Highly Efficient Wireless Energy Harvesting System using Metamaterial based Compact CP Antenna

Kush Agarwal1, Tanuja Mishra1, Muhammad Faeyz Karim2, Nasimuddin2, Michael Ong Ling Chuen2,Yong Xin Guo1, Sanjib Kumar Panda1

1National University of Singapore, email: elekush@nus.edu.sg2Institute for Infocomm Research, Agency for Science, Technology and Research (A*STAR), Singapore

Abstract — This paper presents a highly efficient 2.4 GHz wireless energy harvesting system comprising of a metamaterial based circularly polarized (CP) antenna and a power management circuit. The antenna is designed at 2.4 GHz using a circular slotted truncated corner square patch radiator placed on reactive impedance surface (RIS) for antenna size miniaturization, better impedance matching and to improve the front-to-back ratio. The power management system integrates a matching circuit with a single stage Dickson charge pump and an ultra low power with high efficiency DC/DC boost converter/charger. The Dickson charge pump uses three schottky diodes to minimize the losses at high frequency. The DC-DC converter (BQ25504) is capable of acquiring and managing low power levels. The measured axial ratio (boresight) is below 3-dB for the entire 2.40-2.48 GHz band and the 10-dB return loss band is from 2.35-2.49 GHz. The gain (boresight) of the antenna is around 4.6 dBic at 2.44 GHz. The proposed antenna shows an improvement in front-to-back ratio of around 3 dB with size reduction of approximately 22%. The power management system generates an output voltage level of 1.5 volts at -10 dBm and 4 volts at 0 dBm input RF power respectively. The overall efficiency of the proposed energy harvesting system is above 28%.

Index Terms — Energy harvesting, circular polarized antenna, Metamaterial, Periodic structures, Reactive impedance surface.

I. INTRODUCTION

RF energy harvesting paves a way to utilize RF energy which otherwise is being wasted. Normally, batteries are the dominant energy source for wireless sensor networks (WSN) but they are not the optimal choice for wireless electronics. Most of the modern WSN are equipped with dynamic power management system (DPM) and may require power consumption few times per day during readout operations. Such devices can be efficiently powered from RF sources belonging to existing wireless systems.

Many studies have been done to maximize the harvested power by using micro controller based maximum power point tracking (MPPT) algorithms to control the boost converter operation [1, 2]. However, operation of the control circuitry needs a significant voltage level to operate. Hence at very low power levels, when the output of the rectifier goes down, the operation of the control circuitry become difficult. Also many studies have been carried out for using voltage multipliers in different configurations for rectification and power management [3, 4, 5]. However, when the numbers of stages

of the charge pump goes up, the voltage increment per stage decreases considerably. Also for multistage charge pumps with RF input, the design of power divider circuits make impedance matching more complicated and difficult.

Circularly polarized microstrip antennas (CPMAs) offer the flexibility of insensitivity towards the device orientation or multi-path effects for phase alignment, thus making them more desirable for today’s world wireless applications. Antenna’s volume is an important factor for deciding the overall size of the system and it becomes more critical as the front-to-back ratio and forward gain vary with the ground plane size. In recent years, electromagnetic metamaterials have been intensely studied and used for enhancing the radiation properties of the antennas like frequency bandwidth and direction of antenna radiation for CPMAs with size miniaturization which have resulted in highly efficient compact antennas as in [6, 7].

This paper uses a two stage power management approach. In the first stage, a single stage Dickson charge pump is used which does the work of a rectifier as well as voltage multiplier circuit. In the second stage, an ultra low power DC/DC converter and charger chip BQ25504, is used to boost the output of first stage to a significant level to charge a lithium ion battery. A metamaterial based compact antenna with improved gain and CP bandwidth has been used as the receiving antenna for the wireless energy harvesting system. The overall system architecture is shown in figure 1.

Fig. 1. Overview of proposed RF energy harvesting system

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II. COMPACT CIRCULARLY POLARIZED ANTENNADESIGN

Cross-sectional view of the proposed circular slotted truncated corner square patch (CSTCSP) radiator over the reactive impedance surface (RIS) is shown in figure 2 (a). The antenna consists of a CSTCSP radiator printed on the top of dual-layer FR4 substrate (�r = 4.2 and tan � = 0.02) with thicknesses of h1 = 0.5 mm and h2 = 3.2 mm, in which ground plane lies at the bottom of the structure and RIS is printed at the interface between the two dielectric layers. This RIS layer is realized using an array of 5×5 Jerusalem-cross (J-Cross) unit cell patches printed periodically along the x- and y-axis, thus leading to a symmetrical design along the lateral dimensions of the antenna.

