A Simple and Stable Load Control Algorithm for Time-Varying Harvested Energy in Miniaturized Implantable Devices

One of the most challenging issues in a miniaturized implantable device is to supply power sufficiently and continuously for stable operation of the device. An optical energy transfer is the most practical solution due to its high efficiency among existing wireless power transmission techniques. In general, large-capacity storage unit and responsive power regulation circuits are essential to overcome the fluctuation of harvested energy, but this conventional architecture cannot be implemented in the millimeter scale device due to the volume limitation. In this paper, we propose a control-theoretic load control algorithm to maximize the utilization of time-varying harvested energy while maintaining a minimum voltage ripple in the storage capacitor. The proposed algorithm estimates available current for the next time slot based on the amount of charge in the storage capacitor and that of harvested energy, then matching system load current with the result. This algorithm is simple in that the number of computations at micro-processor is minimized and stable in that the voltage of the storage capacitor is maintained at the target level. We analyze the theoretical stability of the proposed algorithm and validate its superior performance through simulation.