International Journal of Advance Electrical and Electronics Engineering (IJAEEE)
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Wireless Battery Charger (RF/Microwave to DC Conversion)
1Vyjayanthi A S, 2Channabasappa Baligar
1M Tech, 6th Semester VLSI Design and Embedded Systems, VTU Extension Centre, UTL Technologies Bangalore – 22, Karnataka, India.
2Professor, VTU Extension Centre, UTL Technologies Bangalore - 22, Karnataka, India.
Email: 1[email protected], 2[email protected] Abstract: -Because of technological advancements in
electronics it is now possible to charge the portable devices batteries using wireless power transfer technology. This paper deals with wireless battery charging, its current limitations, and exploration on communication possibilities to conserve power. Also, efficiency improvement of wireless power transfer was accomplished with modified receiver architecture.
Wireless power transfer contains transmitter & receiver, the hand held will have the power receiver circuit which charges the battery and communicate the status back to vary the power intensity or charge status. There is a need to use the best matching power frequency to get max power transfer by suitably fine tuning the antenna parameters and getting the maximum efficiency. This paper describes the design of wireless battery charger receiver side module and also modelling of spiral coil and array of coils to achieve maximum & efficient power transfer.
Keywords: Array of coils, Intelligent Battery charging, Power transmitter, power receiver, Spiral coils, Safety mode of charging Wireless power transfer, WPC.
I. INTRODUCTION
The world is moving towards complete wireless, including battery charging for user convenience; even we do not need the power cable too. Wireless power is beginning to show great potential in the consumer market. The ability to power an electronic device without the use of wires provides a convenient solution for the users of portable devices. This technology’s benefits can be seen in the many portable devices, from cell phones to electric cars that normally operate on battery power. Inductive coupling is the method by which efficient and versatile wireless power can be achieved. Power efficiency is a crucial aspect of wireless power transmission. With diminishing resources and the threatening climate change in mind we cannot no longer afford to waste energy, especially for general purpose applications.
However, at low power the efficiency of the system will be low. But compared to a power supply, the result may look different. An additional aspect of saving resources and standby power arises, if one wireless power system replaces several individual supplies.
For ease of use and the benefit of both designers and consumers, the Wireless Power Consortium (WPC) has developed a standard that creates interoperability between the device providing power (power transmitter, charging station) and the device receiving power (power receiver, portable device). A typical application diagram is as shown in figure 1. The WPC standard defines the type of inductive coupling (coil configuration) and the communications protocol to be used for low-power wireless devices [1]. Any device operating under this standard will be able to pair with any other WPC- compliant device. One key benefit to this approach is that it makes use of the coils for communications between the power transmitter and the power receiver.
Figure 1. A typical application block diagram Under the WPC standard, ―low power‖ for wireless transfer means a draw of 0 to 5 W. Systems that fall within the scope of this standard are those that use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver.
The distance between the two coils is typically 5 mm.
Regulation of the output voltage is provided by a global digital control loop where the power receiver communicates with the power transmitter and requests more or less power. Communication is unidirectional from the power receiver to the power transmitter via backscatter modulation [1]. In backscatter modulation, the power-receiver coil is loaded, changing the current draw at the power transmitter. These current changes are monitored and demodulated into the information required for the two devices to work together. The WPC standard defines the three key areas of the system—the
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power transmitter that will supply power, the power receiver that will use the power and the communications protocol between the two devices. These three areas are explored in next sections.
To arrive at the specification for the transmitter and receiver and for greater understanding of this new emerging technology the WPC standard has been referred and important specifications are summarised in the following sections
A. Power Transmitter
The direction of power transfer is always from the power transmitter to the power receiver. The key circuits of the power transmitter are the primary coil, used to transfer power to the power-receiver coil; the control unit for driving the primary coil; and the communications circuit for demodulating the voltage or current from the primary coil. Flexibility of the power- transmitter design is limited to provide consistent power and voltage levels to the power receiver. The power receiver identifies itself to the power transmitter as a compliant device and also provides configuration information. Once the transmitter initiates power transfer, the power receiver sends error packets to the power transmitter requesting more or less power. The power transmitter stops supplying power upon receiving an ―End Power‖ message or if no packets is received for more than 1.25 seconds. While no power is being transmitted, the power transmitter enters a low-power standby mode. The power transmitter, typically a flat surface upon which the user places the power receiver, is connected to the power source. The coils of a WPC- compliant device operate as a resonant half bridge on a 50% duty cycle, with a 19-V DC input (±1 V). If more or less power is needed at the power receiver, the frequency in the coil changes but stays between 110 and 205 kHz, depending on power demands.
