Design and Performance Analysis of Supercapacitor Charging Circuits for Wireless Sensor Nodes

Micro-solar energy harvesting systems have achieved efficient operations through maximum power point tracking (MPPT) and maximum power transfer tracking (MPTT) techniques. However, they may have chargers with relatively high power thresholds, below which they have 0% efficiency. As a result, these harvesters either require much larger panels than necessary, or they fail to sustain extended periods of poor weather. To address this problem, we propose to generalize MPTT to MCZT, for Maximum Charging Zone Tracking, to expand the zones of effective charging. To cover the wide dynamic range of solar irradiation, we propose a programmable charge pump driven by a direct digital synthesizer (DDS). In addition, we dynamically reconfigure the topology of multiple supercapacitors to maximize charging efficiency and minimize voltage-dependent leakage. Experimental results from simulation and measurement show that under the high solar irradiance of 1000 W/m², our MPTT part achieves 40%-50% faster charging time than one without MPTT; and under low solar irradiation of 300 W/m², the boost-up operation of our system enables fully charging the supercapacitors, thereby extending the harvesting time zone from 10:00 am-07:10 pm to 8:20 am-8:00 pm even on a sunny day, all with an MPTT overhead of 1.5 mW.