Microwave Wireless Power Transmission System

Transcription

1 1 Microwave Wireless Power Transmission System Omar Alsaleh, Yousef Alkharraz, Khaled Aldousari, Talal Mustafawi, and Abdullah Aljadi Prof. Bradley Jackson California State University, Northridge November 16 th, 018 Abstract This paper discusses an implementation of a far-field wireless power transmission system using microwaves in the ISM band. In this project, antennas transmit and receive power at a frequency of.45 GHz over distances up to more than 60 cm. The design, fabrication, and testing of a microstrip patch array antenna is illustrated in this paper. Furthermore, three different rectifier designs are discussed and compared based on their performance: a peak rectifier, a full-wave rectifier, and a voltage doubler rectifier. Finally, this paper discusses the overall results of the system including how input power and distance between antennas affect the output voltage of the system. The entire system is able to illuminate a light-emitting diode (LED) wirelessly at maximum distance up to 40 cm between the antennas. Keywords RF; microwaves; ISM; antenna; microstrip patch; rectifier; voltage doubler rectifier; full-wave rectifier; peak rectifier. W I. INTRODUCTION ireless power transmission has been a topic of research interest for several decades. Many technology leaders are investing substantially into this technology to be able to add wireless charging capability to their devices. In general, wireless power transmission technology falls into two categories: near-field and far-field. In near-field technologies, power is transferred wirelessly over short distances using inductive coupling with coils. This category is widely used to add wireless charging capabilities with charging mats to smart phones and other handheld devices. However, techniques used in this category can only transmit power over a very short distance. On the other hand, in far-field technologies, power is transferred via electromagnetic radiation such as microwaves and laser beams. In contrast to near-field, far-field technologies provide power transmission over much longer distances but have considerable propagation losses. This paper describes a far-field wireless power transmission system. In this system, antennas transmit power using microwaves in the ISM band. As shown in Fig. 1, a signal generator will generate power at.45 GHz and will be connected to a transmitter antenna which will transmit the power using microwaves to the receiver antenna. The power received at the receiver antenna will be connected to a rectifier which will convert the received microwave signal into DC power. Then, the converted DC power will be connected to a load such as a light-emitting diode (LED). In Section II of this paper, the transmitter and receiver antennas will be discussed, while Section III will focus on the rectifier circuits tested in this project. The overall system performance will be explored in Section IV, followed by conclusion in Section V. Fig. 1. Microwave wireless power transmission block diagram To ensure safety from high-levels of electromagnetic radiation, the system designed in this project is FCC (Federal Communications Commission) compliant. Part 15 of subchapter A in chapter 1 under title 47 of FCC telecommunication rules and regulations shows the maximum power that can be safely transmitted between the two antennas [7]. For the gain of the antenna used in this project, 8 dbm is the maximum power that can be safely transmitted which will not be exceeded in this project. All the antennas and rectifier circuits are home-built; thus, equipment authorization is not required by FCC. II. ANTENNA To achieve our objective, which is transmitting power using microwaves, a device was required that can transmit, receive and handle a certain amount of power. In other words, an antenna is required to achieve this objective. There are many types of antennas such as dipoles, parabolic, and microstrip patch antennas. Each one of them has its own features, advantages and disadvantages. In this project, a microstrip patch antenna was chosen because it has many advantages that will help this project to be efficient. In particular, microstrip patch antenna has a light weight and small size compared to other antennas. Also, directivity is one of the most important features of microstrip patch antenna. Directivity helps to transmit maximum power possible into one direction. Moreover, if considered fabricating the antenna with large quantities, microstrip patch antenna has one of the lowest fabricating costs [].

