EE 198B Final Report “WIRELESS BATTERY CHARGER” (RF/ Microwave to DC Conversion) Dec 02, 2005 Group Members: Eric Lo Hau Truong Louis Elnatan Alvin Mar Ha Nguyen Adviser: Dr. Ray KwokTable of Contents I. Abstract……………………………………………………2 II. Introduction….…………………………………………….3 III. Design of Overview……………………………………….4 IV. Engineering Procedure…………………………………….5 A. Transmitter…………………………………………….5 B. Antenna………………………………………………..6 C. Receiver……………………………………………….7 V. Final Design……………………………………………….8 A. PSPICE Simulation……………………………………8 B. Final Design Circuit…………………………………...9 VI. Testing and Data…………………………………………..10 VII. Specification………………………………………………12 VIII. Problems and Solutions…………………………………...13 IX. Possible Improvements..…………………………………..15 A. Multiple Rectifiers…………………………………….15 B. High Gain Parabolic Antennas………………………..16 X. Marketing Plan….…..……………………………...……..16 XI. Conclusion………...……………………………………....17 XII. References………………………………………………...18 XIII. Appendix.............................................................................19 1I. ABSTRACT This report covers the basis and design of the wireless battery charger. The wireless charger will convert the RF/ microwave signal at 900 MHz frequency into a DC signal, and then store the power into an AAA battery. The project is divided into 3 parts: transmitter, antenna, and charging circuit. A complete discussion of the specifications of the battery charger is provided after data measurements. This report also includes component list, financial, data results, and other key information. 2II. INTRODUCTION Portable electronic devices are very popular nowadays. As the usage of these portable electronic devices is increasing, the demands for longer battery life are also increasing. These batteries need to be recharged or replaced periodically. It is a hassle to charge or change the battery after a while, especially when there is no power outlet around. Therefore, our team is inspired to design a wireless battery charger. This wireless battery charger is expected to eliminate all the hassles with today’s battery technology. As for now, there are no known companies that are developing the wireless battery charger. This means that there might be a good opportunity in the market for this type of product. Moreover, people tend to spend more money for convenience that meets the price. The outlook of this device is supported by the above predictions. It would be convenient not having to worry about charging or changing the batteries and still have a working device. The advantage of this device is that it can wirelessly charge up the batteries which can save time and money in a long run for the general public. Base on this concept, the design team has come up with a new way to charge the batteries wirelessly. The project is to make a prototype device that converts microwave signals to DC power. Once the prototype has been proved to be working, it is possible to implement this prototype into other applications such as in television remote control, fire alarm, clock, and places that are far to reach to change battery. 3III. DESIGN OVERVIEW This wireless battery charger is designed to operate at 900 MHz. In this project, a power transmitter acts as the power source. It will transmit power to the receiver side. And then, the rectifier circuit in the receiver will convert the RF/ microwave signal into DC signal. After the DC signal is produced, the charging circuit will store the power into the battery. Here is the block diagram of the overall design. Figure 1: The overall wireless battery charger design 4IV. ENGINEERING PROCEDURE A. Transmitter Figure 2: Block Diagram of the Transmitter Figure 3: 900 MHz Video/Audio Transmitter Since the group does not design the transmitter, therefore the design is mainly focus on the receiver side. A power transmitter is bought from a commercial website. It is a 900 MHz video/audio transmitter. Here’s the specification of the transmitter*. Power: 12V DC, 900 mA Output Power: 3 Watts Operating Frequency: 900 MHz Connector Type: SMA – Female Output Impedance: 50 Ω (*Note: Further information is available in the Appendix) 5B. Antenna The antenna plays a very important role. To charge a battery, a high DC power signal is needed. The wireless battery charger circuit must keep the power loss to the minimal. Therefore, there are many considerations to choose the correct parts for the design. The considerations of choosing the appropriate antenna are: 1. Impedance of the antenna 2. Gain of the antenna Taking the above design spec in consideration, the team found Yagi antennas that fit our spec. Below is a picture of the Yagi antenna. Figure 4: A picture of the 9 dBi gain Yagi antenna The impedance of the antenna should match with the output impedance of the power transmitter and input impedance of the rectifier circuit. Non-matching impedance between circuits can cause a tremendous power loss due to signal distortion. Since the output impedance of the transmitter is 50 Ω, the antenna should also have 50 Ω impedance. The higher of the antenna gain yields a better result of the design. However, higher gain will also increase the cost and the size of the antenna. This becomes a major factor in choosing the antenna due to the group’s limited financial resources. After consideration, a 9 dBi Yagi antenna is chosen for the design (see Appendix for specifications) 6C. Receiver The receiver’s main purpose is to charge an AAA battery. A simple battery charging theory is to run current through the battery, and apply a voltage difference between the terminals of the battery to reverse the chemical process. By doing so, it recharges the battery. There are other efficient and