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Fixed Inductor Terminology Glossary

Self-Resonant Frequency (SRF)

The frequency at which the inductor’s distributed capacitance resonates with the inductance. It is at this

frequency that the inductance is equal to the capacitance and they cancel each other. The inductor will

act purely resistive with a high impedance at the SRF point.

The distributed capacitance is caused by the turns of wire layered on top of each other and around the core.

This capacitance is in parallel to the inductance. At frequencies above the SRF, the capacitive reactance of

the parallel combination will become the dominant component.

Also, the Q of the inductor is equal to zero at the SRF point since the inductive reactance is zero. The SRF

is specified in MHz and is listed as a minimum value on product data sheets.

Shielded Inductor

An inductor designed for its core to contain a majority of its magnetic field. Some inductor designs are

self shielding. Examples of these are magnetic core shapes which include toroids, pot cores and E-cores.

Magnetic core shapes such as slug cores and bobbins require the application of a magnetic sleeve or

similar method to yield a shielded inductor.

It should be noted that magnetic shielding is a matter of degree. A certain percentage of the magnetic f

ield will escape the core material. This is even applicable for toroidal cores as lower core permeabilities

will have higher fringing fields than will high permeability toroidal cores.

Toroidal Inductor

An inductor constructed by placing a winding(s) on a core that has a donut shaped surface. Toroidal

cores are available in many magnetic core materials within the four basic types: Ferrite, Powdered Iron,

Alloy and High Flux, and Tape Wound. Characteristics of toroidal inductors include: self shielding

(closed magnetic path), efficient energy transfer, high coupling between windings, and early saturation.

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TCUT20183P Datasheet(PDF) 7 Page - Token Electronics Industry Co., Ltd.