Analysis of Off-Resonant Flexible Tertiary Coils in Biomedical Implants for Back Telemetry Detection

A systematic design technique of flexible tertiary coil in a wireless power transfer (WPT) system for back telemetry detection is presented. The detection (tertiary) coil is co-planar with the primary coil and receives the backscattered load shift keying (LSK) data from the secondary stage. Tertiary coil parameters such as the number of turns, compensation capacitance, and quality factor are determined to maximize the link gain and the end-to-end (E2E) efficiency. The analysis is validated through SPICE simulations and measurements. A novel area-saving inner-tertiary coil is further proposed and critically compared with traditional outer-tertiary coil. The RF electromagnetic radiation or Specific Absorption Rate (SAR) value of the coils was evaluated for possible health hazards using 3D head models and ANSYS/HFSS finite element software. In comparison to a heuristic outer-tertiary coil design, the primary-secondary (operating) distance is improved by 50%, from 12 mm to 18 mm. At an operating distance of 15 mm, the link gain and E2E efficiency are improved by
$\sim$
207% and
$\sim$
18%, respectively.