Power processing issues for micro-power electrostatic generators

For various monitoring and sensing applications it is desirable to power the electronics by scavenging energy from any locally available source. A prototype generator for low frequency (human body) motion has been developed using a micro-machined (MEMS) implementation of an inertial generator based on a moving-plate capacitor. The prototype generates pulses of 300 V on a 10 pF capacitor. This paper examines the design of a circuit and MOSFET device to convert this energy to a low voltage. Because of the very small charge involved, the effect of leakage and parasitic stored charge are important. A silicon-on-insulator design is proposed and is examined through physics based finite-element simulation. The overall effectiveness of the generation process is shown to be composed of several terms which are functions of system parameters such as generator flight time, device area and circuit inductance. It is shown that device area is a compromise between leakage current, charge storage and on-state voltage. It can, for a given generator and inductance, be optimised to provide the maximum energy yield.