Inductors (Coils)

Tech Notes

Selection Guide for Power Inductors in Consideration of Leakage Flux

Magnetic shielding characteristics that lower the leakage flux of power inductors

Small SMD coils used for the power supply of electronic equipment are called power inductors. The mainstream of the products are a wire-wound type in which a copper wire is wound around a ferrite drum core, and an integrally-molded metallic coil type. Compared with multilayer-type, thicker copper wire can be used for it, which can support larger current.
When winding on a magnetic toroidal core (donut-like core) and applying current, the magnetic flux refluxes the inside of the core. A magnetic circuit like this is called a closed magnetic path. In the case of using rod-like or drum-like cores, the magnetic flux goes out of the inside of the core and becomes a leakage flux, which draws a loop returning to the core again. This is called an open magnetic path.
In electronic equipment, if the leakage flux of the inductor is magnetically coupled with another coil, wiring pattern, etc., inductance can change or noise may be caused. Particularly, the leakage flux increases in power inductors in which large current flows, various magnetic shield structures are employed so that leakage of magnetic flux to the outside would not occur as much as possible.
Power inductors are categorized into the types shown below in terms of magnetic shielding;

Table 1 Types of power inductors in terms of magnetic shielding
Non-shielded type A type in which just simply a wire is wound around a drum core and a magnetic shielding is not provided.
Resin-shielded (semi-shielded) type A type in which the wound drum core is encapsulated and molded with resin kneaded with magnetic powder such as ferrite or soft magnetic metal. Although the magnetic shielding effect is limited, it has an advantage of cost reduction.
Full-shielded type A type made structurally closer to the closed magnetic path by covering around the drum core with ferrite shield core. However, magnetic shielding does not always provide complete encapsulation, and flux can leak from the air gap or terminal of the junction between the shield core and drum core. The shield core has various shapes such as a ring type and an L-type, and the strength and distribution of magnetic flux leakage, as well as the cost vary, depending on the structure.
Metallic integral molding type This is not a wire wound type but a type in which an air-core coil is encapsulated in the soft magnetic metal powder kneaded with binder (binding material) and integrally molded. Since there are no air gaps, it can lower the leakage flux than in the usage of full-shielded type. However, magnetic flux leaks from the surface or other locations.

Figure 1 Power inductors of open magnetic path and closed magnetic path

Figure 1 Power inductors of open magnetic path and closed magnetic path

3D graphics of a power inductor leakage flux based on near magnetic field analysis

Which type of power inductor to choose depends on cost, performance, size, etc. and this is a problem that plagues designers. TDK is providing a help for the customers to select the most appropriate power inductors through measuring the leakage flux of power inductor using the near magnetic field measurement system and its visualization.

The basic configuration of the system is shown in Figure 2.
The power inductor mounted on the evaluation board of the DC-DC converter is scanned with the magnetic field probe equipped with the small loop antenna at the half-height of the power inductor, and the magnetic field strength around the power inductor is measured. The data of the obtained time waveform is converted into a spectrum waveform by a spectrum analyzer, and then it is displayed as 3D graphics on a monitor.

Figure2 Configuration of measurement/visualization system of near magnetic field around power inductor

Figure2 Configuration of measurement/visualization system of near magnetic field around power inductor

Metallic integral molding type SPM series with extremely low leakage flux

TDK’s power inductor core product, VLS-EX series, CLF-NI series, and SPM series; 3D graphic images visualizing their leakage flux are shown in Figure 3.