Reconfigurable System for Electromagnetic Energy Harvesting with Inherent Activity Sensing Capabilities for Wearable Technology

This paper presents a power management system for wearable technology based on a custom made electromagnetic (EM) transducer and a front-end circuit for energy harvesting (EH) and activity sensing. Due to the AC-output nature of the EM transducer, this work proposes a reconfigurable rectifier that can switch from passive operation to a more power-efficient active topology depending on the available power in the system. The passive mode operates as a negative voltage converter during start-up, while the active configuration is enabled and driven by a control block after the available voltage at an output storage capacitor surpasses 1.8 V. This work also introduces an activity detection circuit that enables the use of the inherent sensing capabilities of the EM transducer. The combination of both, a reconfigurable rectifier and an activity detection circuit, allows the system to gather information similar to that of an accelerometer regarding the activity of the user, but at net-zero power consumption and lower cost. The EH front-end described in this work was implemented in a 130 nm CMOS process with an area of 0.0254 mm2. Measurements show that the circuit has a peak power conversion efficiency of 92.04%, while the power management system can extend the battery’s charge of a Fitbit Charge HR by four hours for every 30 effective minutes of a sprint interval training routine.