The feeding is done using a coaxial probe at point (-4.75, 0) with respect to patch center. As the feeding probe should not come in direct contact with RIS metal layer, a circular region from RIS is removed to accommodate the coaxial SMA connector. The CP radiation with antenna miniaturization and improved front-to-back ratio is achieved by using RIS layer as a metasurface below the circular slotted truncated corner patch radiator. The design of the J-Cross RIS and the proposed CSTCSP radiator for CP radiation are discussed in subsections A and B, respectively.

A. Reactive Impedance Surface (RIS) Top view of the J-Cross RIS metamaterial surface is shown

in figure 2 (b) under the CSTCSP radiator. The RIS unit cell is first designed by exciting a TEM wave to tune the operating frequency band between the perfectly electric conductor PEC (180o reflection phase) and perfectly magnetic conductor PMC (0o reflection phase) boundary limits. The simulation model of the J-cross RIS unit cell is shown in figure 3 with the reflection phase response for the best compromise between the optimal bandwidth and miniaturization factor. Different metal patch parameters for the J-cross unit cell are varied for keeping the frequency band of operation in inductive RIS region, while keeping the overall unit cell size (a2) = 6.2 mm constant. The optimized values are shown in inset of figure 3.

A J-Cross RIS with a purely reactive impedance of � = jvimproves the three main backdrops of typically used PEC and PMC surfaces as antenna ground planes: (a) by reducing the mutual coupling between the patch radiator and ground plane, it demonstrates better impedance matching over a wider bandwidth, (b) by combining the inductive (capacitive) behavior of RIS with capacitive (inductive) behavior of patch radiator at relatively lower frequency than the resonance frequency, it demonstrates antenna miniaturization, and (c) it improves the front-to-back ratio.

B. Circular Slotted Truncated Corner Square Patch (CSTCSP) Radiator

The CSTCSP patch radiator is designed over RIS for CP radiation as shown in figure 2 (b). The opposite corners of the square patch of side length L = 24 mm are chamfered by length l = 4.4 mm to simultaneously generate two orthogonal resonance modes for CP radiation. A circular slot of radii r = 4 mm is introduced in the TCSP patch for further improving the CP radiation.

The measurement results of the fabricated antenna are plotted for the return loss and axial ratio (AR) with gain at boresight in figures 4(a) and 4(b). The measured AR lies below 3-dB for the entire ISM frequency band of 2.40-2.48 GHz and lies completely within the 10-dB return loss

Fig. 2. Proposed RIS based antenna design (a) cross-sectional view (b) top view

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Fig. 3. Reflection phase graph of J-Cross RIS unit cell with the RIS unit cell (shown in inset) enclosed by PEC walls in direction of E-field, PMC walls in the direction of H-field and illuminated by incident plane wave in negative z-direction.

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bandwidth of 2.35-2.49 GHz. A maximum measured right handed circularly polarized (RHCP) gain of around 4.6 dBic is achieved at 2.44 GHz in the boresight direction. The measured normalized radiation patterns for both the principal planes (xzand yz) are plotted in figures 5(a) and 5(b) at 2.44 GHz respectively. The 3-dB AR beam width is more than 120o for the 3-dB AR bandwidth frequency range. The proposed J-Cross RIS based CSTCSP antenna shows a further 3-dB improvement in front-to-back ratio as compared to square patch RIS based antennas presented in [7] with approximately 22% size reduction when compared to CSTCSP antenna without RIS, thus improving the gain in the boresight direction.

III. RECTIFIER AND POWER MANAGEMENT SYSTEM

The Power management system consists of two stages. In the first stage, the RF power received by the antenna is rectified using a Dickson voltage multiplier preceded by an impedance matching circuit. In the second stage, the output voltage is further boosted to a level to charge a lithium-ion battery as shown in figure 1.

A. Impedance matching The Impedance matching network is the most critical

element in a RF energy harvesting system and should be designed very carefully for optimal output. The matching circuit is designed to match the load impedance to impedance of the source (antenna in this case) which is designed for 50 �and is tuned to 2.4 GHz.

