TDK Ferrite Core Terminology

Below are some common questions TDK has received on some of their terminologies.

What is inductance coefficient AL?

It is the self-inductance that is generated in units of the winding of a coil with a given shape and dimensions wound on a magnetic core, as shown in the following formula.

It is normally represented in units of 10–9H (nH).


L : Self-inductance of the coil when there is a magnetic core (H)

N : Total number of turns of the coil

What is “effective permeability μe”?

It is the permeability from the effective self-inductance in the magnetic core of a closed magnetic circuit (with a leakage flux that can be ignored) as shown in the following formulas.

Formula (1) is used for measurements

formula (2) is used for the dimensions of parts of the magnetic core and for calculations when permeability is added.

L : Effective self-inductance (H)

N : Total number of turns

ℓ : Magnetic path lengths (mm) of each of the same materials and same cross-sectional area parts

A : Each of the cross-sectional areas (mm2)

: C1 = core constant (mm–1)

μ : Permeability of each material

What is “initial permeability μi”?

It is the amplitude permeability of the magnetic core when the intensity of the magnetic field is unboundedly close to “zero”.

  • The symbol and the catalog of this data are in accordance with IEC Publication 60401-3.

  • TDK’s terminology and settings were established by reference to JIS C 2560-1,-2.

Information was provided by TDK FAQ and Digi-key Electronics.