| Our world in the present has faced a significant number of energy-related challenges mainly due to the extremely high demand for energy-related devices since we have become more and more reliant on electricity to perform most daily activities.As a result,the world witnessing a great growth of mobile phones,tablets,laptop computers,and other electronics-based portable devices,which needs power.Hence,the concept of power harvesting has been introduced by researchers as a technique for reaping energy from the external environment of different available resources as thermal,solar,wind,electromagnetic radio frequency(RF)waves,etc.Radio frequency is one of the modern renewable energy sources abundant in space since such waves are broadcast vastly in the urban ambient environment.Therefore,RF energy harvesting has been introduced to extract energy from the ambient environment to make devices self-sustaining concerning the energy required for operation and obtain a theoretically unlimited operating lifespan.RF energy harvesting can be mainly used to replace batteries or increase their lifespans.Although the attractive features of RF energy sources,the power intensity of RF waves in space is low for the following reasons: First,the federal rules concerning communication and wave propagation in space,which limit the signal power intensity of broadcast RF waves for humans health safety.Second,as the distance between the transmitter source and the RF energy harvesting circuit increases,the propagation medium negatively affects the received RF signals’ power intensities.Furthermore,the practical gain factor of either transmitting or receiving antennas affects the signal power intensity of broadcast and received RF waves.On the other hand,diode-based rectifiers are mostly utilized as the essential part of the RF energy harvesting circuit because of their low forward voltage drop and no external power supply requirement for biasing.However,the dynamic input impedance of the diode-based rectifiers is an essential drawback for the RF energy harvesting system due to the non-linearity of the Schottky diode under the RF wave variation concerning either the applied input power level or the frequency value.The dynamic input impedance makes the rectifier mismatched in front of the receiving antenna,which serves as an RF source for the energy harvesting circuit.Hence,the mismatching between the receiving antenna and the rectifier significantly degrades the RF-DC conversion efficiency.Therefore,the low power intensity of RF waves in space and the dynamic input impedance of the diode-based rectifiers are weak points that reduce the harvested DC power level through RF energy.As a result,the harvested DC power level ranges from microwatts to milliwatts,as reported in the literature.The low harvested power range is the main challenge to introducing autonomous microdevices based on RF energy harvesting sources.This dissertation takes the harvested DC power level improvement of RF energy harvesting as the focus of this paper.In this paper,a mathematical model that can predict the input impedance of the rectifier circuit was presented.Moreover,the advanced design system simulator was used in this research.In light of the mathematical model and simulations,a suitable matching network was introduced to solve the mismatching between the receiving antenna and the rectifier under the RF wave variation concerning either the applied input power level or the frequency value.The main research contents of this paper are as follows:First,the research scenario in this article aimed to overcome the negative effect of the rectifier’s dynamic input impedance on the efficiency of RF-DC conversion due to the Schottky diode non-linearity under the variation of the applied input power level to the RF energy harvesting circuit.The low and high input power levels circuits were studied and integrated to achieve the wide input power range.The HSMS-2852 Schottky diode was used in the low input power range single-stage Dickson voltage multiplier circuit,whereas the HSMS-2862 Schottky diode was used in the high input power range circuit.Hence,the proposed matched impedance rectifier has a wide input power range(covers received powers from-40 d Bm to +30 d Bm).Furthermore,it has a simple design and high RF-DC conversion efficiency at the 900 MHz frequency band,aiming to increase the harvested DC power level.Second,in view of the RF energy harvesting array aiming to enhance the harvested DC power level to power up devices properly in modern applications.Via this scenario,the amount of the harvested DC power by the typical single-stage voltage multiplier circuit was limited and insufficient.Hence,the optimized voltage multiplier circuit was adopted in this scenario,working at single frequency bands,and it is composed of two sub-circuits.The proposed offered a simple design with a small size and an improved harvesting power level.Moreover,the optimized circuit provided DC power higher than the traditional circuit with a factor of around three times,thanks to the higher conversion efficiency of the optimized rectifier circuit.The optimized rectifier can be designed to work at any available frequency band anywhere with a limited modification only to the impedance matching elements.The proposed system was evaluated independently at single frequency bands of the GSM-900,GSM-1800,UMTS-2.1,and Wi-Fi 2.45.Third,in view of overcoming the negative effect of the rectifier’s dynamic input impedance due to the Schottky diode non-linearity on the RF-DC conversion efficiency under the variation of the frequency value.Hence,a matched impedance quad-band rectifier with high RF to DC conversion efficiency was introduced.The quad-band RF energy harvesting system circuit covered the GSM-900,GSM-1800,UMTS-2.1,and Wi-Fi 2.45 frequency bands Simultaneously.The rectifier impedance was found via the proposed theoretical model and simulation at each frequency band.Accordingly,a simple impedance matching network utilizing the Smith chart utility was designed for each frequency band.Hence,two scenarios for the quad-band circuit were adopted to harvest the radiated power of the four frequency bands.The first scenario introduced a quad-band voltage multiplier using only a single receiving antenna for the four combined channels.To address the issue of one channel was operating while the others were off,four elliptic band-pass filters were inserted after the receiving antenna for each of the four voltage multiplier circuits.The second scenario involved a quad-band voltage multiplier circuit with four receiving antennas.In addition,the four channels were connected in parallel with the load resistance to combine the harvested DC power.A resistor was added between the output of each channel and the load resistor to avoid the negative effect if one channel produced a higher output than the other channels,which would cause the rest channels to be off.The values of these resistors were equal to enable the output voltage to be the same across all four branches.The improved design showed a harvested DC power higher than the harvested DC power of popular single and dual-band circuits reported in the literature,which enhances the RF energy harvesting based-autonomous microdevices. |
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