A Low Power IPT System With Integrated Magnetics and Hybrid Tuning for Metal Object Detection

Metal object detection (MOD) plays a crucial role in ensuring the safe operation of inductive power transfer (IPT) systems. One of the main challenges in MOD is distinguishing magnetic misalignments from the presence of metal objects, especially when ferrite cores are utilized. This article proposes an IPT system with integrated magnetics and a hybrid tuning circuit designed to differentiate the existence of a metal object from coil misalignments involving ferrite cores. To enable this differentiation, a hybrid tuning method is developed by incorporating an additional coil with series and parallel configurations. This configuration allows the system to resonate at two distinct frequencies. Despite being magnetically decoupled, the extra tuning inductor is physically integrated into the transmitter (Tx) pad, thereby enhancing the system's metal object detection capability. Theoretical analysis demonstrates that the ratio between the two resonant frequencies remains approximately the same even under misaligned conditions, but this ratio varies when a metal object is present. Simulation results further illustrate that both nonferromagnetic and ferromagnetic metal objects can be effectively detected by monitoring the ratio between the two resonant frequencies as part of the handshaking process prior to power transfer. A 10 W working prototype is built, and the experimental results demonstrate that a metal object with an object-to-coil size ratio down to 0.2 can be effectively detected even when the magnetic misalignment is increased to 20%.