The mutual inductance between the transmitter and receiver sides is critical for the efficiency of a wireless power transfer (WPT) system. This article proposes a novel online mutual inductance measurement method based on a local detection coil with a sandwich structure. The mutual inductance and the receiver current are obtained by measuring the induced voltage of the sandwich coil and the output voltage of the inverter, with contactless measurement from the receiver side. Using the ferrite substrate as the skeleton of the sandwich detection coil not only increases the differential mutual inductance between the receiver coil and the sandwich coil but also maintains the differential mutual inductance almost constant when horizontal misalignment occurs. Moreover, the differential mutual inductance is used to calculate the mutual inductance between the transmitter and receiver coils and the orthogonal eigenvalue expansion method is exploited to analyze the sandwich detection coil in a finite region. Then, the factors affecting the differential mutual inductance are analyzed and optimized, and a stable differential mutual inductance is obtained by formulating a standard coil design procedure. Finally, a prototype is built to verify the proposed method. The experimental results show that the differential mutual inductance remains almost constant when the horizontal misalignment reaches 50% of the outer radius of the transmitter/receiver coil, and the maximum calculated error of the detected mutual inductance is 2.18%.

The performance of a conventional Class-E2-based WPT system is sensitive to system parameters such as the coil coupling coefficient and load variation. System efficiency decreases rapidly when the coil coupling coefficient and load deviate from their optimum values. In this paper, an impedance matching method and a design procedure are proposed to maintain high system efficiency over a wider range of coupling coefficient and load variations. The load-pull technique is adopted to identify the high-efficiency load region of a Class-E power amplifier (PA), and a double-L-type impedance matching network (IMN) is proposed to transform the load impedance of a Class-E PA into a high-efficiency working region. Compared to a single L-type IMN, a double-L-type IMN is more flexible and has better tuning performance. A 6.78 MHz Class-E2-based WPT system was built to validate the proposed design method. The experimental results show that the proposed double-L-type IMN can significantly attenuate the decline in Class-E PA efficiency when system parameters dynamically change. With a double-L-type IMN, the WPT system could maintain high efficiency (over 55%) under a wider range of coil coupling coefficient and load variations. The peak system efficiency reached 83.2% with 13.7 W output power. The impedance matching method and design procedure in this paper could provide a practical solution for building a high-efficiency WPT system with strong robustness.