Wireless Power Supply for Implantable Biomedical Device Based on Primary Input Voltage Regulation P. Si, Member IEEE, A. P. Hu, Senior Member IEEE, J. W. Hsu, M. Chiang, Y. Wang, S. Malpas, and D. Budgett The Department of Electrical and Computer Engineering The Bioengineering Institute The University of Auckland, New Zealand Abstract-This paper presents a wireless power supply system for implantable biomedical devices. Magnitude of the input voltage supplied to the primary power converter is dynamically regulated according to the power demand of the device. The major advantage of such a system is that its average power loss is minimized. Unlike methods implemented at implantable secondary (pick-up) side, the magnitude regulation is undertaken at the external primary side. Thus the heating effect and physical size of the implantable secondary can be reduced. The system utilizes parallel tuning circuit to boost the voltage induced in the secondary pick-up, and does not require a tight coupling between the primary and secondary coils. As a result, the system has great tolerance to the variation in the air gap distance between the coils. The characteristics of the magnitude regulated power flow have been thoroughly analyzed, and both simulations and laboratory experiments have verified the proposed system. I. INTRODUCTION Implantable biomedical devices have found applications in a wide range of biomedical areas, including pacemakers, monitoring devices, functional electrical stimulators (FES), left ventricular assist devices (LVAD), and artificial hearts. Supplying power to these devices for long-term operation is a challenging task. Traditionally, implantable batteries and percutaneous link power supply systems are used. However, the batteries have limited energy storage and life span, and the percutaneous links across the skin impose infection risks. Wireless systems have been developed to supply power over relatively large air gaps to implantable biomedical devices [1-5]. Compared to the devices relying purely on implantable batteries, the biomedical devices driven by the wireless power supplies can be much smaller, especially when high power outputs are required. A major issue involved in the wireless power supply systems is power flow regulation. This is particularly important to systems with high coupling variations and load changes. In biomedical applications, it is a common practice to implement a power regulating unit at an implantable pick- up (secondary) side [6-8]. However, this requires extra components which may contribute to high heat generation and will contribute to additional size and weight. To reduce the power losses and physical size of the pick-up circuit, a closed-loop power control strategy is a favorable choice to regulate power flow at the external primary side of the system. In this paper, a wireless power supply system based on voltage magnitude regulation at the external primary side is presented. The power flow of the system is regulated by controlling the input voltage magnitude of the primary power converter according to power demands of load and the dynamic changes of circuit parameters. II. PROPOSED SYSTEM Fig. 1 shows the basic configuration of the magnitude regulated wireless power supply system. At the external primary side, a current-fed push-pull resonant converter is employed due to its advantages of high efficiency, low cost and small physical size. The operating frequency of the system is governed by the zero voltage switching frequency of the push-pull resonant converter, which is determined by [9]: ( )       +         -         - = π ϕ ϕ ω 2 1 4 1 1 2 T C L Q p (1) where φ, Q and T are the initial phase angle, the quality factor, and the time constant of the converter. Inductor L p represents primary coil (track), which is tuned with a capacitor C, forming the primary resonant tank. The pick-up (secondary) coil represented by L S is implanted for inducing power from the electromagnetic field generated by the primary coil. Since L s is tuned with capacitor C t in parallel (C t =1/(L s ω 2 )), the induced voltage in the pick-up coil can boost up according to the boost-up factor of the pick-up [10]. Due to the boost up ability, air core windings can be utilized in the primary and secondary coil while a relatively large gap distance between them is allowed. Additional weight and heating effect caused by magnetic cores are eliminated in such a air-core system which is with parallel tuning. As shown in Fig. 1, for achieving the maximum power transfer capacity, a full bridge rectifier is adopted in the pick-up to convert ac power to dc. Also, a dc inductor L dc is employed at the dc side of the rectifier for increasing power delivery ability. The dc inductor maintains the current flow through the rectifier to be continuous so that the power transferred from ac to dc side becomes more stable [10].