Optimization of both Perturb & Observe and Open Circuit Voltage MPPT Techniques for Resonant Piezoelectric Vibration Harvesters feeding bridge rectifiers

When the characteristics of the input vibrations change with time, also the optimal operating point of a Resonant Piezoelectric Vibration Energy Harvester (RPVEH) changes with time. Such an optimal operating point is characterized by the highest value of the extracted electrical power and is called Maximum Power Point (MPP). This paper is focused on MPP Tracking (MPPT) techniques for RPVEHs. In the most widespread RPVEH AC/DC architecture, which is based on the use of a diode bridge rectifier cascaded by a DC/DC converter, MPPT requires that the voltage V0 at the output of the bridge rectifier is properly regulated. The first issue that is addressed in this paper is the derivation of a simple closed form expression of the power extracted by a RPVEH as a function of the voltage V0 as well as of the optimal value of V0. Moreover, a complete set of general guidelines leading to the proper choice of the values of the parameters of the two most widely used MPPT techniques for RPVEHs applications (the Perturb & Observe technique and the Open Circuit Voltage technique) are provided and experimentally validated. It is experimentally shown that, if the above MPPT techniques are not properly and carefully customized to the particular RPVEH application of interest, they may lead to very poor performances of the system.