B. Voltage multiplier There are many voltage multiplier topologies in the

literature. But most of the modern digital integrated circuits use Dickson charge-pump. The rectifier topology should be chosen such that there is no requirement of an external voltage source. Also, the selected topology should be able to rectify the received RF input and improve the output voltage at the same time. In this work, a single stage Dickson voltage multiplier with RF input is used. The rectifier uses Skywork SMS7630 small signal detectors diodes.

C. DC-DC Converter/Charger This is the second stage of power management circuit which

uses Texas Instrument BQ25504.The chip integrates a DC-DC converter and a battery management feature. Low quiescent current (<330nA) and ability to operate at ultra low source voltage (VIN>80mv) makes it most suitable for low power RF energy harvesting circuits among all other ICs available currently.

IV. EXPERIMENTAL RESULTS

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The designed voltage multiplier and energy harvesting system was tested for different RF power inputs, and the efficiency of the circuit was measured for different load resistance values as shown in figure 6.

TABLE ISUMMARY OF VOLTAGE MULTIPLIER AND OVERALL

SYSTEM PERFORMANCE Input

RF Power

No load Voltage

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Vout (V) η (%)

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Multiplier

1mW 2.20 433.55 1.029 44.6

100μW 0.69 137.35 0.133 18.3

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circuit

1mW 4.06 138.50 2.084 28.9

100μW 1.51 1.15 1.32 15.2

Table I summarizes the performance of the voltage multiplier and RF energy harvesting system. Figures 7 (a) and (b) show the measured efficiency of the voltage multiplier and figures 8 (a) and (b) show the measured efficiency of the overall system respectively at different load conditions and input RF power level. The proposed Dickson voltage

multiplier has an efficiency of as high as 44.6 % at RF input of 1mW and 18.3% at RF input of 100µW.

The overall efficiency of the system was measured to be around 28.9% and 15.2% with 1mW and 100µW of RF power respectively at its input.

V. CONCLUSION

In this paper, a highly efficient wireless energy harvesting system was presented. A metamaterial RIS based compact circularly polarized antenna was proposed with improved front-to-back ratio of around 3 dB, thus resulting in improved gain in the boresight direction. The antenna covered the entire CP band of 2.40-2.48 GHz with size reduction of approximately 22%. The system generated an output voltage level of 1.5 volts at -10 dBm and 4 volts at 0 dBm input RF power respectively. The overall efficiency of the system was as high as 28.9% at 1mW RF power at input.

ACKNOWLEDGEMENT

The work was in part supported by Singapore Ministry of Education Academic Research Fund Tier 1 Project R-263-000-667-112.

REFERENCES

[1] A. Dolgov, R. Zane, & Z. Popovic. “Power Management System for Online Low Power RF Energy Harvesting Optimization”, IEEE Transactions on Circuits and Systems,2010, page 1802-1811.

[2] A. Costanzo, M. Fabiani, A. Romani, D. Masotti, “Co-design of Ultra-Low Power RF Microwave Receivers and Converters for RFID and Energy Harvesting Applications”, Microwave Symposium Digest (MTT), IEEE, 2010. page 856-859.

[3] F. Yuan and N. Soltani, “Design Techniques for Power Harvesting of Passive Wireless Microsensors” 51st IEEE International Midwest Symposium on Circuits and Systems, pp. 289-293, 2008.

[4] Muhammad Adeel Ansari, Waqar Ahmad, Svante R. Signell, “Single Clock Charge Pump Designed in 0.35�m Technology”, 18th International Conference on Mixed Design of Integrated Circuits and Systems, 2011.

[5] Sherlyn dela Cruz, Mark Gerard delos Reyes, Anastacia Alvarez, Maria Theresa de Leon, Christian Raymund Roque, “Design and Implementation of Passive RF-DC converters for RF energy harvesting Systems”, TENCON 2010, page 1503-1508.

[6] K. Sarabandi and H. Mosallaei, “Antenna miniaturization and bandwidth enhancement using a reactive impedance substrate”, IEEE Trans. Antennas and Propagation, vol. 52, no. 9, pp. 2403–2414, September 2004.

[7] K. Agarwal, Nasimuddin and A. Alphones, “RIS based Compact Circularly Polarized Microstrip Antennas”, IEEE Trans. Antennas and Propagation, vol. 61, no. 2, pp. 547–554, Feb. 2013.

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Fig. 8. Overall efficiency vs. load resistance plot: (a) Pin=1mW (b) Pin=100μW

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