17-08-2012, 02:53 PM
WIRELESS BATTERY CHARGER
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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.
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.
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.
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.
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 faster ways to charge the battery, but it requires a large amount of energy which the wireless battery charger can not obtain, yet. Therefore, in our design, we use a straight forward method to charge the battery.
Microwave signal is an AC signal with a frequency range of 1 GHz – 1000 GHz. 900 MHz is in between the RF/ Microwave range. No matter how high the frequency is, AC signal is still AC signal. Therefore, the signal can also be treated as a low frequency AC signal. In order to get a DC signal out of the AC signal, a rectifier circuit is needed.
PROBLEMS AND SOLUTIONS
There are 5 people in the design project team, and it’s a relatively large group compare to others. Each member of the team is responsible to communicate to each other and update the status of the project. In the very early stage of the project, the group had defined some specific roles for each group member, so that each group member had a defined responsibility in the design process. As time went by, the sense of this pre-defined responsibility of individual seemed to get dissolved soon after the EE 198A was over.
The progress of the project had been sluggish since the beginning of the semester and caused some uprising frustration from group members. The communication between group members and the project adviser was decaying as well. The directions of the project that were pre-defined from the previous semester had been changed several times in the beginning of EE 198B. Due to those back-and-forth changes, valuable time had been wasted.
Before the final decision was set on working with the circuitry with 900 MHz frequency, the design team had other choices of frequency range in mind. The first choice was 2.4 GHz from commercial wireless routers. The accessibility is one of the main advantages of using 2.4 GHz wireless routers. However, it fails to provide sufficient power to fit our design requirement and lack of test equipment to measure the circuitry. The second choice was to switch the design specifications using 20MHz frequency. The advantage is that at such low frequency it is possible to use function generator as the transmitter. One big disadvantage is insufficient time to build the power amplifier for the transmitter. The antenna design was another problem that we encountered when we picked this frequency. At 20 MHz, a directional antenna becomes very large compare to 2.4 GHz antennas. Finally, the team decided on 900 MHz frequency for its accessibility, power, and size of the antenna.
CONCLUSION
In conclusion, power loss and efficiency are the major problems for this design project. Our design team has noticed the potential problem whether the converted DC power will be significant enough to charge up the battery. Therefore, the characteristics of the diodes should be mounted directly onto the antenna for minimum power dissipation. In addition to harmonics, the nonlinear diode creates a DC-bias in the resonant circuit which can be extracted without affecting the RF/ microwave characteristics of the resonant circuit. The time varying voltage and current relationship at the physical point of the diode in the cavity determines the loss in the diode and, consequently, the RF/ microwave to DC efficiency.
As the wireless technology is getting popular nowadays, the demand of battery is also increasing. The battery needs to be recharged or changed eventually. Therefore our team is inspired to design the wireless battery charger. This wireless battery charger will eliminate all the hassle with the battery.