I. Introduction
An increasing number of papers focusing on vibration energy harvesters has been proposed in the literature in the last years [1]-[3]. Among the various technologies, the piezoelectric one is the most studied. In Resonant Piezoelectric Vibration Energy Harvesters (RPVEHs) applications, as in all the harvesting applications, the maximization of the extraction of power represents one of the most important goals. In presence of a linear load the conditions for the maximization of the extracted power are defined by the Maximum Power Transfer Theorem [4]. However, typically, in practical applications a RPVEH is used for supplying DC loads (e.g. wireless sensor nodes in IoT applications [5]-[7]) and, for this reason, a power electronics AC/DC interface is placed between the RPVEH and the DC load.