I. Introduction
Piezoelectric vibration energy harvesters are very attractive means for powering wireless sensors networks [1]-[3], because these harvesters exploit the mechanical vibrations available in the environment [4]-[7]. Since wireless sensor nodes need to be supplied by DC voltages, whereas piezoelectric harvesters are AC sources, an AC/DC converter is needed between the harvester and its DC load. Due to the simplicity of both the power stage and the control circuitry, the most widely used AC/DC architecture in low-power harvesting applications is composed by a passive diode bridge rectifier cascaded with a DC/DC converter, as shown in Fig. 1 [8]-[11]. The rectified voltage VDC is controlled by the downstream DC/DC converter in order to dynamically maximize the harvested power, carrying out the so-called Maximum Power Point Tracking (MPPT) [8].