The variable load and coil distance of implantable wireless power transfer system lead to unsteady output voltage, and resonant topology for wireless power transfer system is likely to solve this issue. But existing topologies are unable to meet the demands of implantable wireless power transfer system in practice. This paper presented a method to design resonant topologies for implantable wireless power transfer system in detail and proposed a series of topologies. Wireless power transfer systems based on the proposed topologies can achieve load‐independent output voltage. The output voltage is not constrained by the parameter of coils and its load‐independent feature is irrelevant to the coupling coefficient. Besides, a simple output voltage estimation method can be implemented on the transmitting side to monitor output voltage accurately. The characteristics of proposed implantable wireless power transfer systems are compared and the most attractive resonant topology named LCC/PS topology is pointed out. The proposed implantable LCC/PS wireless power transfer system has high power transfer efficiency from 60% to 76.9% at a coil distance of 5 cm when the load varies from 500 to 2500 Ω. An experimental setup is built and its working frequency is 6.78 MHz. The experimental results with acceptable tolerance show the accuracy of theoretical analysis and the excellent performance of the resonant topologies.

In the multi-load wireless power transfer (WPT) system, the output power and transfer efficiency will drop significantly with the change of distance between transmitter and receiver. Power distribution among multiple loads is also a major challenge. In order to solve these problems, a novel multi-load WPT system based on parity–time symmetry (PT-WPT) is proposed in this paper. Firstly, the multi-load PT-WPT system is modeled based on the circuit model. Then, the transmission characteristics of the multi-load PT-WPT system are analyzed. It is found that constant output power with constant transfer efficiency can be maintained against the variation of coupling coefficient, and the power distribution relationship among loads is only related to the coupling coefficient. Further, power distribution under different coupling situations is analyzed in detail to meet different power demands. Finally, taking a dual-load PT-WPT system as an example, the system parameters are designed and the circuit simulation is carried out. The simulation results are consistent with the theoretical analysis, which shows that PT symmetry can be applied to the multi-load WPT system to achieve constant output power, constant transfer efficiency, and power distribution simultaneously.

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