Modeling and Design of Cold-Start Charge Pumps for Photovoltaic Energy Harvesters

This article presents modeling and design optimization of switched-capacitor charge pumps for use as cold-start circuits in photovoltaic energy harvesters. For a successful cold-start from photovoltaic energy, a certain output voltage and current must be generated by the charge pump while jointly minimizing the required input voltage and current from the energy harvesting source. We model the operation of the cold-start charge pump including its oscillator, derive expressions for required input voltage and current based on output requirements, and demonstrate a Pareto-optimal design space that defines the tradeoff between the required input voltage and current to achieve cold-start. We then assess the impacts of process and temperature variation, and suggest a general design methodology for cold-start charge pump circuits. Finally, we validate models with measurements from a design fabricated in 55nm CMOS.