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Enhanced Passive RF-DC Converter Circuit Efficiency for Low RF Energy

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Enhanced Passive RF-DC Converter Circuit Efficiency for Low RF Energy sensors Article Enhanced Passive RF-DC Converter Circuit Efficiency for Low RF Energy Harvesting Issam Chaour 1,2,*, Ahmed Fakhfakh 3 and Olfa Kanoun 1 1 Chair of Measurement and Sensor Technology, Technische Universität Chemnitz, 09126 Chemnitz, Germany; [email protected] 2 National Engineering School of Sfax, University of Sfax, Sfax 3038, Tunisia 3 National School of Electronic and Telecommunication (ENET’Com), Sfax 3018, Tunisia; [email protected] * Correspondence: [email protected]; Tel.: +49-371-5313-4399; Fax: +49-371-531-834-399 Academic Editor: Davide Brunelli Received: 9 January 2017; Accepted: 6 March 2017; Published: 9 March 2017 Abstract: For radio frequency energy transmission, the conversion efficiency of the receiver is decisive not only for reducing sending power, but also for enabling energy transmission over long and variable distances. In this contribution, we present a passive RF-DC converter for energy harvesting at ultra-low input power at 868 MHz. The novel converter consists of a reactive matching circuit and a combined voltage multiplier and rectifier. The stored energy in the input inductor and capacitance, during the negative wave, is conveyed to the output capacitance during the positive one. Although Dickson and Villard topologies have principally comparable efficiency for multi-stage voltage multipliers, the Dickson topology reaches a better efficiency within the novel ultra-low input power converter concept. At the output stage, a low-pass filter is introduced to reduce ripple at high frequencies in order to realize a stable DC signal. The proposed rectifier enables harvesting energy at even a low input power from −40 dBm for a resistive load of 50 kW. It realizes a significant improvement in comparison with state of the art solutions. Keywords: radio frequency energy transmission; RF-DC Converter; rectifier; power efficiency; voltage multiplier; Villard topology; Dickson topology 1. Introduction The use of energy harvesting technologies in wireless sensor networks (WSN) enables reducing installation and maintenance efforts for autonomous systems [1]. It helps to overcome many challenges, such as difficult direct access, flexible use, and working under harsh environmental conditions. Nevertheless, in some cases, using only energy harvesting [2] is not sufficient because of the lack of availability of ambient energy, aging effects, and changing ambient conditions. Wireless power transmission provides a potential solution to support energy harvesting as an additional source and is able to bridge a relatively long distance between a sensor node and its power source [3]. The transmitted radio frequency (RF) power can reach a level of microwatts (µW) to milliwatts (mW) DC, which is a useful range for charging batteries or powering battery-free devices. The main challenge of RF energy transmission is the low level of the received power and the RF-DC power conversion efficiency (PCE). In this paper, we investigate possibilities to improve the efficiency at the receiver side. In order to be able to convert RF energy even from low power ambient RF sources, it is necessary to optimize the system design, especially for low RF energy density. Therefore, we focus on the rectifier circuit, due to the interesting possibilities to reduce energy losses. This paper is structured in three main sections. In Section2, a short description of the related challenges and solutions for RF power transmission is presented followed by a discussion on RF Sensors 2017, 17, 546; doi:10.3390/s17030546 www.mdpi.com/journal/sensors Sensors 2017, 17, 546 2 of 14 energy conversion and storage. Section 3 describes the Villard voltage rectifier circuit for RF-DC converters and discusses the component selection. Finally, the novel approach for an RF-DC voltage multiplierSensors 2017, rectifier17, 546 circuit is detailed, illustrating the simulation results and undertaking the proved2 of 14 circuit performance. energy conversion and storage. Section3 describes the Villard voltage rectifier circuit for RF-DC 2.converters RF Energy and Conversion discusses theand component Storage selection. Finally, the novel approach for an RF-DC voltage multiplierRF transfer rectifier requires circuit sophisticated is detailed, illustrating circuits for the conversion simulation and results storage and the undertaking available RF the ambient proved energycircuit performance.on the receiver side [4]. As shown in Figure 1, this can be reached by the optimization interface between the rectenna (rectifying antenna), and typical storage unit for the WSN. The main aim is to 2. RF Energy Conversion and Storage reach a high overall efficiency by minimizing discontinuities and signal reflections [5]. For that, a reactiveRF matching transfer requires circuit connects sophisticated the antenna circuits to for the conversion rectifier under and storage optimized the availableoperatingRF conditions ambient [6].energy on the receiver side [4]. As shown in Figure1, this can be reached by the optimization interface betweenRF wave the rectenna propagation (rectifying models antenna), are available and typical to evaluate storage the unit received for the WSN.RF power. The mainThe received aim is to powerreach a(P highr) by overallthe antenna efficiency placed by at minimizing a distance R discontinuities from the transmitter and signal antenna reflections can be expressed [5]. For that, as in a Equationreactive matching (1) [7]. circuit connects the antenna to the rectifier under optimized operating conditions [6]. FigureFigure 1. 