This paper proposes a voltage sense circuit and a current sense circuit for extracting the fundamental AC voltage and current in variable-frequency inductive power transfer (IPT) systems. Particularly, the proposed circuits are designed for an EV wireless charger. A current source is generated via a high resistance and a current transformer in these two sense circuits, respectively. The main part of both sense circuits uses the “double resonance” and “critical damping” principles to gain a stable voltage gain and a linear phase response in the operating frequency band. The AC output voltage is processed by a rectifier for amplitude measurement and by a comparator for phase detection. Meanwhile, the harmonic components are effectively filtered due to the band-pass property of the sense circuits, hence their impact on the AC output voltage is negligible. The operating principle, parameter design method and parameter sensitivity analysis are discussed in detail. The feasibility of the proposed sense circuits is verified by calculations and experiments.

Energy efficiency and leakage magnetic field (LMF) are two important issues in electric vehicle inductive chargers. In this work, the maximum achievable coil efficiency and the corresponding LMF strength are formulated as functions of system parameters, and figures of merits (FOM) are proposed for assessing the efficiency and LMF performance of the coil assembly pair. The impacts of the coil assemblies’ geometric parameters on both FOMs are examined with the aid of finite element analysis (FEA), and measures to improve the FOMs are suggested based on FEA results. A manual optimization process is conducted on a coil assembly pair. Compared with the initial design, the optimized one results in a higher DC-to-DC efficiency and lower LMF strength while consuming less copper. The performance improvement is verified by FEA results and experimental data measured on an 85 kHz electric vehicle inductive charger prototype. The key measures for coil assembly optimization are summarized.

An equivalence-principle-based method (EPM) for calculating the magnetic field around the coil assemblies applied in electric vehicle inductive chargers is proposed. The ferrite core and the aluminum shielding plate are substituted with surface equivalent sources (ESs), the distribution of which is solved from surface boundary conditions. The magnetic field is the sum of the incident field (induced by the coil current) and the scattered field (induced by ES). The impacts of ferrite core and aluminum plate are quantitatively characterized using their respective source–field relations. The optimization of coil assemblies to improve the efficiency is selected as an application of EPM. The key factors that affect efficiency, e.g., mutual inductance and stray losses, are approximated using simple functions of ES. An exemplary circular coil assembly pair is manually optimized via EPM. Comparisons with finite element analysis (FEA) prove the accuracy and speediness of EPM. The other benefits of EPM are also discussed. Due to the immeasurability of ES, experimental test results of two coils are compared with FEA results to indirectly validate the accuracy of EPM.

Active rectification is a common option for improving efficiency and impedance matching in inductive power transfer systems. One of the technical challenges in active rectification is the synchronization of phase angle. In this work, the rectifier input impedance, which is calculated from the AC current and AC voltage, is used as the control objective during phase synchronization. When the impedance angle matches the target value, synchronization of phase is automatically fulfilled. The details of a PI-controller-based phase synchronization algorithm is introduced, and the PI coefficients are manually optimized. Experimental results demonstrate the good performance of the proposed phase synchronization method.