Designing with binocular ferrite cores – Σ(A/l)

Designing with ferrite binocular cores can be frustrating as there are different formats in which data is provided, and data for different mixes on the same dimensioned cores appear inconsistent.

#61 mix

This article documents calculated geometry Σ(A/l) derived for a number of Fair-rite cores from their specified Al (at µi).

This is a relatively low µ core at measurements at 10kHz (as stated in the specifications) will not be affected by self resonance, and so the specified Al should be reliable. Al is a controlled parameter for these #61 products.

Fair-rite partMin Al (nH)Min Σ(A/l) (m)Min Σ(A/l)+20% (m)

Σ(A/l) is a simple function of Al and µi (Al=Al/(4πe-7*µi)).

Fair-rite specifies Min Al for the parts in the table but no tolerance, though they show a 20% tolerance on the controlled impedance, so the fourth column showing Min+20% could be used to imply a typical value. Nevertheless, MinΣ(A/l) from the table might be a better design point.

Many of these parts are dimensionally identical to the BN61-xxxx designations (eg 2861000202 is equivalent to a BN61-202).

The value Σ(A/l) and appropriate µ can be plugged into Calculate ferrite cored inductor – ΣA/l or Σl/A to predict the impedance at a given frequency, but note that at higher frequencies approaching SRF selection of an appropriate value of Cs is necessary for good results.

Other mixes

The binocular products are described as having different controlled parameters depending on the mix, so application of the Σ(A/l) implied from the #61 data to the other mixes (using the appropriate µ’,µ” data) that might be tweaked from the nominal material characteristic is prone to some error, even though they are likely to be pressed in the same dies. In any event, backtracking from a controlled Z at 25MHz or more is fraught with problems due to test fixture strays and other things.

Experience with #43 binocular cores is that impedance may be significantly higher than predicted from core Σ(A/l) and µ’,µ” data even are relatively low frequencies. This may be due to tweaking to meet other / controlled parameters.


As always, predictions need to be verified by measurement.