Designing a wireless charger for electrical vehicles

Wireless energy transfer (WET) has become popularized recently and it is being planned to be used by plenty of technologic devices such as electrical cars, medical applications, laptops, mobile phones, even sensors and so on. The main causes of lately intense interest of WET are the power electronics technology that is almost sufficiently advanced and the sharp increase in the usage of the electrical devices variously powered and sized, which have battery, in the daily life and industry. The main purposes of WET researchers are transferring as much power as possible, taking into account high system efficiency in spite of low mutual inductance between coils, human and animal health. It is obviously accepted that the WET systems can not be replaced with plug-in systems, unless these purposes are carried out. The main purpose of this project is that designing and developing a wireless energy transfer system that can charge wirelessly the battery of an electrical vehicle by using magnetic coupling method and control the system efficiency adaptively. The number of connectivity standards is increasing day by day and the usage of connectivity has been becoming complicated, as a solution, high efficient wireless charge device which can prevent from danger revealing due to wired charger, also increases the mobility of electrical cars and provides some significant simplicity for users, as well. It can be basicly said that the magnetic coupled method principle and the transformer working principle are the same. The current flowing through the first coil creates magnetic fields, and then this magnetic field induces a voltage in the second coil because of faraday's law. As a result, the time - varying current is necessity to accomplish coil-to-coil wireless energy transfer. In resonance frequency, the magnetic flux density of the transmitted energy has been increased via the total reactance of magnetic circuit has been eliminated. Moreover, high frequency contributes to more power transfer. Last of all, resonance power converter is very convenient in order to fulfill these needs. In addition, the stress on the switching element has been decreased by designing the full bridge series resonance power electronic circuit. The equivalent circuit of WET system can be found in the literature. Nevertheless, the components are assumed to be ideally. In this dissertation, the efficiency expression is derived assuming all the components are non-ideally for series resonance converter and parallel resonance converter. The grid voltage is rectified by using KBFC3510 full bridge rectifier at which voltage and current rate is 700 V 35 A. After rectifying, 1000 V 1 mF capacitor is used to absorb voltage ripple from the rectified voltage. Four IXFH50N50P N-channel MOSFETs are used to invert dc voltage. Almost sinusoidal current is obtained by switching MOSFETs in the resonance frequency. The gate signals are produced by TMS320F28335 ezDSP development kit. However, the gate signal produced by TMS320F28335 does not have enough voltage and power, so that its power and voltage are increased by using FOD 3180 optocoupler. These all components compose the power resonance converter. The PCB of this converter is drawn using ALTIUM DESIGNER. The power is wirelessly transferred from primary coil to secondary coil. The almost sinusoidal voltage induced in the secondary coil is rectified by using high frequency full bridge rectifier, the diodes of rectifier is chosen VFT4045BP Schootky diode since the voltage drop of selected Schootky diode is 0.4 V. After the high frequency rectifier, 63 V 3300 μF capacitor is used to absorb voltage ripple. The mutual inductance between coils is crucial in order to get higher efficiency. There are some iterative methods that provide to find analytically the mutual inductance between coils. These methods are named respectively, Sidhu method, Neumann method and Approximated Neumann method. However, when the coils are not aligned each other, the mutual inductance between coils can be found easily. Different method must be applied to find the mutual inductance between misaligned coils. This special method is called as a Grover method. Then, the matlab codes are written for all methods and the codes are added to the Appendix. The coupling factor is a value of how the coils is connected each other. The coupling factor is ratio of mutual inductance value and coil's inductance value. The coupling factor serves the two purposes. The first one is that the higher coupling factor causes the higher efficiency. The second one is about operating frequency. In the wireless energy transfer system, there are two optimum frequency. In these frequencies, the system work most efficiently. These frequencies depends on resonance frequency and coupling factor. The issue of the designing coil being extremely important for the project has been intensified. The quality factor is the most important factor for designing coil. Calculating inductance value with small error is crucial for defining the quality factor of a coil. There are a lot of methods based on iteration or approximated formulas. These methods can be count respectively Sidhu's method, Lundin's method and Wheeler's method. The Matlab codes are written for all methods and the codes are added to the Appendix. Skin and proximity effects inevitably increase the self-resistance of coils which leads to the lower quality factor. Skin effect results in that current density in surface of wire is more than at the center of cable. Moreover, proximity effect causes that flowing current induces another current flow in a near wire magnetically and this distributed and induced current produces irregular current density. For this reason, the study presents some limits such as the critical height and critical winding pitch of coils. These limits lead to decrease in proximity effect on resistance of coil in high frequency. The solenoid coil having specially structural features provides high efficient system taking into account skin effect and proximity effect. The solenoid bobbin's radius is 35 cm and the height of the solenoid bobbin is 3.5 cm. A spiral coil's inductance value is same as solenoid coil. The spiral bobbin is also designed and is wounded to compare the solenoid coil. Ansys MAXWELL program is used for the best coil design. The system resonance frequency is changed by the mutual inductance between the primary coil and the secondary coil. As a result, the resonance frequency varies with parking status of electrical vehicle. The used controller maximizes the efficiency in accordance with parking style. Theoretical point of this view is based on measuring the dc current and the dc voltage of the secondary side, then calculating the power to the load, after that this power data is sent via bluetooth to the primary side controller, DSP. In addition, the dc current and the dc voltage of the primary side are measured and the input power is calculated. Then the efficiency of total system is calculated in the DSP. If the efficiency is lower than critical value, the system closes oneself. HC05 bluetooth module and HCPL 7520 optocoupler are used to accomplish control of system. Finally, state space analysis of wireless energy transfer system is done, the all components are assumed non ideally and the transfer function of wireless energy transfer with nonideally components is proposed. The test of designed circuit, coils and control algorithm are performed. The maximum efficiency of total system is 30 percent. According to experimental results, the system with the designed solenoid coils is at least 7% higher efficiency than the system with the spiral coils.