Load-Independent Split-S-SSHI With Envelope Tracking MPPT for Piezoelectric Energy Harvesting

Piezoelectric energy harvesting interfaces are mainly categorized into single-stage conversion interfaces with high efficiency and cascaded conversion interfaces with high output power. However, the single-stage conversion interface has limited output power and lacks an MPPT configuration, while the cascaded conversion interface suffers from low end-to-end harvesting efficiency due to the cascaded conversion process. This paper proposes a load-independent Split-S-SSHI (SS-SSHI) interface with an envelope-tracking MPPT that can simultaneously achieve high output power and high end-to-end efficiency. In addition, the split S-SSHI eliminates the need for a rectifier capacitor and enhances the response speed of the MPPT. The proposed harvester is fabricated in a 0.18-$\mu $m CMOS process with a low quiescent current of 39 nA. Measurements indicate a maximum MPPT efficiency and end-to-end harvesting efficiency up to 99.5% and 91.9%, respectively, and the maximum output power reaches 9.7 times that of a conventional full-bridge rectifier.