MASTER'S THESIS Optimization of Wireless Power Oskar Rönnbäck 2013 Master of Science in Engineering Technology Engineering Physics and Electrical Engineering Luleå University of Technology Deptartment of Computer Science, Electrical and Space Engineering Optimization of Wireless Power Oskar R¨onnb¨ack Lule˚aUniversity of Technology Dept. of Computer Science, Electrical and Space Engineering December 5, 2013 ABSTRACT Today, the limit of wireless devices lays in the way they are powered. Imagine a device that doesn't need a charger or even a battery, which instead gets the power wirelessly over the air. To make such a device possible the transfer distance of currently known systems have to be increased. That will be the aim of this thesis, to investigate how to increase the transfer distance of a wireless power system, WPS, purposed to charge low power electronic devices. In order for the system to be usable certain design limits are set to restricts the size of the coils, flat spiral coils with diameter < 90mm and wire diameter < 2mm, and thereby also narrowing the scope of the thesis. This thesis starts with a presentation of the theoretical framework behind wireless power, including techniques for modeling a complete system. The framework is then broken down to its basic components which generates expressions with geometrical and material properties as variables. These expressions are implemented in Matlab creat- ing a simulator, which finds optimal values of geometrical and material properties that maximizes the transfer distance. The simulator is set up and ran for each system, 2, 3 and 4 coils, this because each system behaves differently and all have some desirable properties. The findings are implemented in Comsol which provides verification and illustrates the electromagnetic fields that are generated. The results from Comsol and Matlab are similar and shows that a 2-coil system can transfer power with 40% efficiency over a distance of ≈ 150mm. While 3- and 4-coil systems significantly improve the transfer distance and can transfer power with the same efficiency over a distance of ≈ 350mm. As a last step were WPS's built using the findings from the simulations. The coils were made according to the optimal parameters and capacitors were added to tune them to the same resonance frequency. An E-class amplifier was designed and built to excite the transmitting coil in the real system. The measurements made are the power delivered to the amplifier and the power delivered to the load. From that the efficiency of the complete system can be calculated. The measurements made in this thesis don't hold up to the simulations in the sense of transfer distance. The main reasons for that is that the amplifier is included in the measured PTE and not in the simulations and that the coils are not perfectly built or tuned. iii PREFACE This thesis work were conducted as the last part of the Master Programme in Engineering Physics and Electrical Engineering at Lulea University of Technology, LTU. During my project work course I first came into contact with wireless power and I thought is was a fascinating technology. Seeing the possibilities for wireless power it is clear that it will play a huge part in the future of electronics. I was not aware of any research in this area in Sweden, therefore I made the thesis work as a project on my own initiative which I carried out at LTU. I would like to thank Kalevi Hyyppa for his understanding and guidance and my family for always supporting me. Oskar Ronnback v CONTENTS Chapter 1 { Introduction 1 1.1 Wireless power today . .1 1.2 Benefits of wireless power systems . .2 1.2.1 Environmental . .2 1.2.2 Social . .2 1.3 Baseline . .3 1.4 Delimitations . .3 1.5 Goal . .4 1.6 Outline . .4 1.7 Frequently used variables and abbreviations . .4 Chapter 2 { Theory 7 2.1 Resistance . .7 2.1.1 Resistance in a wire . .7 2.1.2 Litz wire resistance . .8 2.2 Inductance . .8 2.2.1 Self inductance . .8 2.2.2 Inductance of pancake coil . .9 2.2.3 Inductor quality factor . .9 2.2.4 Mutual inductance . .9 2.2.5 Coupling coefficient . 10 2.3 Induction . 10 2.4 Resonance . 11 2.4.1 Electrical Resonance . 11 2.5 Wireless power using magnetic resonance . 12 2.6 Coupled Mode Theory . 12 2.6.1 Lossy model with source excitation . 13 2.6.2 Model of lossy 2-coil coupled system . 13 2.6.3 Wireless Power Transfer Efficiency . 14 2.6.4 WPT expanded to 3- and 4-coil systems . 14 2.7 Reflected Load Theory . 15 2.7.1 Expanded for m-coil systems . 15 2.8 Unified theory . 16 Chapter 3 { Simulations 17 3.1 Matlab simulations . 17 3.1.1 Finding the optimal PTE . 18 3.1.2 Simulation results . 19 2-coil system . 