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WIRELESS POWER TRANSMISSION
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.
DIWAKAR BANSAL 0619231015
SANTOSH KR. VAISH 0619231041
ANKIT SINGH 0619231007
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:
Â¢ Elimination of cords on the ground that make tripping hazards.
Â¢ Allows no wire installation and mobility on table.
Â¢ A necessary step towards consumer wireless power.
The entire interface has the following features:
Â¢ Feedback control for driving frequency to maximize efficiency.
Â¢ DC power output for computers and cell phone charging that allows for elimination of large power bricks.
Â¢ Slight mobility offered for different table heights and positions.
The specifications original specifications from our design proposal are as follows in Error! Reference source not found..
TABLE 1: DESIGN SPECIFICATIONS
Transmission Efficiency >30 %
Overall Efficiency >25 %
Output Voltage 18 VDC Ã‚Â± 1.8 V
5 VDC Ã‚Â± .5 V
Frequency Within 10 kHz of optimal
Power Abilities Laptop and cell phone
1.3 Block Diagram
The block diagram for the transmission setup is shown in Error! Reference source not found..
Figure 1: Block Diagram
1.4 Block Descriptions
The different blocks shown in the block diagram were implemented separately and then integrated together. Below are descriptions of each block and specifications for each.
1.4.1 DC Source
The DC source takes in the input from the wall voltage which is a 60 Hz sinusoid. Using diodes, the voltage is rectified and passed through a PI filter. The original design specified a 1 % voltage ripple, but this ripple requirement was excessive and difficult to meet at such a low frequency. The final design chosen had a voltage ripple of less than 5 % and was more than suitable.
1.4.2 Full Bridge Inverter
The full bridge inverter is a circuit that uses four switches, a DC source, and a load. The four switches are setup in an H-bridge with the middle being the load. In this case the load is the top coil. Two of the switches are connected from the high side DC source to opposite sides of the coil. The remaining two switches are connected from the low side of the DC source to opposite sides of the coil. High side switches have opposite duty cycles and the low side switches are connected such that the DC source is applied across the load. The result is a square wave being applied across the coil. The switches are MOSFETs that have the capability to carry the max current and can block the full DC voltage.
1.4.3 Gate Drivers
Gate drivers are used to turn on and off the switches. The gate drivers take in a timing signal and output a voltage high enough and with enough current to drive MOSFETs on and off at the same frequency of the timing signal.
1.4.4 PIC Microcontroller
The gate drivers are controlled by a PIC microcontroller. There is feed back into the PIC about how much power is being drawn. The PIC attempts to increase the frequency until the power out of the DC source is at a maximum. This should correspond to the resonant frequency because the increase in input power means there is an increase in output power. The PIC outputs 10 digital logic pins as either high or low. The pins are then converted to a voltage by the Digital to Analog Converter (DAC).
1.4.5 DAC/ VCO
The output of the DAC is a voltage between 0 and 5 V. This voltage is an input to a Voltage Controlled Oscillator (VCO), which produces a square wave that increases in frequency as the input voltage increases. The range of frequencies is determined by a bias voltage and an external capacitor, and is set around our expected resonant frequency.
1.4.6 Current Sensing
The current sensing circuit is used to tell the PIC how much current is being pulled from the DC source. It uses a precision .15 resistor on the output of the DC source and that voltage is feed into an op-amp circuit that produces a voltage proportional to the current. This voltage is designed to be within the range of inputs for the PIC.
1.4.7 Coils and Air Gap
The coils are each made out of 100 turns of 20 AWG magnet wire. They are separated by about 2 m and have a diameter of about 1 m. The power transfer between them is done through resonant magnetic coupling.
A transformer is used to scale down the voltage to around 18 V. This is done before the signal is converted to DC because high frequency transformers are small and relatively efficient.
1.4.9 Rectifier and Filter
A similar circuit is used to convert the AC signal from the transformer to a DC signal. Different diodes are used due to the high frequency nature of the signal. Smaller capacitors are used in the filter because the frequency is much higher than the 60 Hz signal filtered the top filter. The capacitors are also ceramic because electrolytic capacitors have a much lower self resonant frequency, after which they begin to behave like inductors.
1.4.10 Buck Converter
A buck converter is used to convert the 18 V for the computer down to 5 V for charging the cell phone. It was ordered to save us the trouble of making our own and its ability to keep the voltage regulated with a large input voltage range.
2. DESIGN PROCEDURES
The overall concept for mutually inductive coils is an idea from an MIT experiment used to transmit power to power a light bulb. The size of the inductors was increased and the number of turns increased due to ideal equations in hopes of lowering resonant frequency and increasing transmission efficiency. PSPICE simulation was done wherever possible to verify design before actual testing.
2.1 DC source
The DC source was designed with a rectifier and filter circuit. A full bridge rectifier was chosen because they have less ripple than a half bridge rectifier because the frequency is twice as fast. This means the filter has to supply the voltage for only half as long so it has less time to decay. Figure 2 shows the difference in the two rectifiers.
