A Low-Power MCU-Based MPPT Architecture With a Fast Impedance Measurement for Broadband Piezoelectric Energy Harvesting

This article presents the combination of a fast and robust maximum power point tracking algorithm called two-load characterization method (TLCM) and an efficient energy harvesting interface called resistive short circuit (RSC) for broadband piezoelectric energy harvesting. The proposed architecture is implemented using a low-power microcontroller (STM32L4) with its associated discrete electronics. The TLCM consists of an embedded impedance measurement that computes the piezoelectric impedance after two successive phase and voltage measurements. It calculates the optimal load impedance as the complex conjugate of the generator output impedance. It is up to 13 times faster than classical Perturbation and Observation algorithms. The complex conjugate impedance is then emulated by the RSC interface: a Flyback converter emulates a resistive load while a variable duration short-circuit following the zero voltage instant emulates a capacitive load. This two-parameters electrical interface allows a 9.4% harvested power bandwidth, which is 35% more than for a fixed resistor. The whole system is tested at 0.5 m/s
²
peak ambient acceleration and between 26 Hz to 30 Hz in standalone operation. In those conditions, the proposed system showed an end-to-end efficiency in the 65% to 80% range while consuming 21.5 μW to 28.5 μW, with a measurement duty cycle of 13%. It harvests a net maximum power of 157 μW with a peak extracted power of 235 μW.