Equivalent inductance of a ferrite core

[Bruce Sparrow]

#25752

I have a ferrite core that has proven to reduce radiated emissions in our medical device. The problem is, the ferrite core is too big and I’d like to move the core inside our device where the trouble frequency is generated. I understand if I put an equivalent inductance in series with the cable wires this will have the same effect as the external ferrite. How do you determine the equivalent inductance of a ferrite core? I am using Fair Rite core 0446167281.

[Bruce Sparrow]

#25754

Ferrite cores designed for EMI suppression show inductive characteristics up to some frequency but then turn resistive. This is ‘complex permeability’. Devices made from these materials are not considered inductors but rather suppressors and are not rated in henries but in impedance (at frequency). The resistance, as the name implies, has little phase lag so power is absorbed and dissipated as small amounts of heat. With your part this mode begins above 100MHz. If you look at the Fair-Rite web site for that part an impedance curve is shown. So you really can’t replace it with an inductor, per se.

Fair-Rite makes solid core equivalent parts to the snap on version that you have. The equivalent would be 2661626402.The solid core equivalents tend to be noticeably smaller, for the same impedance, because they do not have to compensate for the losses generated by splitting the core in half. In this case, over 40% smaller.

The frequencies that the part is suppressing is not identified nor is how much suppression is required so if the solid core part still isn’t small enough then some experimenting will be required to get another solution. Smaller beads around only the offending wires might be considered.

[fr]

#25755

Thank you for your reply. The “little phase lag” is due to the inductance of the core. I want to know that inductance and I am hoping Fair Rite can provide that??? Using a smaller core on each individual wire is an option, but the application requires the patient carry our device on their hip. Lugging around a small box 24/7 that’s full of ferrites becomes annoying and exposes the device to damage mainly due to weight and size.

[Bruce Sparrow]

#25756

*** Corrections: the part you mentioned becomes resistive above about 25MHz, not 100mHz. Equiv solid core is 2646626402 ***
Sorry for the mistake and any confusion it may cause.

The inductance value is highly frequency dependent, at the frequencies of concern. We can, however calculate the inductance (at a frequency) from the impedance curve. For example, XL at 20mHz is about 65 ohms. Using Z=2PI*F*L we get 517nH. Note at that same point the resistive losses are also about 65 ohms. The resulting vector impedance is 90 ohms. Above 200mHz the inductance drops to 0 quickly and the component is purely resistive.
Impedance curve: https://www.fair-rite.com/product/round-cable-snap-its-446167281/

Feel free to give me a call if I can be of help trying to get the smallest solution.

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