As is known, energy harvesting systems (also known as energy-scavenging systems) for harvesting energy from intermittent environmental energy sources (i.e., ones that supply energy in an irregular way) have aroused and continue to arouse considerable interest in a wide range of technological fields. Typically, the energy harvesting systems are designed to harvest, store, and transfer energy generated by mechanical sources to a generic load of an electrical type.
Low-frequency vibrations, such as for example mechanical vibrations of disturbance in systems with moving parts, can be a valid energy source.
The mechanical energy is converted, by one or more appropriate transducers (for example, piezoelectric or electromagnetic devices) into electrical energy, which can be used for supplying an electrical load. In this way, the electrical load does not require batteries or other supply systems that are cumbersome and have a low resistance to mechanical stresses.
FIG. 1 is a schematic illustration, by means of functional blocks, of an energy harvesting system of a known type.
The energy harvesting system 1 of FIG. 1 comprises: a transducer 2, for example of an electromagnetic or piezoelectric type, subject during use to environmental mechanical vibrations and configured for converting mechanical energy into electrical energy, typically into AC voltages; a scavenging interface 4, for example comprising a diode-bridge rectifier circuit, configured for receiving at input the AC signal generated by the transducer 2 and supplying at output a DC signal for charging a capacitor 5 connected to the output of the rectifier circuit 4; and a DC-DC converter 6, connected to the capacitor 5 for receiving at input the electrical energy stored by the capacitor 5 and supplying it to an electrical load 8. The capacitor 5 hence has the function of element for storage of energy, which is made available, when required, to the electrical load 8 for operation of the latter.
The rectifier 4 comprises, in the embodiment of FIG. 1, a diode bridge comprising four Schottky diodes 4a, 4b, 4c, 4d. The minimum voltage VIN at input to the rectifier 4 of FIG. 4 for a rectified output voltage VOUT to be generated is given by VIN>2VD, where VD is the threshold voltage of each Schottky diode. When the input voltage VIN is lower than the minimum voltage required at input by the rectifier 4 for its operation, the rectifier 4 is in a blocked state and does not produce an output voltage VOUT.
When the input voltage VIN equals the value 2VD, the rectifier 4 conducts, and a charge appears on the capacitor 5, which is charged. It is evident that for voltages VIN at input to the rectifier 4 lower than the threshold 2VD the capacitor 5 is not charged, and the environmental energy converted by the transducer 2 into electrical energy is dispersed.
It should be noted that the Schottky-diode bridge rectifier can, under certain operating conditions, guarantee an output VOUT also at low input voltages VIN (lower than 1 V). However, this is valid only for low current densities (typically less than 10 mA). In energy harvesting systems the currents generated can vary a lot in time, and can assume very low values as well as very high (peak) values, in an unforeseeable way.
A rectifier 4 of the type illustrated in FIG. 1 consequently does not guarantee a sufficient continuity of operation in different operating conditions. Moreover, Schottky diodes have a response markedly depending upon the temperature, which limits their use to controlled-temperature environments.
FIG. 2 shows an energy harvesting circuit 10 of a known type according to a further embodiment.
Unlike the system of FIG. 1, the system 10 of FIG. 2 comprises a rectifier 14 of a magnetic type.
The magnetic rectifier 14 comprises a transformer 16, including two primary windings 16a, 16b and a secondary winding 16c. The coupling factor m is chosen greater than 1. The energy harvesting system 10 further comprises a diode 12 connected in series to the secondary winding 16c. 
Moreover, each primary winding 16a, 16b is set in series to a respective unidirectional switch 15a, 15b. When the voltage VIN is maximum, the switch 15a is closed and the capacitor 5 is charged. When the voltage VIN is minimum the switch 15b is closed and the capacitor 5 is discharged.
The equivalent threshold voltage of the system, fed back to the input, transducer 2 side, is thus equal to the voltage VD of the diode 12 scaled by the factor “m”.
Hence, as compared to the system 1 of FIG. 1, in which the threshold of the rectifier 4 is fixed at 2VD, the system 10 of FIG. 2 enables storage of energy in the capacitor 5 starting from lower input voltages VIN. However, this embodiment is particularly costly on account of the use of magnetic elements. Moreover, since said system cannot be easily built in an integrated form, the consumption of area is high.
There is a need in the art to provide an improved rectifier circuit and an environmental energy harvesting system comprising the rectifier circuit that will enable the aforesaid problems and disadvantages to be overcome.