wireless power transmission-free project report
In the age of wireless communication and portable music players the demand for powering those devices wirelessly is ever prevalent. The advantages of portability and wireless communication are greatly hindered by the fact that the devices themselves must be plugged into the walls to charge. The next generation in portable devices is a device that receives power wirelessly. The first step in wireless power is providing power to a computer charging pad wirelessly. The market for this device would be businesses with large conference rooms. The device would allow users to plug their phones and computers into the conference room table without large power bricks and cords running everywhere. The pads can conveniently be placed under the table and inside the ceiling so there are no visible wires that could ruin the aesthetic feel of the room. The ease of installation and convenience of this device would make the marketability of this product quite large and if finished could be seen in thousands of conference rooms. If the efficiency of coupling could be increased slightly further, wireless power transmission could become a standard means for charging a mobile device.
The overall goal of this project is to design and successfully implement a wireless power transmission system to be used in a conference room. The system will work by using resonant coils to transmit power from an AC line in the ceiling to a pad on the table. The pad will output DC voltages in order to charge computers and cell phones. There are several benefits for the use of such a system:
• Elimination of cords on the ground that make tripping hazards.
• Allows no wire installation and mobility on table.
• A necessary step towards consumer wireless power.
The entire interface has the following features:
• Feedback control for driving frequency to maximize efficiency.
• DC power output for computers and cell phone charging that allows for elimination of large power bricks.
• Slight mobility offered for different table heights and positions.
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The full bridge circuit is a generic circuit found on page 212 of [5]. The switches were chosen based on max frequency, current carrying capabilities, and voltage blocking. The speed is important because our switching frequency is several MHz at high voltage and a reasonable amount of current. The gate drivers were chosen because they have the appropriate frequency requirements and are designed to drive MOSFETs, also they have an inverting and non-inverting signals. This means that it can drive all of the MOSFETs. At first, the gate drivers and MOSFETs chosen were not fast enough to be able to handle the high switching frequency needed to make our coils resonant. Professor Krein provided a solution in the form of the EL7222 gate drivers and IRF6875 MOSFETs. The speed was verified with datasheets [4] and [3] respectively.
2.3 PIC, DAC, and VCO
The PIC was chosen because it was readily available, has an analog input for feedback, and it can operate at a speed that is fast enough to control the frequency from [9]. The PIC outputs a 10 bit logical signal that is converted to a voltage by a DAC. The DAC was chosen similar to the PIC in that it was readily available, has the ability to convert the digital pins to the analog voltage range needed. The initial DAC that chosen would not correctly output the analog voltages needed. After some searching and some help from Dan Block, a DAC was found that would output the voltage range needed [1]. The C-code already generated for the PIC did not have to be changed with the operation of this new DAC. The output voltage of the DAC is an input to the VCO. The VCO was chosen because of its frequency range. The voltage bias pin was set such that the increase in frequency per increase in voltage is a maximum based on [11]. The external capacitor was chosen such the maximum output frequency is more than twice as high as the expected resonance frequency.
2.4 Current Sensing
The Current Sensing circuit was found in the data sheet for the op-amp used [8]. The Transistor part numbers were changed based on what was available in the parts shop. The fact that the circuit had to handle a common mode voltage of around 170 V made the circuit more complicated than normal current sense circuits. The zener diode and the p-type MOSFETs in the design allow the high side voltage to be as high as 500 V, while only making the voltage supply to the op-amp around 62 V and taking care of any common mode voltage problems by referencing to a voltage other than ground.