On-Chip Photovoltaic Energy Conversion in Bulk-CMOS for Indoor Applications

Modeling and design aspects of on-chip photovoltaic energy conversion, voltage boosting and storage in bulk CMOS are investigated under the constraints of indoor illumination and small form factor. A power-supply architecture consisting of a photovoltaic converter, a clock generator, a charge-pump and a storage capacitor is considered. Maximization of the recharging rate of the capacitor at the low end of irradiance is adopted as the aim of design optimization. First, photodiodes available for conversion in bulk CMOS are comparatively evaluated, and potentially the most efficient ones are experimentally characterized for the typical range of indoor irradiance. Next, a generic model is developed and experimentally verified for a clock generator powered by a converter built with these photodiodes. This is followed by the modeling and experimental verification of a Dickson charge-pump whose single-device rectifier is free of body effect and near-ideal in forward characteristic. These models are based on the subthreshold operation of devices as necessitated by optimization under the limitations of bulk-CMOS stacking and indoor irradiance. Guidelines for design optimization are derived from these models. Experimental work is based on two test chips fabricated in 0.18-mu m bulk CMOS with deep n-well option.