Study On A Maximum Power Point Tracking Circuit Of Piezoelectric Energy Harvesters With Automatical Matching

In recent years,with the development of wireless communication technology and sensor technology,wireless sensor network has been widely used in various fields.The traditional power supply of wireless sensor nodes are battery-powered,but it is unable to work for a long term because of the power in the battery is limited with small size.So it needs to replace the battery regularly.Replacing the battery regularly will cause the battery pollution,which is neither environmentally friendly nor economic.It does not meet the requirements of sustainable development.Replacing the battery regularly is very dangerous,because of many sensor nodes are arranged in high pressure,swamps,jungles,Gobi and other complex geographical environment.In order to solve the problem of wireless sensor node's power in complicated geographical environment,it is necessary to collect energy in the environment for the wireless sensor nodes.At present,the energy,which can be collected in the environment,is vibration energy,wind energy,solar and electromagnetic energy and so on.The energy harvester,which uses the piezoelectric principle,takes part in a large part.The piezoelectric energy acquisition system can be used to collect the vibration energy,wind energy,tidal energy,magnetic field energy and other clean and environmental energy,so the study of piezoelectric energy harvesters have been paid more and more attention.The design of energy harvester based on piezoelectric principle has many advantages,such as simple structure,no heat,no electromagnetic interference,easy to manufacture and realize the miniaturization and integration of structure.However,the output voltage of the piezoelectric material is high and the output impedance of the piezoelectric material is high.Meanwhile,the energy density of the environment is low and the energy is not stable in environment.So the output energy of the piezoelectric energy harvester cannot drive the wireless sensor node directly,which needs energy management circuit to manage the energy.This paper designs the maximum power point tracking(MPPT)circuit which can match different sizes of piezoelectric energy harvesters automatically according to the different output characteristics of different size piezoelectric energy harvesters.The energy management circuit is mainly composed of up-conversion matching circuit,feedback control circuit,energy storage circuit and rectifier circuit.When the equivalent capacitance of piezoelectric transducers with different sizes changes in a wide range,the circuit can adjust the conduction time of the switch according to the charging power of the storage capacitor.Then the circuit can change the equivalent input resistance and the equivalent input inductance of the management circuit,which can make the management circuit to match different piezoelectric transducers automatically.So that the storage capacitor can charge the power at the maximum power point.Piezoelectric materials with different sizes have different equivalent capacitances Cp,and the traditional management circuit cannot automatically realize the conjugate matching of piezoelectric materials with different sizes.This paper designs a maximum power point tracking circuit matching the piezoelectric energy harvesters with different sizes automatically.The management circuit can adjust the duty cycle of the square wave oscillation circuit by detecting the charging power of the storage capacitor.The output of the oscillation circuit controls the turn-on time of the impedance matching switch,and then changes the input impedance of the management circuit achieving the conjugate match to the different sizes of the piezoelectric transducers.Compared with the traditional MPPT circuit,this circuit realizes not only the resistance matching to a single transducer,but also the conjugate matching of different size transducers.Experimental results show that this circuit can realize the conjugate matching for piezoelectric transducers when the equivalent capacitance Cp varies within the range of 1~30nF.The charging power of the storage capacitor reaches the maximum charging power at an equivalent capacitance of 30 nF and a storage capacitor voltage of 0.8V.The maximum charging power is 90.6?W,which is 1.32 times higher than that in the up-conversion matching circuit.This circuit achieves the impedance matching for different sizes of piezoelectric transducers automatically.