Figure 2: Rectifier Plots
The capacitors were chosen with relatively high values and the design choice was verified in PSPICE with a simulation
2.2 Full-Bridge Inverter and Gate Drivers
The full bridge circuit is a generic circuit. 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.
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. 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. 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.
2.4 Current Sensing
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.
2.5 Coils and Air Gap
The coils were designed using a series of ideal equations. If the coils are treated as windings around a transformer the reluctance can be calculated using equation(1.1).
Using the dimensions given and the relative permeability of air the reluctance is . The reluctance can be used to find the mutual inductance using equation(1.2).
The resonant frequency is given in equation(1.3).
Therefore, in order to reduce the resonant frequency using the mutual inductance the number of turns should be maximized, the area should be maximized, and the distance between the coils should be minimized. The distance between the coils was varied to observe the effects on coupling due to coil distance but would eventually be set around six feet to simulate the distance between a tabletop and ceiling. A value of 100 turns was chosen because it was a large value, but not large enough to start contributing too much unwanted factors from series resistance and winding capacitance. The diameter was set to 1 meter because it is large value but not too unreasonable of a size for a pad on or under a desk. The actual expected frequency can be calculated by finding the capacitance which is given by equation(1.4).
Substituting in values for (1.3) and (1.4) results in a resonant frequency of 395.814 kHz. This is a very rough value because of losses in the air and non-ideal elements in the circuits. The actual measured natural frequency is around 3.4 MHz when measured using a signal generator with amplitude 20 Vp-p.
The transformer was designed based on the turn ratio needed to scale down the voltage and current requirements to prevent magnetic flux saturation of the core. The saturation magnetic flux is given by(1.5).
The core losses were attempted to be minimized using(1.5). The number of turns was kept to a minimum to prevent losses from series resistance in the windings.
2.7 Rectifier and Filter
The rectifier was chosen using a single diode to prevent loss because there is only current flowing through one diode and the frequency is fast enough that the full-wave is not need. The diode chosen had its frequency verified by looking at . The capacitor was picked such that its resonant frequency is above 6 MHz because it will not act like a capacitor above this frequency. The inductance from the connections dominates the impedance, and a smaller capacitor was chosen than in the top filter because the self resonant frequency is higher. Ceramic capacitors tend to have a lower capacitance than electrolytic capacitors, but in this case the frequency is high enough that a lower capacitance is acceptable.
2.8 Buck Converter
The component was selected because it can handle the power necessary for the cell phone, outputs 5 V, and meets the power requirements for the cell phone.
A series inductor and a capacitor to ground will filter the signal output by the buck converter making it a cleaner signal.
3. DESIGN DETAILS
3.1 DC Source
The DC source comprises of a rectifier circuit using 1N1188 diodes and a PI filter.
Figure 3: Wall Voltage to DC Rectifier and Filter (DC Source)
The 1N1188 diodes were chosen because they can carry more than 1 A and can block up to 400 VDC. They were also readily available in the parts shop. The voltage ripple from this circuit is hard to calculate on paper due to the fact that it is a third order filter. The inductor was chosen at a standard part value and verified in PSPICE that it can regulate the current properly.
A PSPICE simulation was run with Dbreak diodes in place of the 1N1188, because there is no PSPICE model available for the 1N1188. The diodes should not noticeably affect the output voltage.
The output signal is connected by switches operating at the speed that the full-bridge inverter is expected to operate at. The purpose of the switches is not to test the full-bridge inverter circuit, but make sure that the output voltage is properly regulated.
This circuit diagram in Figure 4 is what was used to simulate. The switches were used to make the circuit more like the real circuit that would be running in the demo.
As Figure 5 shows, after about 40 ms the voltage is very steady. It has a ripple of less than 1 V. When the circuit in Figure 3 was built, the regulation was not as steady as shown in the simulation of Figure 5 so the capacitance values were changed to 1000 Ã‚ÂµF.
Figure 4: Simulation Circuit for Rectifier and Filter
Figure 5: Simulation Results
3.2 Full-Bridge Inverter/Gate Drivers
This inverter takes in the voltage from the DC source and through using the PIC and gate drivers, outputs signal in the form of a square wave with a frequency that is controlled by the PIC and is adjusted based on induced current in the coil. The gate drivers are ICs that take in the signal from the VCO and output the right amount of voltage to turn on and off the power MOSFETs in the full-bridge inverter.
3.3 PIC Microcontroller, DAC, and VCO
The DAC requirements were that it had 10 bits, parallel inputs, and transparent output. Finding a part that meet these restrictions was difficult.
Figure 6: DAC and VCO Circuit Diagram
The PIC will control the frequency of the signal that is driving the coil. It will also adjust the frequency based on the current that is sensed through the top coil to get the most power transfer through the coils
Figure 7:PIC Pin Out Diagram
3.4 Current Sensor
A very low resistance resistor will be put in series with the coil. The voltage is then measured across it to determine the current through the coil based on the voltage drop and resistance. The extra parts in this circuit are to protect the op-amp. The op-amp was not rated for 150-170 V common mode voltage, but it was found on the datasheet that this circuit would work .
