How to select ferrite cores

This page reviews the points to keep in mind when selecting a ferrite core.

Shape

Point Select a core that fits the cable.
In the case of a ring core, the form factor depends on Ae/Le as shown below. Since Ae/Le is proportional to impedance, the most advantageous shape will obey the following criteria in order:
To make Ae/Le as large as possible  →  1) Reduce inner diameter (B)
2) Increase length (H)
3) Increase outer diameter (A)


Example
  P/N ΦA (mm) ΦB (mm) H (mm) Z (Ω at 100MHz
1 FSRH142200RN000T 14.0 8.0 20.0 127
2 FSRH091160RN000T 9.0 4.3 16.0 145
3 FSRH050100RN000B 4.7 1.4 10.0 120

   → Nearly the same performance

Number of turns of wire

Points 
  • At low frequencies (up to 300MHz), the effect is proportional to the number of turns of wire.
  • At high frequencies (300MHz and greater), do not exceed 3 turns.

Point 1: Low frequency region
The impedance depends on the core as follows:
  • It is proportional to the effective cross-sectional area (Ae)
  • It is inversely proportional to the effective magnetic path length (Le)
  • It is proportional to the square of the number of turns of wire (N) of the cable


Point 2: High frequency region
If the number of turns of wire is low, the stray capacitance has little impact, and the impedance extends into the high frequency band.
As the number of turns of wire increases, the impact of stray capacitance increases, causing the resonance points to move to the low frequencies and reducing impedance in the high frequency band.
In some cases, increasing the number of turns of wire can harm performance.


Example

Materials

Point If you use Murata’s ferrite cores, there's no need to worry.
Murata mass produces its own Ni-Zn ferrites.

The raw materials are carefully selected to improve performance and ensure stability.

By changing the mixing ratios and other properties, a variety of materials can be produced. Today, there is no significant difference between materials for noise frequencies of 30MHz and greater, but there are differences for the low frequency regions.

Point 1: When low frequency noise removal is required
Select materials with a large μ’.

Point 2: When high frequency noise removal is required.
There is little difference between materials.


For general radiation protection, if you stick with Murata products there's no need to worry about the type of materials.

Summary

Following these points will enable you to select the optimum ferrite core.

Shape
Select the core with the smallest possible inner diameter (this is the most important point).
Make sure the length exceeds the outer diameter.
For effective results, the priority order is (1) inner diameter, (2) length, and (3) outer diameter.

Number of turns of wire
Basically, the effect is proportional to the square of the number of turns of wire.
For frequencies above 300 MHz, do not wind more than 3 turns of wire.

Materials
If you use Murata products, the materials make no difference at higher frequencies (above 30MHz).