Fully Integrated Inductor-Less Flipping-Capacitor Rectifier for Piezoelectric Energy Harvesting

This paper presents a fully integrated piezoelectric energy harvesting interface without external components. Instead of relying on bulky external inductors with high quality factor as in the conventional parallel-synchronized-switch harvesting-on-inductor (P-SSHI) approach, we propose a flipping-capacitor rectifier (FCR) topology to achieve voltage inversion of the piezoelectric energy harvester through a reconfigurable capacitor array. This fundamentally preserves a fully integrated solution without inductors while achieving a high-energy extraction capability. Measurement results from FCR₁ using discrete components shows an output power enhancement of up to 3.4×, which is close to the theoretical prediction. We also fabricated a sevenphase FCR₃ with four MIM capacitors and 21 switches using a 0.18-μm 1.8/3.3/6 V CMOS process, occupying an active area of ~1.7 mm². Additionally, we implemented an active rectifier based on a common-gate comparator with phase alignment to ensure high-speed operation while minimizing the diode voltage drop. A phase generate-and-combine circuit eliminates redundant switching activities. Systematic optimization of the three main energy loss mechanisms during the finite flip time: 1) phase offset; 2) incomplete charge transfer; and 3) reduced conduction time, is also introduced. Measurement results show that the output power enhancement can reach up to 4.83× at an excitation frequency of 110 kHz.