A 40 nA, 84% Efficient, PLL-based Rectifier-Less Power Switching Converter for Low-Voltage Piezoelectric Energy Harvesting

We proposed a PLL-based rectifier-less interface circuit to control the power switches for harvesting energy from a low-voltage piezoelectric transducer with input power levels of 100 nW to 40 mu W. In this proposed method, charge-pump PLL is first locked with the input frequency of the PEH to detect its peak output voltage for maximum power extraction. Once locked, power transfer occurs where PLL is operated in an open-loop (only VCO is functional) to minimize the power loss. In 180 nm CMOS technology, the power switching converter was designed and extracts power with a minimum piezoelectric open-circuit voltage of 0.17 V. At an input power of 10 mu W and a vibration frequency of 88 Hz, it achieves a maximum power efficiency of 84%. The control unit consumes an average current of 40 nA from a 1.5 V output voltage over a broad input power range.