20 3-coil system . 21 4-coil system . 22 3.2 Comsol . 23 3.2.1 Simulation setup . 23 3.2.2 Simulation results . 24 2-coil system . 24 3-coil system . 26 4-coil system . 28 Chapter 4 { Electronic design 31 4.1 Tuning of the coils . 31 4.2 Source . 31 4.3 E-class amplifier . 32 4.4 Simulation . 33 4.4.1 Component selection . 33 4.4.2 PSpice . 34 Chapter 5 { Real testing 37 5.1 Measurement setup . 37 5.2 Test procedure . 37 5.3 Measurements on 2-coil system . 38 5.4 Measurement on 3-coil system . 39 5.5 Measurements on 4-coil system . 40 Chapter 6 { Discussion 41 6.1 Simulations . 41 6.2 Real tests . 42 6.3 Conclusions . 42 6.4 Health issues . 43 6.5 Future work . 43 viii CHAPTER 1 Introduction Wireless power is an old concept, Nikola Tesla experimented with it in the late 1800's. He was considering it as an alternative to building the electric grid. History tells us that wireless power were never realized at a consumer level and the concept was almost forgotten. Induction stoves and transformers transfers power "wirelessly" and have been around for some time but they all work over negligible distances. In 2007 scientists at MIT issued a press release describing how to transfer power wirelessly using magnetic resonance and presented results of transfer distances up to a couple of meters [1] . Since then interest in this technology have boomed and it is easy to see why. Wireless power could be used in a wide range of applications stretching from mobile devices cell phones, tablets, laptops, sensors, medical implants to electric cars, trains and buses. Estimations indicate that wireless power could be a billion dollar industry within the next 10 years. 1.1 Wireless power today Today, six years since that press release hardly any products have hit the market. At least not using magnetic resonance or who can transfer the power over a significant distance. The scientists at MIT that was behind this technology started a company, WiTricity, to commercialize their discovery. Since then most of their work are kept secret and protected by hundreds of patents. Their primary targets are OEM's that can embed their technology directly into their products. But no such products have been released yet. They have four own products, all using magnetic resonance [2]. Three of them are low power development kits aimed to showcase the technology for developers. The fourth one is a high power system for charging electrical cars. The products that are starting to pop up are charge pads/mats most of them uses the qi standard which is created by the Wireless Power Consortium [3], WPC. The WPC consists of over 140 members including industry leaders in mobile phones, batteries and consumer electronics. Their qi standard is made for low power wireless charging, <5W, 1 2 Introduction and specifies coil geometries, frequencies, communication, control and electric sources. The standard enables some design freedoms and is said to work with both direct induction and magnetic resonance. Most products today uses the first technique and the maximum transfer distance for a qi product today is 4cm. The WPC are working on a standard for medium power < 120W, but the specification for that is not made public yet. Their goal is to make worldwide standards for wireless power which is compatible for all devices, similar to Wi-Fi. Many of the large companies are doing their own research in this area e.g. Apple, Qual- comm, Duracell and Texas Instrument. But most is kept secret and the only available products are a few development kits and short distance charge pads/mats. There are research going on in a wide range of other applications also, from medical implants, consumer electronics to electrical cars and electric roads [4]. The medical implant research focuses on low power transfer using small coils. A 2-coil solution using direct induction, similar to a transformer, has been present for some time. Research for implementing a 4-coil system which is much more efficient and can work over longer distances have been made [5]. 1.2 Benefits of wireless power systems 1.2.1 Environmental One can argue that wireless chargers are not environmental friendly because they have lower efficiency than regular chargers i.e. will consume more power while charging. Be that as it may, wireless chargers can be made with high efficiency similar to regular chargers. But the biggest benefit will come if a global medium-range-wireless standard is implemented. A standard that enables charing of all mobile devices, phone, tablet, laptop, sensors etc, using the same charger. Then all devices don't need to have an own charger, which in turn will save a lot of resources and energy.