Abstract
Nowadays, the development of green energy systems becomes more and more important. In addition to renewable energy, energy harvesting is another way to replace conventional fuels. There are two popular portable generators to derive energy: a mechanical-to-electrical generator and a portable solar board. Nevertheless, the source of power is small and unstable in both cases. Thus it is hard to collect the generated energy efficiently. As a result, this work proposed a Buck Li-ion battery charger IC for portable generators which overcomes the problems of efficiency and stability. According to the chemical characteristics of Li-ion battery, there are three different charging modes correlating with battery voltage, namely the trickle current (TC) mode, the time varying energy from portable generators makes the charger difficult to maintain an efficient CC mode. In this work, a current mirror in the charger circuit is utilized to sense the charging current during positive duty cycle and maintain its value by using a pulse width modulator (PWM). Once a charging routine begins, if the portable generator provides suitable charging current, then the duty cycle is incremented to100% by the pulse width modulator. Conversely, if the portable generator provides too much power, then the duty cycle is decremented to limit the excessive current. This chip was fabricated in a TSMC 0.35μm CMOS technology. The simulation results show that the charging efficiency is about 90% and the measured performance indicates that the charging efficiency is 86.1%. The IC is suitable for energy harvesting applications.