In this paper, we designed and demonstrated a smartwatch strap wireless charging system for the first time. First, we designed a flexible printed circuit board (PCB) coil, shielding material, and receiver (Rx) circuit in a watchstrap. In the design process, we proposed a model for the flexible PCB coil with a bending radius of 40 mm and shielding materials. We used a flexible PCB coil that has 215 μm thickness with dimensions of 54.5 × 16 mm. In addition, ferrite core and sheet are applied on the transmitter (Tx) and Rx coils. We verified the proposed model through a three-dimensional (3-D) electromagnetic (EM) simulation and measurement in the frequency and time domains. The proposed flexible PCB coil inductance modeling results showed 7.5% and 3.4% errors when compared to the 3-D EM simulation and measurement results, respectively. Furthermore, we demonstrated the smartwatch strap wireless charging system using an LG Watch Urbane. A resonance frequency of 100 kHz with the series–series tuning topology is used in accordance with the Qi specifications. Finally, we achieved 30% dc–dc power transfer efficiency and exposed magnetic field of 270 mG, 1 cm away from the system through measurements.

中文翻译：

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带柔性 PCB 线圈和屏蔽材料的智能手表表带无线电源传输系统

在本文中，我们首次设计并演示了智能手表表带无线充电系统。首先，我们在表带中设计了柔性印刷电路板 (PCB) 线圈、屏蔽材料和接收器 (Rx) 电路。在设计过程中，我们提出了弯曲半径为 40 mm 的柔性 PCB 线圈和屏蔽材料的模型。我们使用了厚度为 215 μm、尺寸为 54.5 × 16 mm 的柔性 PCB 线圈。此外，铁氧体磁芯和薄片应用于发射器 (Tx) 和 Rx 线圈。我们通过三维 (3-D) 电磁 (EM) 仿真和频域和时域测量验证了所提出的模型。与 3-D EM 仿真和测量结果相比，建议的柔性 PCB 线圈电感建模结果分别显示 7.5% 和 3.4% 的误差。此外，我们使用 LG Watch Urbane 演示了智能手表表带无线充电系统。根据 Qi 规范，使用串联-串联调谐拓扑结构的 100 kHz 谐振频率。最后，我们通过测量实现了 30% 的 dc-dc 功率传输效率和 270 mG、距离系统 1 cm 的暴露磁场。