2 A. Design To design a microstrip patch antenna, different parameters must be considered to design the antenna properly, as shown in Fig.. The dielectric constant ε r of the material used to fabricate the antenna and the thickness of that material are not free variables, so it has to be chosen before doing any calculation. A lower dielectric constant will lead to a wider impedance bandwidth. All of the parameters must be optimized for the type and the height of the material. Parameter Frequency TABLE I SINGLE MICROSTRIP PATCH ANTENNA PARAMETERS Dielectric constant ε r. Thickness Width of substrate Length of substrate Width of patch Length of patch Width of feed line Notch width Value.45 GHz mm 75 mm 64.9 mm 65.5 mm 39.1 mm 4.88 mm 0.8 mm Fig.. Microstrip patch antenna [3] Considering the parameters shown in Fig., the width was calculated using: c W = f (ε r+1) where c is the velocity of electromagnetic waves in free space, which equals to 3x10 11 mm/s, and f is the operating frequency in GHz. Then, the effective dielectric constant ε reff was calculated using: [1 + 1 h 1 W ] ε reff = ε r ε r 1 Fig. 3. Single microstrip patch antenna geometry The single microstrip patch antenna has a gain around 7 dbi and a reflection coefficient less than -10 db between.4 GHz and.47 GHz, as shown in Fig. 4. To be able to focus more power to increase the distance, a higher gain is required. To achieve that, a x1 microstrip patch antenna array was designed for better gain and bandwidth. However, the size of the antenna is doubled. where W >1. Finally, the length was calculated using: h c L = L f ε reff where L can be found using [3]: W h +0.64) (ε reff 0.58)( W. h +0.8) L = 0.41 (ε reff+0.3)( h The microstrip patch antenna was designed to operate at.45 GHz. the parameters were calculated for that frequency using the formulas mentioned earlier. The calculated parameters, as shown in Table I, are approximated, and they must be optimized with the simulator to have better results and the best reflection coefficient possible. A reflection coefficient is the amount of power reflected from the antenna, and it needs to be matched to transmit or receive maximum power, where less than -10 db is considered well matched. The length of the patch will affect the chosen operating frequency [1]. The geometry of the antenna is shown in Fig. 3. Fig. 4. Reflection coefficient versus frequency for a single microstrip patch antenna

3 3 The x1 microstrip patch antenna was designed using a standard microwave T-junction power divider technique, as shown in Fig. 5, where the feed line splits into two other feed lines. A 50 Ω feed line splits into two 100 Ω line, and each 100 Ω feed line connects to a 70.7 Ω feed line. This method helps to match the impedance accordingly. The x1 microstrip patch antenna parameters is shown in Table II. TABLE II MICROSTRIP PATCH ANTENNA ARRAY PARAMETERS Parameter Value Frequency.45 GHz Dielectric constant ε r. Thickness mm Width of substrate 167 mm Length of substrate 75 mm Width of patch 76 mm Length of patch 39.1 mm Width of feed line 4.88 mm Notch width 0.8 mm Fig. 7. Reflection coefficient versus frequency for a x1 microstrip patch array antenna Fig. 5. x1 microstrip patch antenna array geometry B. Fabrication & testing The ease of fabricating microstrip patch antenna is one of its own advantages. After the array was designed and the results were satisfying, the antennas were fabricated to be tested. A photograph of the fabricated microstrip patch array antenna is shown in Fig. 6. The array was fabricated with an RT/Duroid 5880 with a thickness of 1.57 mm using the milling machine at CSUN. The microstrip patch arrays were tested in the CSUN anechoic chamber. It was observed that the simulation results were slightly different compared to the measured results after fabrication. The measured results showed a reflection coefficient of -16 db as shown in Fig. 7. The normalized radiation pattern is shown in Fig. 8. The measured gain of the antenna is 10 dbi. The simulated and measured results were different due to manufacturing tolerance and simplifications used for the simulated model. Fig. 6. A photograph of the microstrip patch array antenna Fig. 8. Normalized radiation pattern of the x1 microstrip patch array antenna III. RECTIFIER CIRCUITS This section illustrates the comparison of peak rectifier, full wave rectifier, and voltage doubler in terms of performance. The diode has enormous impact on the rectifier s performance since it is the main source of power loss. To be suitable for this microwave power transfer project, the diode must have low turn-on voltage to operate efficiently. As a result, the diode Avago HSMS-80 was chosen for each rectifier in this project. The smoothing capacitor was SMD Multilayer ceramic capacitor. This model was used due to its High-Q and low effective series resistance (ESR). Advanced Design System (ADS) by Agilent Technologies Inc. was used to simulate and design the circuits. The components were simulated accurately considering parasitic parameters from manufacturer's data sheets. The components were simulated taking into

