Micro system designs - that feature sensing technology, miniature radio transceivers, low power consumption and tiny volumes are now gaining more attentions in both commercial and research communities. By utilizing the energy harvested from the environment, the device lifetime can be extended to virtual infinity as long as the energy source exists. Harvested energy is either stored in energy buffer for later use or directly used
to power computation circuit. Here, the environmental energy can be from solar, vibration, heat or radioactive decay of matters, while solar energy is most popular because of its ubiquitous spreading, high power density, etc. Low power consumption is one of the design criteria for modern VLSI design because of the popularity of portable devices equipped with wireless communication capability. Most of the current effort of low power design is to reduce the power consumption of the applications subject to a performance requirement under a fixed energy source and constant supply voltage environment.
Previous researches have shown that the output power from the PV cells is greatly affected by the input lighting power and the output load. And lots of works have been done in order to maximize the PV cells power output. The PV cells under the strong sunlight were directly connected to a handheld device. By operating the device at different modes, the load is matched with lighting variations and the energy extracted from the PV cells is maximized. A DC-DC converter was connected to the PV cells. By adjusting the converter switching duty cycle, the PV load was changed and its maximum power point was tracked.
For lots of PV applications such as environment monitoring or wireless sensing devices, the deploying condition is actually unknown in the design phase. With low light intensity, the PV voltage may be too low to charge up a battery. Thus, the power management system should cater for the large difference in light intensities and the voltage step-up scheme is needed.
Traditional DC-DC converter has been proposed to step up the voltage in recent works . However, the inductors in the converter are costly if being integrated on chip. Moreover, most of the strategies tried to maximize the PV cells output power and neglect the converter power loss, while the solar power transfer efficiency (measured at the
Converter output) is the actual design issue that should be considered.
In this work, we present an inductor-less solar power management system. A charge pump is used at low light conditions to step the PV voltage up to a reasonable value to charge a rechargeable battery. Furthermore, we target to maximize the absorbed or regulated power to the battery or the circuit using a using a novel MPPT tracking method. We first analyze the power system behavior based on an ideal PV cells model. We study the load matching problem of using charge pump for the solar power management system and estimate the optimal...