Wireless power transfer with self-tuning LCC inverter using PWM-controlled switched capacitor is proposed in order to compensate for the variations of TX inductance. Usually, receivers contain magnetic and conductive materials for shielding and mechanical support. The inductance of TX coil is sensitive to the variation of TX-RX alignment because the interaction between TX coil and RX magnetic-conductive materials is also varied. It is shown that the detuning of TX inductance value exacerbates the losses in LCC inverter. In proposed method, the PWM duty ratio of a single capacitor adjusts the effective output impedance of LCC inverter. The PWM feedback loop is designed such that the power factor and real-part impedance of LCC inverter is maximized. The PWM switch is turned-on at zero voltage and turned-off with low dv/dt, minimizing switching losses. Such soft-switching tunable capacitor can handle higher power compared to traditional hard-switching capacitor, pn-junction capacitor, or analog-IC capacitor. Other advantages of proposed technique are the continuity in achievable capacitance value using only one capacitor. The feedback can fully be implemented by analog components, obviating the necessity of digital samplings, ADC/DACs, and microcontrollers. 54 W is transferred at 76.6% and 60.7% with and without the proposed method, respectively.

The paper proposes optimal driving conditions of two transmitters (TXs) in magnetic wireless power transfer, where the couplings from each TX to an RX are not identical. First, for the case of identical TX coil currents, a criterion for deactivating a specific TX element is proposed. Specifically, one TX element among the two TXs often has weaker coupling coefficient than the other TX element. It is found that the TX element with weaker coupling should be deactivated if its coupling coefficient is less than 0.414 times of the stronger coupling. Second, for the case of adjustable TX coil currents, the optimal current ratio between the two TX coils is proposed. It is found that the TX coil current ratio should be equal to the coupling coefficient ratio, e.g., the TX coil with twice coupling with an RX should carry twice current. The results are widely applicable regardless of absolute coupling strength and load condition because it turns out that the results only depend on the ratio of couplings. Finally, it is discussed that the required component values of an LCC inverter should be adjusted based on the current ratios and coupling between adjacent TXs.

We propose a maximum efficiency tracking for multiple-transmitters system that optimizes both the current ratios of transmitter (Tx) coils and the effective Rx load. It is found that unlike single-Tx case, the optimal load impedance of multiple-Txs system depends on the current ratios among Tx coils. Meanwhile, the Tx current ratios should be controlled to be the same with magnetic coupling ratios between each Tx and Rx. Hence, we propose a two-step tracking method that first optimizes the Tx current ratios and then optimizes the Rx load impedance. To match the Tx current ratio with magnetic coupling ratio, it is analyzed that the reflected resistances at every Txs become identical each other when the Tx coil current ratios are equal to the magnetic coupling ratios. It is also revealed that the reflected resistance at specific Tx is reduced (increased) when its Tx current is increased (reduced) relative to other Tx currents. The proposed feedback equalizes all reflected resistances based on this property. After that, the second step of the proposed tracking is to optimize the Rx load while fixing the Tx current ratios by globally scaling up or down the strengths of whole Tx currents. Maximum 478W is delivered at 79% efficiency.

A platform has been presented for wireless powering of receivers (Rx's) that are arbitrarily distributed over a large area. A potential application could be powering of small Rx implants, distributed over large areas of the brain. The transmitter (Tx) consists of three overlapping layers of hexagonal planar spiral coils (hex-PSC) that are horizontally shifted to provide the strongest and most homogeneous electromagnetic flux coverage. The three-layer hex-PSC array is driven by a three-phase time-division-multiplexed power Tx that takes the advantage of the carrier phase shift, coil geometries, and Rx time constant to homogeneously power the arbitrarily distributed Rx's regardless of their misalignments. The functionality of the proposed three-phase power transmission concept has been verified in a detailed scaled-up high-frequency structure simulator Advanced Design System simulation model and measurement setup, and compared with a conventional Tx. The new Tx delivers 5.4 mW to each Rx and achieves, on average, 5.8% power transfer efficiency to the Rx at the worst case 90° angular misalignment, compared with 1.4% by the conventional Tx.

We propose a fast maximum efficiency tracking, which instantly optimizes all. TX coils' currents using runtime-extracted mutual inductances, for simultaneously activated multiple TXs whose output levels are different each other. For fast control, we propose a theory to runtime extract all the mutual inductances between multiple TXs and an RX. Here, one prerequisite of extracting the values of mutual inductances is to acquire its relative ratios between the mutual inductances. It is proposed that the relative ratio can be obtained by reflected resistances and coil currents of each TX, which are ac quantities but can easily be measured using dc input property of LCC inverter. Using the ratio information in conjunction with RX dc output, the exact values of mutual inductances can be extracted. The aim of the proposed method is to enable fast (no iteration) maximum efficiency tracking for multiple TXs, which entails the optimization of the relative ratios among TX currents, as well as the RX load optimization. A 200-W RX moves at 10 km/h, and the controller can track the optimum target TX currents in response to time-varying magnetic couplings.

A receiver that can adjust the output voltage of coil is proposed to simultaneously charge as many receivers of different couplings as possible. One large coil of traditional receiver is split into two small coils while total coil volume is kept constant. The receiver has three output modes: half voltage, same voltage, and boosted voltage. Firstly, the boosted mode can produce the RX coil voltage that is ~60% higher than the traditional receiver with identical coil volume. It is useful for low-coupling and low-TX-output condition. Secondly, the half-voltage mode protects the semiconductor devices from overvoltage by outputting half of the traditional receiver with identical volume. It is necessary for high-coupling and high-TX-output condition with otherwise high voltage stress. Finally, same-voltage mode produces the same coil voltage as with traditional receiver. By the mode selection, relatively constant RX coil voltage can be maintained at both high coupling and low coupling conditions. This enables a single TX to charge both the strongly-coupled RX and the weakly-coupled RX simultaneously. In the experiment, the proposed 30 W receiver can operate across 1.9~7.5 cm whereas the conventional receiver is restricted to 3.5~5.5 cm.