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A metal-rim-connected inductive coupler with series-none compensation topology is proposed for smartwatch applications. By cross-connecting the receiving coil to the metal rim with a 1 mm slot, the direction of the induced current on the metal rim is transformed to be the same as the current flowing on the receiving coil, leading to a strong magnetic coupling between the transmitting coil and receiving coil. Considering the space limitation in the smartwatch, non-compensation components are needed inside the smartwatch and only a series capacitance is integrated on the transmitter side. A prototype of the proposed inductive coupler has been built and the wireless power transfer through metal rim has been validated via experiment. The experimental results show that the prototype achieves 5 W output power with 87.4% coil-to-coil efficiency.

Fast charging of electric vehicles (EVs) has been the trend recently. For conductive charging, normal charging can be realised by an on-board charger, while fast charging can be realised by a DC charger which is usually off-board the vehicle. However, for wireless charging, there is a need of a transmitter on the ground and a receiver on the EV side. Therefore, there will be a high-power receiver and a low-power receiver in one EV to achieve dual charging capabilities. To reduce the EV-side cost, weight, and volume, this paper proposes a wireless charging system with a shared receiver compatible of fast wireless charging (FWC) at a small air gap and normal wireless charging (NWC) at a large air gap. The relationship between the coil size and the power level is investigated and a suitable receiver coil size is selected for FWC. The LCC-LCC topology is selected due to its characteristic of output power proportional to the coupling coefficient. Design procedures of the receiver and transmitters are investigated. The simulations and the experimental results obtained from the downscaled prototype verified the effectiveness of the compatibility design.

An underwater wireless power transfer system with a curly coil structure is proposed to adapt to the cylindrical symmetric hull of the autonomous underwater vehicles (AUVs). The unipolar and bipolar curly coils are optimised to minimise the weight of the receiver with the same output power. It is revealed that the bipolar curly coil structure has a heavier receiver than the unipolar curly coil structure. However, the electromagnetic field radiation in the AUV of the bipolar curly coil structure is much smaller than that of the unipolar curly coil structure, which means that the bipolar curly coil structure has a smaller influence on the electronics components in the AUV. Therefore, the bipolar curly coil is adopted for the prototype. The series−series (SS) and double-sided inductor−capacitor−capacitor (LCC−LCC) compensation topologies for the bipolar curly coil structure are also investigated. A prototype was built and the experimental results showed that distorted coil currents are generated in the SS compensation topology, while the LCC−LCC compensation has a nearly sinusoidal coil current. The efficiencies of the SS and LCC−LCC compensation topologies are approximately the same, at ∼95%, which indicates that the proposed curly coil structure is applicable.

Accurate modelling is necessary for designing a wireless power transfer (WPT) system and currently, first harmonic approximation (FHA) is widely used. However, it is not accurate for WPT systems with a strong coupling, such as fast charging of electric vehicles with a coupling coefficient of 0.80, compared to the conventional wireless charging with a coupling coefficient of 0.15-0.30. This study develops accurate models for WPT systems with series-series (SS) and LCC-LCC compensations. For the SS compensation with a strong coupling, the transmitter and receiver currents are distorted, leading to much larger values than the estimations from FHA, which determines the selection of power switches and resonant capacitors. For the LCC-LCC compensation, the transmission coil currents are only highly distorted with rich third-order harmonics at the vicinity of the 0.889 coupling coefficient, leading to low efficiency and large coil current ratings. For the experimental prototype, the efficiency drop can be over 3%, which is significant, especially for high-power systems. The WPT system with the LCC-LCC compensation should avoid operation in the vicinity of this particular coupling coefficient. Furthermore, experiments are conducted, and the results perfectly match the calculations, demonstrating the accuracy of the proposed models.