Winding Structure of Air-Core Planar Inductors Capable of Reducing High-Frequency Eddy Currents

High-frequency converters with high power density have shown promise in recent years. Air-core planar inductors built with printed circuit boards can eliminate problems associated with the use of magnetic materials, such as iron loss and magnetic saturation. However, conventional air-core planar inductors have large stray capacitance and suffer from eddy current loss. Stray capacitance affects the self-resonant frequency, and eddy currents affect the resistance and inductance of the inductor. This paper proposes a stranded wire structure to reduce stray capacitance and eddy currents and realize a smaller stray capacitance than that in the conventional structure. In addition, eddy currents can be reduced by splitting windings of the proposed stranded wire structure. Seven types of air-core planar inductors were built, and their impedance, resistance, and inductance were measured to evaluate the effects of different structures on stray capacitance and eddy currents. Furthermore, the temperature distribution of planar inductors was measured with a power amplifier. The experimental results demonstrate the utility of the proposed stranded wire structure.