Figure 8: Current Sense Circuit
An inductor made with about 100 turns and a diameter of around 1 m. It will also have a current limiting resistor in series to make sure nothing burns up. An inductor like the top coil that will receive the electromagnetic waves transmitted by the top coil and have a current and voltage induced to power the devices. The inductance of either coil was around 27 mH. This was lower than calculated but still relatively high. Preliminary tests were done on the coils to find their resonant frequency. Multiple frequencies were found, including 3.4 MHz, 6 MHz, and one around 9 MHz. The most resonant of these being the 6 MHz signal, but the 3.4 MHz was chosen for the target frequency, due to the fact that it is easier to find parts for and will work nearly as well.
These frequencies were far from the expected frequency. This could be due to a multitude of factors including skin effect of the 20 AWG wire, imperfections in the windings, incorrect permeability numbers, incorrect estimates of capacitance, and fringing among others. The high frequency made the designs a bit more restricted.
The transformer was not made because we were never able to get a voltage on the bottom coil so it was hard to figure out a turn ratio and what the saturation current would be.
3.7 Half Wave Rectifier
A diode that will take the signal induced in the bottom coil and cut off the negative side of the AC, helping to create a DC signal.
Filter: A series 1mH inductor and a capacitor to ground that will filter the signal output by the rectifier making it a smoother signal.
Figure 11: Circuit Diagram for Transformer, Rectifier, and Filter
3.8 DC/DC Buck Converter
A Buck-Converter will step the 18 V for the laptops down to 5 V for a phone. Initial tests found that the regulation by this buck converter is outstanding, changing 1 mV or less through the recommended input voltage range.
4. COMPONENT USED
The final product is designed to operate off a wall outlet. The only other considerations for cost are placing the bottom coil under the table top. The final design would also need some voltage supplies (5 and Ã‚Â±12).
TABLE 2: ESTIMATED COMPONENT USED
Part Block Quantity
Current Sensing Resistor Current Sensor 1
62 V Zener Diode Current Sensor 1
Operational Amplifier Current Sensor 1
and 2M Resistors 1/4W Current Sensor 1
Current Senor MOSFETS Current Sensor 2
Diodes DC Supply 4
1mH Inductor DC Supply 2
Capacitor 1000uF DC supply 2
40PIN PIC PIC 1
20 MHz Oscillator PIC 1
Voltage Controlled Oscillator DAC/VCO 1
DAC DAC/VCO 1
Gate Driver Gate drivers 2
Full-Bridge Inverter MOSFETs Full-Bridge Inverter 4
700m Magnet Wire (20AWG) Top and bottom coils 1
Scaffolding Wood Top and bottom coils 1
Transformer Transformer 1
Diode Bottom Filter 1
Capacitor 1uF Bottom Filter 1
Resistor 2.7K Buck Converter 1
Buck Converter Buck Converter 1
Capacitors 100uF, .01uF, and 470Uf Buck Converter 1
100uH Inductor Buck Converter 1
Diode Buck Converter 1
Â¢ Proved that power can be transmitted via resonantly coupled coils (theoretically)
Â¢ Multiple resonant frequencies found at many coil spacing
Â¢ PIC able to regulate frequency based on current measured
Â¢ 60 Hz DC wall voltage filtered with a small voltage ripple
Â¢ 3 MHz signal able to be filtered with a small voltage ripple
Â¢ MOSFETs were not able to operate fast enough to drive the coils at resonance.
Â¢ Isolation problems for the inverter.
3. SIXTH INTERNATIONAL SYMPOSIUM NIKOLA TESLA October 18 â€œ 20, 2006, Belgrade, SASA, Serbia
TABLE OF CONTENTS
1.1 Objectives 1
1.2 Specifications 2
1.3 Block Diagram 2
1.4 Subprojects 3
1.4.1 DC Source 3
1.4.2 Full-Bridge Inverter 3
1.4.3 Gate Drivers 3
1.4.4 PIC 4
1.4.5 DAC/VCO 4
1.4.6 Current Sensing 4
1.4.7 Coils and Air Gap 4
1.4.8 Transformer 4
1.4.9 Rectifier/Filter 5
1.4.10 Buck Converter 5
2. DESIGN PROCEDURE 6
2.1 DC Source 6
2.2 Full-Bridge Inverter/Gate Drivers 6
2.3 PIC, DAC, and VCO 7
2.4 Current Sensing 7
2.5 Coils and Air Gap 7
2.6 Transformer 8
2.7 Rectifier and Filter 9
2.8 Buck Converter 9
3. DESIGN DETAILS 10
3.1 DC Source 10
3.2 Full-Bridge Inverter/Gate Drivers 11
3.3 PIC, DAC, and VCO 12
3.4 Current Sensing 13
3.5 Coils and Air Gap 13
3.6 Transformer 14
3.7 Rectifier and Filter 14
3.8 Buck Converter 14
4. COMPONENT USED 15
4.1 Parts 15
5. CONCLUSIONS 16
5.1 Accomplishments 16
5.2 Uncertainties 16