22.5 A 93%-power-efficiency photovoltaic energy harvester with irradiance-aware auto-reconfigurable MPPT scheme achieving >95% MPPT efficiency across 650µW to 1W and 2.9ms FOCV MPPT transient time

With more and more functions in modern battery-powered mobile devices, enabling light-harvesting in the power management system can extend battery usage time [1]. For both indoor and outdoor operations of mobile devices, the output power range of the solar panel with the size of a touchscreen can vary from 100s of µW to a Watt due to the irradiance-level variation. An energy harvester is thus essential to achieve high maximum power-point tracking efficiency (ηT) over this wide power range. However, state-of-the-art energy harvesters only use one maximum power-point tracking (MPPT) method under different irradiance levels as shown in Fig. 22.5.1 [2–5]. Those energy harvesters with power-computation-based MPPT schemes for portable [2,3] and standalone [4] systems suffer from low ηT under low input power due to the limited input dynamic range of the MPPT circuitry. Other low-power energy harvesters with the fractional open-cell voltage (FOCV) MPPT scheme are confined by the fractional-constant accuracy to only offer high ηT across a narrow power range [5]. Additionally, the conventional FOCV MPPT scheme requires long transient time of 250ms to identify MPP [5], thereby significantly reducing energy capture from the solar panel. To address the above issues, this paper presents an energy harvester with an irradiance-aware hybrid algorithm (IAHA) to automatically switch between an auto-zeroed pulse-integration based MPPT (AZ PI-MPPT) and a slew-rate-enhanced FOCV (SRE-FOCV) MPPT scheme for maximizing ηT under different irradiance levels. The SRE-FOCV MPPT scheme also enables the energy harvester to shorten the MPPT transient time to 2.9ms in low irradiance levels.