Along with the selection of other components, ferrite core selection is also an important step in design process. Let see some of the important selection criteria;
Ferrite Core Selection
Below steps will clear the process of ferrite core selection;
1) Shape of cores
• Effective cross sectional area and Effective magnetic path length of ferrite core are the main parameters of ferrite core for deciding shape of cores.
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| How to Select a Ferrite Core - Ae and Le value |
• Cross section area (Ae) should be large and Magnetic path length (Le) should be small.
• Impedance of ferrite core depends on shape of ferrite core.
• Formula for calculating inductance using Ae and Le is;
L = N²µ (Ae / Le)
µ = Permeability
• As an example if we compare shown ferrite cores, core A (it’s Ae is large) is the best performing as compare to core B.
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| How to Select a Ferrite Core - Shape of Core |
• Core A has the highest impedance as compare to core B.
2) Winding turns on a core
• Impedance can be increase by number of turns within a core increase.
• If the working frequency is under 200MHz, ferrite core supporting lower frequency should be use. Also we can set turns.
• If the target frequency is over 200MHz, adding more ferrite cores or use a ferrite core which support’s higher frequency.
• We have to keep winding turns less, because more turns can create problem of stray capacitance.
• When working at higher frequency, this stray capacitance decreases impedance.
• This stray capacitance generates between individual wire turns of the coil. It didn’t cause energy loss but change behavior of coil by causing the coil to become self-resonant.
• More turns, more stray capacitance.
Conclusion
Important role in noise suppression is played by ferrite. Proper selection of ferrite will solve all the issues related to design and make circuit design easy.