A Low Voltage Low Power Maximum Power Point Tracking Circuit For Energy Harvesting

As portable devices has greatly improved the quality of human life,people are increasingly dependent on electonic equipments.As a result,it have been rapidely developed.For example,wearable devices such as smart wristbands,smart glasses;medical devices,such as human wireless monitoring system;unattended positioning and surveillance equipment used in military fields,etc.These new devices which have the characteristics of miniaturization cannot be powered by cable or battery.This factor restrict their application.So energy harvesting is one of the most effective methode to solve this problem and has gotten great concern and improvment.There are several energy sources that are common in the environment can be harvested: solar energy,wind energy,heat energy and vibration energy.The vibration energy draws great attention in the current research because of its wide distribution and high energy density.Comparing to other energy source,vibration energy is different for its AC output signal.Its output energy is related to the load.Therefore,the rectifier circuit and load matching circuit is needed in order to output maximum energy.The traditional maximum power point tracking circuit is mainly designed for solar or wind energy,which has a high working voltage and DC-DC converter.Most of commercial chips use small-scale MCU to do a large number of operation and calculation,which will increase the consumption of space and energy for small power management module.In this paper,an analog MPPT(Maximum Power Point Tracker,MPPT)method,which is disturbance & observation,is added to an active power factor correction circuit to obtain the maximum energy efficiency.The MPPT strategy used in this paper is: the output voltage and current of rectifier are sensed to get the input power,and then the calculated input power is delayed to compare with the real-time signal.The result of comparison controls the input impedance of PFC changes in the direction in which the output power becomes larger.The system includes MPPT module,error amplifier,peak current comparator,zero crossing detection circuit,bandgap reference,and so on.The advantage of this circuit is that the MPPT generates a two-bits signal to control the change of duty cycle.One bit controls the direction of disturbance to determine whether the duty ratio is larger or smaller;Another bit controls the step of the disturbance of duty ratio.If the power changes greatly,the disturbance changes with a large step,in order to achieve faster tracking speed;if the power change is small,the disturbance changes with a small step,in order to ensure the tracking accuracy.Power consumption of MPPT module is 13.375?W,such little power consumption will not affect the judgment of MPPT of the circuit's power change direction.An equivalent model of piezoelectric which consists of an AC current source and a parallel capacitor is used in this paper.The peak current of current source is 3m A and the capacitance is 6?F.After the operation of MPPT module,the output voltage of rectifier is 0.8V,the Boost-PFC circuit outputs 2.5V DC voltage.The final output current is about 0.71 m A,the output power is 1.78 m W,and the energy efficiency is 89.1%.Compared with the circuit without MPPT module,which has a output power of 0.55 m W,it can be concluded that MPPT module works well to improve the output power.