B. Power Receiver
The power receiver is typically a portable device. The key circuits of the power receiver are the secondary coil, used to receive power from the power-transmitter coil;
the rectification circuit, used to convert AC to DC; the power conditioning circuit, which buffers the unregulated DC into regulated DC; and the communications circuit, which modulates the signal to the secondary coil. The power receiver is responsible for all communications of its authentication and power requirements, as the power transmitter is only a
―listener.‖ While design of the power transmitter is restricted to keep it WPC-compliant, much more freedom is permitted for designing the power receiver.
The coil voltage at the power receiver is full-wave rectified, with a typical efficiency of 70% for a 5-V, 500-mA output. Because communication between the two devices is unidirectional, the WPC selected the power receiver to be the ―talker.‖ Inductive power transfer works by coupling a magnetic field from primary to secondary coils. Uncoupled field lines rotate
around the primary coil and do not represent loss as long as the field lines don’t couple a parasitic load (for example, eddy-current loss in metal).
C. Communications protocol
The communications protocol includes analog and digital pinging; identification and configuration; and power transfer. A typical start-up sequence that occurs when a power receiver is placed on a power transmitter proceeds as follows:
a. An analog ping from the power transmitter detects the presence of an object.
b. A digital ping from the power transmitter is a longer version of the analog ping and gives the power receiver time to reply with a signal-strength packet. If the signal strength packet is valid, the power transmitter keeps power on the coil and proceeds to the next phase.
c. During the identification and configuration phase, the power receiver sends packets that identify it and that provide configuration and setup information to the power transmitter.
d. In the power-transfer phase, the power receiver sends control error packets to the power transmitter to increase or decrease the power supply. These packets are sent approximately every 250 ms during normal operation or every 32 ms during large signal changes.
Also during normal operation, the power transmitter sends power packets every 5 seconds.
e. To end the power transfer, the power receiver sends an ―End Power‖ message or sends no communications for 1.25 seconds. Either of these events places the power transmitter in a low-power state [1].
II. MODELING OF SWITCH MODE POWER SUPPLY FOR EFFICINECY
IMPROVEMENT
Modern day smart phones, tablets and other consumer devices are being designed with USB ports for charging and other communication. Switch-mode topology is ideally suited for fast charging from USB ports. The idea is to modify the receiver side circuitry to have the switch mode charging for mobile devices for faster charging. For switch mode charging we make use of Switch Mode Regulator (Switcher), which uses pulse width modulation to control the voltage, Low power dissipation over wide variations in input and battery voltage. It is more efficient than linear regulators but more complex. It needs a large passive LC (inductor and capacitor) output filter to smooth the pulsed waveform [2]. Component size depends on current handling capacity but can be reduced by using a higher switching frequency, typically 50 kHz to 500 kHz, Since the size of the required inductors and capacitors is inversely proportional to the operating frequency [3] and also it is the same frequency range at which the wireless charging evaluation kits are being designed to operate as per the wireless power consortium standards. However,
switching heavy currents gives rise to EMI and electrical noise. This has to be carefully taken care of by better design parameters. All of these factors have contributed to the need for the development of an efficient wireless battery charging receiver side module system.
In the 1960s and early 1970s, power supplies were linear designs with efficiencies in the range of 30% to 50%.
With the introduction of switching techniques in the 1980s, this rose to 60% to 80%. In the mid-1980s, power densities were about 50 W/in3. With the introduction of resonant converter techniques in the 1990s, this was increased to 100 W/in3 [4]. When high speed and power hungry processors were introduced during the mid- 1990s, much attention was focused on transient response, and industry trends were to mix linear and switching systems to obtain the best of both worlds.
Low dropout (LDO) regulators were introduced to power noise-sensitive and fast transient loads in many portable products. In the late 1990s, power management and digital control concepts and many advanced approaches were introduced into the power supply and overall power management.
A basic topology of the switch mode power supply (SMPS) for wireless power receiver system has been modelled using Liner Technology Corporation’s LT SPICE IV, circuit simulation software and the simulated circuit is shown in figure 2. A highly efficient DC-DC buck boost converter, LTC 3789 is utilised for design of SMPS in this work.
