Electric-energy generation using variable-capacitive resonator for power-free LSI: efficiency analysis and fundamental experiment

A power generator based on a vibration-to-electric energy converter using a variable-resonating capacitor is experimentally demonstrated. The generator consists of a complete system with a mechanical variable capacitor, a charge-transporting LC tank circuit and an externally powered timing-capture controller. A practical design methodology to maximize the efficiency of the vibration-to-electric energy generation system is also described. The efficiency of the generator is estimated based on three factors: the mechanical-energy loss, the charge-transportation loss, and the timing-capture loss. Through the mechanical-energy analysis, the optimum condition for the resonance is found. The parasitic elements in the charge transporter and the timing management of the capture scheme dominate the generation efficiency. These analyses enable the optimum design of the energy-generation system. An experimentally fabricated and measured generator theoretically has a maximum power of 580 nW; the measured power is 120 nW, so conversion efficiency is 21%. This results from a 43% mechanical energy loss and a 63% charge-transportation loss. The timing-capture scheme is manually determined and externally powered in the experiment, so its efficiency is not considered. With our result, a new system LSI application with an embedded power source can be explored for the ubiquitous computing era.