4 4 consideration the parasitic parameters from manufacturer's data sheets. The fabricated circuits were designed with Roger RT/Duroid 5880 substrate (ε r =. and thickness of 0.79 mm). A. Peak rectifier In a peak rectifier, the diode is forward biased and current flows through the load resistor during the positive half cycles. During the negative half cycles, the diode is reverse biased. Thus, the rectifier acts as an open circuit and current will not flow through the load resistor. Therefore, the output will be only the positive cycles [8]. The circuit diagram is shown in Fig. 9, and a photograph of the fabricated rectifier is shown in Fig. 10. Fig. 11. Full-wave rectifier circuit diagram Fig. 9. Peak rectifier circuit diagram Fig. 1. A photograph of full-wave rectifier Fig. 10. A photograph of peak rectifier B. Full-wave rectifier The classic full wave rectifier circuit is illustrated in Fig. 11. The main purpose of the full wave rectifier is to convert the AC signal to DC. As shown in the schematic, it is made of two parts. The first part consists of 4 Schottky diodes that operate as follows: during the positive half cycles, the diodes D and D3 become forward biased, the current flows through them into the load resistor. During the negative half cycles, the diodes D1 and D4 become forward biased and the current flows through them into the resistor. The second part of the schematic is the smoothing capacitor. To reduce the ripple voltage of the output, a smoothing capacitor is connected in parallel with the load resistor. The capacitor reduces the ripple by storing and discharging energy in between the cycles which will result in smoother DC output. A photograph of the fabricated rectifier is shown in Fig. 1. C. Voltage Doubler rectifier The functionality of the voltage doubler rectifier is to double the input voltage, so the output voltage would be twice the peak input voltage and it can be represented as V out= (V Peak-V Diode). It consists of two capacitors and two Schottky diodes, as shown in Fig. 13. The voltage doubler rectifier can be constructed of two circuits which are the clamper and rectifier circuits [4]. The clamper circuit appears during the negative cycles when the diode D 1 is forward biased. The rectifier circuit appears during the positive cycles when the diode D is forward biased. C 1 and D 1 rectify the signal during the negative cycles, whereas C and D rectify the signal during the positive cycles. However, the voltage that can be stored on C 1 during the negative cycles is transferred to C during the positive cycles. As a result, the voltage on C can be twice V Peak minus V Diode [5]. Fig. 14 shows the fabricated voltage doubler rectifier. Fig. 13. Voltage doubler rectifier circuit diagram

5 5 Fig. 14. A photograph of voltage doubler rectifier D. Results of rectifier circuits The three rectifiers were simulated and measured at.45 GHz at different load resistances and power inputs. Different load resistances were used to simulate the output voltage at a fixed power input of 7. dbm for the three rectifiers. As shown in Fig. 15, the voltage doubler rectifier had the best results. Furthermore, the rectifiers had similar results for input power less than 4 dbm, as shown in Fig. 16. However, the voltage doubler rectifier has better results for input power greater than 4 dbm. Thus, the voltage doubler was selected for the microwave power transfer system. IV. OVERALL SYSTEM PERFORMANCE A. System Simulator Application To help simulating the system and predicting its performance, a system simulator application was built using MATLAB. The application takes several inputs and provide the power received at the receiver antenna as an output. The inputs to the application are input power, transmitting antenna gain, receiving antenna gain, distance between the two antennas, operating frequency, and polarization angle. Then, for the second stage of the application, it calculates the DC output voltage depending on the rectifier type. The system simulator application graphical user interface is shown in Fig. 17 below. The application uses the Friis transmission equation to calculate the received power output which considers two antennas in free space with no obstruction nearby [9]. The equation is as follows: P R = P TG T G R c (4πRf) Where: P T is the input power at the transmitter antenna. G T is the transmitter antenna gain. G R is the receiver antenna gain. R is the distance between the two antennas. f is the operating frequency. c is speed of light constant. Fig. 15. Simulated load resistance versus output voltage at 7. dbm for the three rectifiers Fig.17. System Simulator Application GUI The application also indicates whether the user is in safe area according to FCC regulations by calculating the power density. The maximum power density allowed for general population according to FCC regulations is 1 mw/cm at the operating frequency of this project [6]. The power density is calculated with the following equation: S = PG 4πR Where: S is the power density. P is the input power at the transmitting antenna. G is the gain of the antennas. R is the distance to center of radiation of antenna. Fig. 16. Measured input power versus output voltage for the three rectifiers with a 1.1 kω load. B. Overall system results In this section, the voltage doubler rectifier was used along with the two antennas to measure the overall system performance. As shown in Fig. 18, the system consists of a

