At What Frequencies Should Air-Core Magnetics Be Used?

Magnetic components for power conversion (inductors and transformers) are often designed with magnetic cores, which add core loss but reduce the required number of turns, copper loss, and usually total loss. At very high frequencies, poor core materials make this tradeoff less advantageous and air-core magnetic components are often preferred. The boundary between the magnetic-core-preferred and air-core-preferred regimes has not yet been theoretically identified, and intermediate frequency ranges (e.g., 5–30 MHz) see both cored and air-core examples. In this work, we calculate various expressions that suggest that cored inductors can outperform their air-core counterparts up to many tens of megahertz on a volumetric basis and about 10 MHz on a mass basis, based on the properties of currently available magnetic materials. We experimentally demonstrate the boundary frequencies by comparing the quality factors of optimized magnetic-core and air-core toroidal inductors. Formally demonstrating the advantage of magnetic-core over air-core inductors suggests more advantageous design strategies at tens of megahertz, with significant impact on applications at industrial, scientific, and medical frequency bands (6.78, 13.56, and 27.12 MHz) and other radio frequencies.