High-performance Far-field RF Energy Harvesting Integrated Circuit Research

To replace batteries and power low-power devices,ambient energy harvesting technology has emerged.As one of them,RF energy harvesting has good advantages as which RF energy is ubiquitous in cities and can be easily transmitted to places where other energy sources（such as sunshine,temperature gradients,vibrations）cannot be transmitted.However,RF energy harvesting system has the challenge of low ambient energy density.The common solution is to improve the sensitivity or conversion efficiency of the system.In this paper,from the perspective of the later,the interface between the RF rectifier and the DC-DC converter is optimized so that the RF rectifier maintains the maximum power output,thus improving the conversion efficiency of the system.The main research of this paper can be divided into the following aspects.1、The nonlinear characteristics of RF rectifier output are studied.Firstly,the discrete impedance matching network between antenna and RF rectifier is designed by ADS software.Then the output parameters of the matched RF rectifier（antenna-matched network-rectifier）are tested.Finally,the variation of the tracking ratio（the ratio of maximum power point voltage to open circuit voltage）in the input power range is obtained by the output parameters.2、Based on the variation range of tracking ratio,an adaptive maximum power tracking（AMPPT）technology suitable for RF energy collection system is designed,which combining the fraction open-circuit voltage（FOCV）and the perturb and observe（P&O）method.Based on PWM+PFM hybrid modulation,the technology adaptively adjusts the tracking voltage perturbator on the basis of half of the open-circuit voltage to adjust the output voltage of the RF rectifier,and at the same time observe the charging time of the output capacitor of the DC-DC converter.When the charging time reaches the minimum,it is considered that the output power of the RF rectifier reaches the maximum,and the tracking voltage is the maximum power point tracking voltage.The system circuit is designed using180 nm CMOS process to verify the adaptive maximum power tracking technology.The tracking efficiency varies from 83.3%to 99%(@P_AV=-8 d Bm),indicating good tracking capability.In addition,the conversion efficiency of the RF rectifier varies from 55%to 67%,and the DC-DC conversion efficiency varies from 78.34%to 86.46%(@P_AV=-8 d Bm,I_OUT=140μA).The minimum conversion efficiency of the system is 46.88%,and the peak efficiency can reach 58.57%(@P_AV=-8 d Bm,I_OUT=140μA).3、In order to solve that the rectifier cannot maintain the maximum power transmission due to the change of the equivalent input impedance of the DC-DC converter when load changes,the design of the DC-DC converter is optimized by treating the input and output ports as mutually independent and decoupled,and adding energy storage capacitors.The circuit was designed using180 nm CMOS process.The simulation results show the improvement of the system conversion efficiency with the same load current.In addition,when the load current I_OUT is 294μA（heavy load）,the conversion efficiency of the DC-DC converter varies from 86%to 90%(@P_AV=-13 d Bm)and the system conversion efficiency varies from 50.15%to 60.37%(@P_AV=-8 d Bm)as the input power changes.when the input power is-5d Bm,the DC-DC conversion efficiency varies from 75.9%to88.99%(@I_OUT=154μA)and the system conversion efficiency varies from 49.97%to58.58%(@I_OUT=154μA)for load variation.