Figure 2. SMPS circuit schematic
The dual resonant circuit shall have the following resonant frequencies:
………… (1) … (2)
Each capacitor can then be calculated using Equations 3& 4:
………..(3) ……… (4)
Where fS is 100 kHz +5/-10% and fD is 1 MHz ±10%.
C1 must be chosen first prior to calculating C2.
The quality factor must be greater than 77 and can be determined by Equation 5:
………. .. (5)
Where R is the DC resistance of the receiver coil. All other constants are defined above.
Where:
1. VIN is a square-wave power source that should have a peak-to-peak operation of 15-19V.
2. CP is the primary series resonant capacitor (i.e. 100 nF ).
3. LP is the primary coil of interest.
4. LS is the secondary coil of interest.
5. CS is the series resonant capacitor chosen for the receiver coil.
6. CD is the parallel resonant capacitor chosen for the receiver coil.
7. CB is the bulk capacitor of the diode bridge (voltage rating should be at least 25 V and capacitance value of at least 10μ F)
The diode bridge is constructed of Schottky diodes.
The simulated output voltage, current and power waveforms are plotted as shown in figures 3 & 4.
Figure 3. Output Voltage & current waveform
Figure 4. Output power waveform
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III. MODELING OF SPIRAL COIL
The range of wireless power is mainly function of coil dimensions and the core shape. The reliable power transfer is usually facilitated when distance between coils doesn't exceed 1/4...1/2 of the coil diameter for flat coils [5].
Planar spirals over a highly scalable geometry are appropriate for wireless power transfer via strongly coupled inductive resonators [6]. We analyse a simple model to identify design considerations for a specific material & dimensions. Two aligned spiral model simulated using the HFSS design environment is as depicted in figure 5 and top-down view is as shown in figure 6.
Two coils were modelled to test the coupling efficiency and design optimization. Device dimensions were do = 55 mm; n = 3; w = 4.5 mm: and di=do = 0.35 (considered optimal). Coil#1 (transmitter) & coil#2 (receiver) were modelled on a 5 mm: thick Rogers 4350B substrate (tan _ = 0:0037). The devices were provided with excitation ports at the entry point and excited.
The primary coil acts as the transmitter and is excited by the source. The transmitter coil is resonated at the operating frequency of 110-210 KHz. The loosely coupled secondary coil acts as the receiver. The receiver output is fed into the integrated circuitry which optimizes the power to the battery of charging device.
The filed plot of energy transfer between transmitter and receiver is as shown in figure 7.
Figure 5. Spiral model
Figure 6. Top-down view (substrate layer hidden)
Figure7. Field plot of energy transfer between Spiral coils using HFSS
When we have analysed the simulated data while simulating the simple planar spiral coils for different dimensions, and found that, the substrate material, thickness, coil geometry, coil material, spacing between coils and quality factor does plays a crucial role in optimizing the efficiency of wireless power transfer between the spiral models.
The strong coupling is concentrated at the centre of the coils as depicted in the field plot diagram shown in figure 8. The lumped elements for coils are extracted by using s-parameter values as shown in figures 8, 9 and 10 in order to evaluate simulated inductive power between the coils. However, the tool is not supporting resonant frequency range of 110 kHz-210 kHz. To obtain simulation results, the operating frequency range is considered in terms of GHz.
For this simulation, strong coupling is achieved for distances of the order ~2 do [6].
The radiation pattern, s-parameter polar plot and the field energy plot are given below in the following figures for the simulated planar coils. In the radiation pattern we can observe that there is strong radiation on lateral side of the model.
Figure 8. Radiation pattern between Spiral coils (aligned)
Figure 9.S-parameter magnitude
Figure 10. S-parameter polar plot
Figure 10. S-parameter magnitude in dB Variation of electric field intensity around the transmitter and receiver was also simulated in this work.
In most of the cases, the transmitter and receiver orientation plays a vital role in the wireless power transmission. The maximum power coupling between the transmitter and receiver is achieved under no case of misalignment [7]. Significant coupling under resonance is efficient considering proper orientation of transmitter and receiver coils. The orientation of the receiver coil with respect to the transmitter coil is simulated as given in figure 11.
