Many of my articles call for finding the complex permeability of a ferrite components from manufacturer’s data.
Let’s explore an example used in a recent article, Another small broadband RF transformer using medium µ ferrite core for receiving use – 50:450Ω.
The core used was a Fair-rite ferrite core of #43 material, and the magnetising impedance of a 5t winding needed to be found.
Above is a chart from Fair-rite’s catalog. Permeability is a complex quantity and is frequency dependent. One could scale from the graph, the values for µ’ and µ” at the frequency of interest.
For much of my work, I digitised such charts and used a cubic spline interpolation of key points to give a better estimate than scaling from the graph.
Above, the manufactuer’s chart with digitised points added.
More recently, Fair-rite published a table of measured values, presumably from which they had created the chart.
I have published an online calculator that contains data for some of the most common ferrite mixes, and functions to interpolate the data to arbitrary frequencies of interest.
For example, to find the magnetising impedance of 5t on a BN43-202 at 0.5MHz, we:
- consult the calculator to find µ’ and µ”, then
- use those in another calculator to find the magnetising impedance.
1. Find µ’ and µ”
Above, Ferrite permeability interpolations output.
2. Find magnetising impedance
Above, Calculate ferrite cored inductor (from Al) output.
References / links
- Duffy, O. 2015. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. https://owenduffy.net/files/EstimateZFerriteToroidInductor.pdf.
- Snelling, E C. Soft ferrites properties and applications. Iliffe books 1969.