A Novel Control Scheme for Class E Wireless Power Transfer Systems in Biomedical Implants

Abstract

Active biomedical implants offer the potential to improve the quality of life for patients suffering from medical conditions such as total heart failure, retinitis pigmentosa, and age-related macular degeneration, which are typically incurable or require extensive surgical intervention. Wireless power transfer (WPT) systems provide a solution for active biomedical implants that require high power or are constrained by spatial limitations, rendering the use of implantable batteries impractical. Although WPT systems exhibit higher losses compared to their wired counterparts, these losses can be mitigated through the use of resonant converters. However, resonant converters are sensitive to variations in output power, coupling factor, and load resistance, which are inherent characteristics of WPT systems. This thesis presents the analysis and design of two Class E WPT systems and introduces a novel control scheme. The proposed controller ensures zero voltage switching (ZVS) across a wide range of input voltage, input power, coil separation, and load resistances through the indirect control of switching frequency and duty cycle. This control strategy is implemented in a prototype Class E WPT system designed for the estimated load conditions and constraints of a retinal prosthesis. Extensive experimental results are presented to show the system’s ability to maintain ZVS over an input voltage range of 12 - 18 V, coil separation of 3 - 9 mm, and load resistance of 200 - 2500 Ω, while supplying 250 - 1900 mW of power. Although the efficiency fluctuates due to the suboptimal tuning of the system, a peak efficiency of 75% is achieved. Unlike existing solutions that manage variations to only one parameter while assuming the others remain constant, the proposed system provides the necessary flexibility required to achieve ZVS in real-world applications of WPT. These findings lay the groundwork for a compact, efficient, and versatile WPT system, suitable for a wide range of biomedical implants and other applications where load, coupling factor, and output power may vary.