Optimal Resonant Condition for Maximum Output Power in Tightly Coupled WPT Systems Considering Harmonics

Previous studies for calculating the maximum output power (MOP) of wireless power transfer (WPT) systems typically employ fundamental harmonic analysis, which is accurate for loosely coupled WPT systems since high-order harmonics are largely suppressed. However, in practical applications, such as automated guided vehicles, the transfer distance is limited, leading to tight coupling. Inevitably, harmonics are introduced, causing inaccuracies in evaluating the MOP ability and associated operating conditions. In this article, a harmonic model is developed for MOP calculation using a series-series compensated system as an example. On this basis, the critical coupling coefficient is defined to establish the threshold for tightly coupled condition. Under this condition, optimal frequency selection is implemented to improve the output power of system rather than limiting it to operate at the inherent resonant frequency. Furthermore, higher output power is attainable by tuning compensation parameters to enable different resonant states in primary and secondary sides, while ensuring zero-voltage switching. The contribution of high-order harmonics to the total power is quantified. At a transfer distance of 5 mm and coupling coefficient of 0.7, the experimental results validate the accuracy of the harmonic model across a broad load range from 5 to 50 Ω using a 400-W prototype.