I. Introduction
Many energy harvesting devices, based on different transduction mechanisms such as piezoelectric, thermoelectric, photovoltaic, and electromagnetic ones, have been proposed in the literature [1]–[2]. In this paper, the attention is focused on Resonant Piezoelectric Vibration Energy Harvesters (RPVEHs) [3]–[4]. In the great majority of practical and commercial cases, the power electronics interface between the RPVEH and the rechargeable battery and/or the DC load is represented by a diode bridge rectifier with a DC/DC converter connected at its output and aimed at the so called Maximum Power Point Tracking (MPPT) [5]–[10]. Two are the strategies that are usually adopted to carry out the MPPT, and hence to extract the maximum average power from the RPVEH, in presence of vibrations characterized by sinusoidal accelerations with frequency equal to the mechanical resonance frequency of the RPVEH. They are the “Resistance Emulation” (RE) technique and the “Voltage Adaptation” (VA) technique. In the RE technique, the power electronics interface emulates a resistance, the Maximum Power Point (MPP) resistance , at the RPVEH terminals [11]–[13]. In the VA technique, the power electronics interface regulates the level of the bridge rectifier DC output voltage to the MPP constant value [14]–[16]. Such techniques are indeed used also in other very widespread energy harvesting applications [17]–[22] but the emphasis in this paper is on RPVEHs applications, even if the conclusions which will be drawn hold general validity.