Piezoelectric Energy Harvesting System With Maximum Power Point Tracking Control

The application of microelectronic systems,such as wireless sensor networks(WSNs),which combining the functions of perception,computing and communication together to promote the interaction between human and environment,quickly become a popular emerging research field.So far,battery supply is also the main power supply of WSNs nodes.Energy constrained is still one of the most important issues in WSNs though battery powred technology provides a preferable cost-effective energy srorage technology.Energy harvesting technology can be used to solve the problems that the battery supply faced,such as service life time and environmental restrictions.Among these environmental energy sources,piezoelectric energy sources have the advantages of high power desity and limited environment restriction,which attract the attention of many researchers.For piezoelectric energy harvesting technology,the key technology is the design of rectifier interface circuits.The rectifier interface circuits need a higher conversion efficiency to effectively utilize the energy harvested from the piezoelectric energy harvester(PEH).In addition,the output power curve of a PEH shows that the output power of the device varies greatly with the change of the load resistance.In order to harvest the maximum power of the piezoelectric energy device all the time,a Maximum Power Point Tracking(MPPT)circuit with has been proposed.To ensure the efficiency of energy harvesting,the MPPT circuit needs high tracking accuracy and high power conversion efficiency.Since the output power of most PEHs can provide is less than m W which is insufficient to supply for some micro electronic devices or integrated circuits directly.How to manage the piezoelectric energy effectly is also one of the key technologies to get even.A power enhanced active rectifier with offset-controlled comparator for self-powered PEH system is presented in the paper.The non-ideal effects of the traditional active diode(such as reverse leakage current and output oscillations)have been analysised in detail,and then a novel offet-controlled comparator is porposed.Thus the input-referred offset voltage of the proposed comparator can be less than 1m V.The current detected accuracy of the proposed offset-controlled comparator is improved by more than 10 times over a traditional comparator.Output oscillations of the proposed comparator are dramatically reduced attributing to the high precision of current detection.In addition,the comparator is also able to prevent the reverse leakage current.The simulation and measurement results show that the rectifier can realize self-powered without the need for external auxiliary circuit.When the open circuit voltage of the PEH is greater than 1.5V,the FOM of the rectifier is higher than 0.9,and the peak power conversion efficiency of the rectifier is 90%.The proposed rectifier has the advantages of area cost,easy to application.For a PEH,the output power will change with the load.And the MPPT circuits based on open circuit voltage algorithm will reduce the the input voltage range.Implemented in SMIC 0.18 ?m standard CMOS process,a MPPT circuit based on disturbance & obaervation algorithm(P&O)is proposed.To broaden the input range,the MPPT circuit converter adopts a Buck-Boost structure.A programmable capacitor array turn-on time generating circuit is introduced to regulate the equivalent input impedance of the Buck-Boost converter to match with the output impedance of the PEH.The Buck-Boost converter operates in DCM mode that the power conversion efficiency of the MPPT circuit can be improved.When the opencircuit voltage of the PEH changes from 2V to 7.2V,the tracking efficiency of MPPT is no less than 84%.When the input voltage of the MPPT circuit(VREC)is 2.5V,the peak tracking efficiency of the MPPT circuit is 98%.The input power range of the MPPT circuit is 33?W-2m W.The power consumption of the proposed MPPT circuit is less than 15?W,and the peak power conversion efficiency of the MPPT circuit is 84%.In addition,an intermittent power management unit is presented in this paper.The core circuit is an efficient Buck converter with fast transisent response and adaptive on time control.When the load changes from 5m A to 100 m A,the undershoot voltage of the output is about 5m V,and the recovery time is less than 8?s.When load current is greater than 5m A,the power conversion efficiency is no less than 65%.The peak power conversion efficiency is about 87% when the load current is 25 m A.Last,the PEH system designed in this paper has 65% peak power conversion efficiency when the open-circuit voltage is in range of 2V-7.2V.