Smartwatch Strap Wireless Power Transfer System with Flexible PCB Coil and Shielding Material


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 x 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.


Electrical and Computer Engineering

Research Center/Lab(s)

Electromagnetic Compatibility (EMC) Laboratory

Keywords and Phrases

Electromagnetic fields (EMFs); flexible printed circuit board (PCB) coil modeling; magnetic shielding material; smartwatch strap; wireless power transfer (WPT)

International Standard Serial Number (ISSN)


Document Type

Article - Journal

Document Version


File Type





© 2019 Institute of Electrical and Electronics Engineers (IEEE), All rights reserved.

Publication Date

01 May 2019