A New Method for Measuring Winding AC Resistance of High-Efficiency Power Inductors

Abstract

Precise estimation of high-frequency winding resistance is essential for designing high-efficiency power inductors. The ac resistance for high-efficiency inductors can be estimated using analytical methods or simulation-based approaches but it must be verified with measurement. Impedance analyzers as a common method for measuring ac resistance of high-frequency inductor windings perform noticeably poor for high quality factor $Q$ inductors (high-efficiency inductor with a small resistive element). This paper proposes a new simple and highly effective method to measure the ac resistance of high-frequency power inductors (up to at least 110 MHz). In this method, a thin auxiliary winding is added to the inductor to create access to a measurement node, which effectively reduces the series inductance seen by the impedance analyzer to the parasitic leakage inductance of the inductor. Thereby it effectively eliminates the impact of inductor main magnetizing inductance in the impedance measurement. The proposed measurement method is compatible with existing four terminal pair fixture of impedance analyzers. Using a proposed modified test fixture for Agilent 4294A, the ac resistance of two high- $Q$ sample inductors is measured. Experimental results show a very substantial improvement with respect to the traditional four-terminal test fixture. A convincing agreement with the two-dimensional finite element method simulation results in COMSOL is also demonstrated.