Figure 11. Coil modelling (misaligned coils)
Figure 12. Field-intensity Vs time
The graph as in figure 12 gives the comparison of electric field intensity between the transmitter and receiver coils with and without the misalignment. We can clearly observe that the intensity is high for coil coupled without any misalignment. The orientations of the coils are vital for the maximum energy to be coupled by the receiver [7]. The misalignment of modelled coils gives us the idea about electric field intensity distribution around the receiver coil. It can be inferred from the graph that the field intensity is maximum only under the case of proper orientation.
IV. MODELING OF ARRAY OF SPIRAL COILS
While high power transfer efficiency is critical for low power systems, area-constrained systems can require larger power transfer through smaller area coils at an acceptable loss in efficiency. With a fixed distance
between two coils, larger coils result in larger k and higher efficiency. However, using larger coils requires more silicon area, and it ultimately decreases the power transfer density. Therefore, a parallel power transfer scheme can be taken into consideration in order to increase power density and maximize the amount of power delivery through the same area, as illustrated in figure 13. The radiation pattern of 3 X 3 array coil with single and two coils excited are as shown in figures 14 and 15.
Figure 13.Simulation setup for parallel inductive spiral coils: 3x3 array of 50mmX50mm coils
Figure 14. Radiation pattern for 3X3 array of coils with single coil excited.
Figure 15. Radiation pattern of two coils excited.
The modelling and simulation of power transmitter and receiver coils has been carried out using trial/student version of the simulation tool. The tool works better at higher frequency range. At resonant frequency range of 140 khz, the tool was taking more than 72 hours to complete the simulation for 3x3 array. As a result, more accurate representation and evaluation of the wireless power transfer coils /structure is not fruitful.
V. HARDWARE CONFIGURATION OF THE PROPOSED WIRLESS BATTERY CHARGING RECEIVER SIDE MODULE.
The objective is to provide a small, general wireless battery charging receiver system utilizing switch mode technology. Ideally, the wireless charging receiver system will be able to charge mobile devices under voltage variation conditions, and also ensures faster charging with safety features.
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Figure 16 illustrates the concept of modified receiver circuitry with an efficient switching regulator and other functional units.
Figure 16. Configuration of proposed wireless battery charging receiver side module block diagram.
In the modified receiver architecture, the power inductively coupled to receiver coil is fed to tuning circuit followed by rectification full wave bridge and detection circuit. Voltage regulation, smoothing is performed by the highly efficient DC-DC regulator. In order to charge the battery, a charge controller IC is utilized. USB host connector is provided as a means to connect mobile/smart phones for charging purpose. MSP 430 launch pad integrated with 16 X 2 LCD display is interfaced externally & manually for monitoring and display of battery charging status.
There are several basic steps involved in producing hardware (PCB). Most designs begin with a hand drawn schematic and design plan. With these, the circuit is prototyped and tested to verify that the design works correctly. Then, using software, an electronic version of the schematic is created. A net list file is created from
the electronic schematic and used in other software to create the physical layout of the PCB. Next, the components are placed and routed in the physical layout software and Gerber files are created. These Gerber files are used in a prototyping system to mill, drill, and cut the PCB substrate. The components are then placed and soldered to the substrate. Finally, the board is tested to verify that it works as expected.
Then, with the PCB still undowered, we have used a multi meter to verify the correct pin connections through the traces. When all verification seems well, then DC supply is applied with testing and troubleshooting to follow as necessary. All repairs such as soldering or de- soldering of components, etc to the board were conducted by Lab technician only, as it is very easy to damage the PCB when soldering or conducting other repairs. Finally a fully functional wireless battery charging receiver side prototype as shown in figure 17 is ready for conducting tests for efficiency improvements as aimed in this paper.
Figure 17. Wireless Battery charging PCB prototype
VI. SOFTWARE DESIGN
In this work software activity is limited to monitor the battery charging process and communication of status on charging or completed charging to output device. In this work we have chosen 16 character X 2 lines LCD display for displaying the charging status instead of hyper terminal.
For smart work like controlling of battery charging and status updates MSP430 low power microcontroller of G2553 series has been chosen. Texas Instrument’s single-chip digital base band microcontroller MSP430 family was designed specifically for low-power embedded systems. The customizable platforms help to achieve a lower component count, save board space, and reduce power consumption. MSP430 microcontroller is a good fit for Li-Ion battery charging solution because of integrated peripherals.
LaunchPad has an integrated DIP target socket that supports up to 20 pins, allowing MSP430 Value Line devices to be dropped into the LaunchPad board. Also,
an on-board flash emulation tool allows direct interface to a PC for easy programming, debugging, and evaluation.
