4.5 Shim Inductor

Off-the-shelf DC storage chokes can store some DC current and allow a certain amount of high-frequency ripple current. For most such components, the maximum ripple current is 40% peak-to-peak of the maximum DC current (usually specified at a 100kHz reference). Unfortunately, many data sheets don’t cover all operating points. As the switching frequency increases, this ripple capability degrades, and this data are not captured in inductor data sheets.

Shim inductors do not belong to the family of conventional storage chokes. Shim inductors store current over the full RMS current operating range, with allowance for huge ripple current (from -I_PEAK to +I_PEAK and a total ripple current of 2xI_PEAK). This means huge high-frequency energy must be stored cycle by cycle. The only possibility is to work with ferrite air-gapped cores or low permeability ring cores. The 2xI_PEAK current with high harmonics content presents a massive challenge here. The only possibility to conduct such a current is to use Litz wire which will reduce skin effect loss. Such devices must keep a low number of winding layers, two to three max, otherwise the proximity effect will cause the copper losses to increase again. This causes these products to be custom-made and design-specific, and as such, unique and slightly more expensive.