6 6 signal generator that is connected to the transmitting antenna. Then the receiver antenna receives the transmitted microwave signal and passes it to the rectifier which converts it into DC power to light up the LED. Fig. 18. A photograph of the microwave wireless power transmission system The overall system results were measured in two different setups. For the first set-up, the output voltage was measured for a fixed signal generator power of 3 dbm and varying distance between the two antennas with no load resistance. For this measurement, the two antennas must directly face each other to maximize the power delivered to the receiving antenna. Any slight rotation of the antenna will result in power loss. As shown in Fig. 19, the output voltage reaches above 5 V for short distances and decreases with longer distances as expected. In fact, a 1 V output was obtained up to a distance of approximately 70 cm. For the second set-up, the distance between the two antennas was fixed to 30 cm and the output voltage was measured for varying input power. Fig. 0 shows how the output voltage increases with higher input power. In this setup, an LED was used to illustrate the results visually. The maximum distance for which the system could illuminate an LED was 40 cm at an input power of 3 dbm. Fig. 19. Measured distance versus output voltage at a fixed signal generator power of 3 dbm Fig. 0. Measured input power versus output voltage at fixed distance of 30 cm V. CONCLUSION A far-field power transmission system was built and tested successfully as intended. The antennas were designed and fabricated to achieve the purpose of transmitting power wirelessly. Afterwards, the three rectifier circuits were built and tested for better performance in terms of converting microwave signal to DC power. The overall system succeeded in powering up an LED wirelessly over a distance of up to 40 cm. However, achieving high DC levels while staying in the safe electromagnetic limits was challenging especially when transmitting power in longer distances due to the FCC limitations. Far-field wireless power transmission may be widely relied on to transmit power wirelessly in the future. Microwave power transmission can be used in many different applications such as charging smart phones and other handheld devices. Technology leaders can enhance the antennas by decreasing its size and increasing its efficiency. In addition, companies can improve the efficiency of the rectifiers to get a better overall result. Such an application may be developed and seen in a market product in the following years. REFERENCES [1] D. M. Pozar, A Review of Aperture Coupled Microstrip Antennas: History, Operation, Development, and Applications, University of Massachusetts, Amherst, MA, rep., [] V. Mohan Kumar, N. Sujith, and S. K. Behera, Enhancementof Bandwidthand Gainof a Rectangular Microstrip Patch Antenna, A thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of Technology in Electronics and Communication Engineering, pp. 1 53, 010. [3] M. P. Civerolo, Aperture Coupled Microstrip Antenna Design and Analysis, In Partial Fulfillment of the Requirements for the Degree Master of Science in Electrical Engineering, pp , Jun [4] A. Sedra and K. Smith, Microelectronic circuits, 6th ed. New York: Oxford University Press, 009, p. 1. [5] D. Harrist, "Wireless Battery Charging System Using Radio Frequency Energy Harvesting", Submitted to the Graduate Faculty of The School of Engineering in partial fulfillment of the requirements for the degree of Master of Science, p. 13, 011.

7 [6] RF Safety FAQ, Federal Communications Commission, 11-Oct-016. [Online]. Available: [7] ecfr - Code of Federal Regulations. [Online]. Available: 15&rgn=div5#se _115. [8] Peak Detector, All About Circuits. [Online]. Available: [9] P. Bevelacqua, The Friis Equation, Friis Equation - (aka Friis Transmission Formula). [Online]. Available: 7