IAR embedded work bench, which is an integrated development environment (IDE) has been utilized for coding and compiling. IAR kick start, a code-limited version, which is available for download is utilized. This IDE will run full-featured on the available MSP430 Value Line devices, as this device will not encounter the 4kB size limit of IAR, or the 16kB size limit of CCS.
Figure 18. LCD 16X2 Character display flow diagram Software activity is usually started with a flow chart which depicts the flow of the programand also eases the coding of the software. Flow chart for monitoring of Charging status and display of charging status for the wireless battery charger is as shown in figure 18.
Once the code is written, it is flashed in the MSP430 launch pad and programme is complied and run in the IAR embedded work bench. The LCD screen has started
displaying the message of ―Charging ―of the load/mobile connected to the prototype module using USB connector and the LCD display is depicted as shown in figure 19.
Figure 19. 16X2 LCD display unit indicating charging status
VII. TESTS & RESULTS
The wireless battery charging receiver side module/prototype functionality testing is done as described in the procedure below [8-9]. The list of required hardware and software for testing the wireless battery charging prototype is as given below.
1. Tools required for testing
The setup tools used to test the wireless battery charging prototype designed and fabricated in this work are as listed below.
Equipment
1) bqTESLA Transmitter
Power for the wireless battery charging prototype receiver is supplied through a Texas instruments bqTESLA transmitter bq500110 EVM-688/9 EVM or WPC-certified transmitter. The input ac voltage is applied to the receiver through the coil located in the receiver bottom.
2) Voltage source
Input power supply to the bqTESLA transmitter is typically 19 Vdc ±200 mV at 500 mA maximum is applied using programmable power supply of Gwinstek PST-3202 make. It is a three channel highly accurate DC power supply. To simulate an external adapter, an additional channel programmed for 5 V at the 1-A is used.
3) Meters
Output voltage can be monitored at TP7 with a voltmeter. Input current into the load must be monitored with an appropriate ammeter. Transmitter input current and voltage can be monitored also but the meter must use averaging function for reducing error due to communications packets.
4) Loads
A single load is required for 5 V with a maximum current of 1 A. The load can be resistive or electronic.
A 100 ohm rheostat is used.
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5) Oscilloscope
A multichannel Tektronix make TPS 2014 model, 100 MHz Digital storage oscilloscope with appropriate probes is used to observe the RECT voltage and other signals.
2. Equipment Setup
1. With the power supply off, connect supply to the bqTESLA™ transmitter.
2. Place the Wireless Battery charging prototype receiver on the transmitter.
3. Connect load to J201, monitor current through load with ammeter,
4. Typical output voltage is 5 V, and the output current range is 0 mA to 1 A.
3. Equipment test Setup
The diagram of Figure 20 shows the equipment test. At no load the system is drawing an input current of 82 ma as shown in figure 21.
Figure 20.Equipment test setup
The following test is conducted to validate the wireless battery charging technology using the above mentioned test setup.
4. Load Step
The procedure for load step is as follows:
1. The test bench is setup as described in Step 2 and is as shown in figure 23.
2. The TX is powered with 19 V DC supply.
3. The Rx is placed and properly aligned on the transmitter.
4. Provided a load step from no load (high impedance) to 5 Ω or 1000 mA , 10 Ω or 500 mA, 20 Ω or 250 mA and 50 Ω or 100 mA (if using a current source load), etc.
5. Monitored the load current, rectifier voltage, and output voltage.
Figure 21. Test setup with no load, input current is 82 ma
5. System Efficiency
The efficiency is measured from input of TX EVM, HPA688 to output of wireless battery charger prototype receiver EVM, with Vin = 19-V input. Due to the communication packet that occurs at an approximate 250-ms rate, averaging of input current and voltage is required for good accuracy [9]. The efficiency calculation for wireless battery charging receiver side module is given in table 1 below.
Table1: Wireless battery charging receiver side efficiency calculation
The graph in the figure 22 shows the plot of efficiency v/s output power
Figure 22. Wireless battery charger receiver module efficiency v/s output power
Note: For measuring receiver efficiency alone, we have considered 100% power available at the Tx coil.
6. Tests Results
The design improvement is done for efficiency of 1 amp.
But we could test only upto 100mA of load current.
