5/1.3/6.0 mm) unless otherwise indicated.

I sought to clarify my interpretation of this clause by asking Fair-rite …whether the published material permeability curves / tables apply to suppression product. Can I use the published permeability curves / tables to predict inductor impedance reliably for suppression products?  Fair-rite's Michael Arasim advised…

Yes the published permeability curve can be used to predict impedance. There will be some variance in the shape of the curve due to individual part size as well as process and material variation. The level of this variance will change depending on the individual material but, the curves themselves are all produced using the same sized toroidal core for each material. One thing of note; Inductive rated parts in theory will be controlled more tightly to adhere to the complex permeability curve since their acceptance criteria is generally going to be inductance and loss factor based. Impedance rated parts are accepted based on a minimum impedance at select test frequencies. Since the impedance is a complex value influenced by both the inductive and resistive components of the complex permeability; In theory you could see more variance in each component and still hit the impedance rating. Prior to production, all lots of our materials are screened to ensure that they will adhere (within a tolerance) to the published material data.

So, the following notes and tools are applicable to Fair-rite inductive and suppression products, but one might expect more variation in the real and imaginary components of impedance with suppression products, and loss in cores used for transformers.

My own experience is that the difference is not huge, but it explains the somewhat wider variation observed with suppression products and the need to verify designs by measurement.