1. RFRF energy energy harvesting harvesting sensor sensor design. design. RF wave propagation models are available to evaluate2 n the received RF power. The received C 1 αR power (P ) by the antenna placed atP a distance P G G R from the transmitter e antenna can be expressed as(1) in r r t t r 4 πf R Equation (1) [7]. 2 n where Pt is the power transmitted which correspondsC to the1 input−aR power to the transmitter antenna, Pr = PtGtGr e (1) Gt and Gr are, respectively, the gains of receiving4 pandf transmittingR antennas, C is the speed of light, andwhere f isP ttheis theRF powersignal transmittedfrequency. α which is the corresponds effective decay to the coefficient input power in air to and the transmitteris equal to antenna,0.001. n denotesGt and G ther are, path respectively, loss exponent the and gains is ofequal receiving to 2 in andfree transmittingspace. As it is antennas, shown PrC dependsis the speed on multiple of light, factors:and f is the the RFpower signal of frequency.RF source, asize/performanceis the effective decay of receiving coefficient antenna, in air and transmission is equal to 0.001.frequencyn denotes and thethe distance path loss from exponent the RF and source is equal [8]. to 2 in free space. As it is shown Pr depends on multiple factors: the powerSince special of RF source,regulations size/performance exist for RF power of receiving transmission, antenna, it makes transmission sense to use frequency the free andlicense the frequencydistance from bands, the or RF industrial, source [8 ].scientific, and medical bands (ISM). They should be classified as either nonspecificSince special short-range regulations devices exist (SRD), for RF wideband power transmission, data transmission it makes systems, sense to or use radio the freefrequency license identificationfrequency bands, (RFID) or industrial,applications scientific, [9,10]. For and example, medical 867.6–868 bands (ISM). MHz They band should is one be of classified the RFID as frequencyeither nonspecific ranges short-rangeused in ultra-high devices (SRD),frequency wideband SRD applications. data transmission For this systems, band, or it radio is allowed frequency to transferidentification until 500 (RFID) mW applicationseffective radiated [9,10 ].power For example,(ERP) in Europe 867.6–868 with MHz 200 bandkHz of is coupling one of the channel RFID spacingfrequency [11]. ranges used in ultra-high frequency SRD applications. For this band, it is allowed to transfer untilMany 500 mW rule effective parts specify radiated RF power (ERP)transmission in Europe limits with in 200 terms kHz of of ERP coupling or equivalent channel isotropically spacing [11]. radiatedMany power rule parts(EIRP). specify ERP and RF powerEIRP are transmission defined in limitslinear in terms terms as of the ERP product or equivalent of the antenna isotropically gain andradiated the input power power. (EIRP). ERP and EIRP are defined in linear terms as the product of the antenna gain and theA lot input of calculations power. and analyses are required to investigate the efficiency and performance of RF powerA lot transfer. of calculations After this and approximate analyses are requiredstudy, it tois investigatebasic to note the that efficiency increasing and frequency performance leads of RFto power transfer. After this approximate study, it is basic to note that increasing frequency leads to a reduction in the received power level, hence, the decrease in the yield. To solve this problem, studying Sensors 2017, 17, 546 3 of 14 Sensors 2017, 17, 546 3 of 14 a reduction in the received power level, hence, the decrease in the yield. To solve this problem, studying the rectification systems characterized by low power loss and high efficiency to recover the the rectification systems characterized by low power loss and high efficiency to recover the maximum maximum RF energy transmitted is needed. RF energy transmitted is needed. 3. Basic Rectifier Circuit for RF Applications 3. Basic Rectifier Circuit for RF Applications An impedance matching circuit between the received aerial and rectifier circuit is necessary to An impedance matching circuit between the received aerial and rectifier circuit is necessary to increase the voltage gain and further reduce reflection and transmission loss. For low-power and increase the voltage gain and further reduce reflection and transmission loss. For low-power and sensing applications, the main aim to satisfy after rectification is to recover the maximum amount of sensing applications, the main aim to satisfy after rectification is to recover the maximum amount of power and reduce the power loss caused by the rectifier circuit.
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