Efficiency is comparable to reference design of Ti’s evaluation module. This module has been designed to get better efficiency at or >1 amp load current and an output voltage of 5 V. We could show the efficiency improvements with 100 mA of load current since the receiver side power management wireless IC is pre- programmed for 100 mA of output current. And the programming of wireless IC is not allowed to User, we could achieve only up to 100 mA. With this result we could charge the mobile battery as it can be connected to the USB connector provided in the design module without any hindrance. Once the mobile is connected the charging indication is displayed on the LCD display unit integrated with module and also in the connected mobile device. The entire test setup and results output are indicated in the following figures 23 and 24.
Figure 23. Wireless Battery charger receiver side module with test setup & results output
Figure 24. Wireless battery charger receiver side module charging a mobile
The MSP 430 launch pad interfaced with LCD display unit is connected to PC through USB cable. Using IAR
embedded work bench with spy bi-wire setting we have loaded code for monitoring the charge status and display of charging status on LCD in the MSP430 launch pad.
Once the charging is started it will be indicated on LCD and when charging is complete same will be indicated on LCD screen and the LEDs provided near the battery charger IC will be off. Thus, power transfer from transmitter to receiver coil is suspended and undue power dissipation is avoided and energy is conserved.
The active wireless power communication scheme, end of power transfer and modulation of received data (power) and controlling of power transfer are performed by the wireless power transfer monitoring IC bq5101x . This wireless battery charging technology could be used to power a battery possibly on or embedded in the rotor shaft or in any hermetically sealed enclosure with improved efficiency and very lesser power loss and thus ensuing a hassle free battery charging. And also it can be utilized for charging of cars and other battery operated vehicles, Global positioning Devices, Smart phones, Digital cameras, MP3 players etc.
VIII. CONCLUSION
With the reference design module, wireless power transfer efficiency is measured and then carried out the design and development of wireless battery charger receiver side module. This module transfers power wirelessly using near field resonant inductive coupling.
In order to increase the absolute power transfer amount, power density is critical & it depends on the distance &
coil technology.
Wireless power transfer and hence charging of Li-ion battery /mobile device has been accomplished in this work. It is an efficient and power conserving topology and thus energy saving is ensured through intelligent and safe mode charging, which is most sought after the ever increasing power hungry portable device development technology. The wireless battery charging receiver module has been tested successfully for its functionality using Texas Instrument’s transmitter evaluation kit. These features show that though it has been developed primarily for application in the portable device segment, it can be used for any application that requires fast, safe, hassle free and efficient charging.
The system is reliable and works without wireless power transfer failure and maximum power transfer efficiency of 27% is achieved in this work. Overall efficiency of wireless power transfer system is realized for an output current of 100 mA, because of the battery charger and receiver side power monitoring ICs pre-programmed for an output of 100 mA and that is what could be tested.
Under this limitation performance test is conducted and results are as tabulated in table 1, and also at low power the efficiency of the system will be low. If we can establish data communication with the transmitter and request for more power & get the power from transmitter then probably we can get the designed power and calculate the designed efficiency to get the feel.
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REFERENCES
[1] Wireless power specification, Part 1; version 1.0.3, September 2011.
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[3] Abraham I. Pressman, Keith Billings & Taylor Morey, ―Switching power supply design‖ third edition, Mc Graw Hill 2009.
[4] Nihal Kularatna, ―Electronic Circuit Design, from concept to Implementation‖, CRC Press, 2008.
[5] An introduction to the wireless power consortium standard and TI’s compliant solutions, 1Q 2011 Analog Applications Journal.
[6] Avraham Kleina, Nadav Katz, ― Strong coupling Optimization with planar spiral resonators ,
Racah Institute, Hebrew University of Jerusalem, Givat Ram, Israel, arXiv:1111.5271v1 [physics.
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[7] R.Selvakumaran, W. Liu, B.H Soong, Luo Ming and S.Y Loon, School of Electrical & Electronics Engineering, Nanyang Technological University, Singapore 639798, ―Design of Inductive Coil for Wireless Power Transfer‖, 978-1-4244-2853-3, PP.584-589, IEEE, September 2009.
[8] Data sheets of Qi compliant Wireless power kit- Texas Instruments SLUSAEOA-November 2010- Revised April-2011.
[9] Data sheets of integrated wireless power receiver, Qi compliant -Texas Instruments SLUSAY6- March 2012.
