Research On Inductive Power Transfer System With High Efficiency And High Misalignment Tolerance

Wireless power transfer(WPT)is an emerging power supply technology based on the principle of near-field coupling,which uses magnetic field instead of traditional conductor to realize wireless power transfer.In some special applications,such as the tunnels,mines and seabed,cables are vulnerable to corrosion,which can cause explosion and fire in serious cases.However,WPT technology can solve the problems caused by “plug in and out” of traditional wire power transmission,and has the advantages of safety,reliability and flexibility.Generally,the loads of WPT system are battery packs,which have a wide range of voltage variation.It is of great importantance to maintain high system efficiency within a wide load range.In addition,since the relative position of the primary and secondary coils is flexible,it is difficult to avoid coil misalignments in practical applications.Considering the influence of the metal conductor on coil parameters,the WPT system inevitably deviate from the resonance state,resulting in the reduction of system efficiency.Although the use of wireless communication module can detect the output power in real time,the bit error of data transmission will cause the instability issue,even damage the device.Hence,this paper is committed to solve the above-mentioned problems,and pursue high efficiency and high misalignment tolerance.In order to make the WPT system operate in resonant condition,phase shift(PS)control is usually adopted.However,when the phase shift angle becomes large,soft switching of the primary inverter can not achieved,which inevitably results high switching losses.Additionally,large voltage spikes may occur on the power switches.To solve the hard switching problem,variable frequency and phase shift hybrid control method is proposed in the existing literature.However,when the switching frequency deviates from the resonant frequency,the reactive power increases rapidly.Besides,to meet the requirements of soft switching and low reactive power losses,simultaneously,two PI feedback loops are required,which increases the control complexity of the system.As a result,this paper presents a series-sries(SS)compensated WPT system based on auxiliary LCL network,which can realize zero voltage switching(ZVS)in the whole load range.The time domain model is established and the parameter design method is given as well.The experimental results show that the SS compensated WPT system based on auxiliary LCL network operates with unity power factor.Meanwhile,the proposed auxiliary LCL network is not only suitable for SS compensation topology,but also suitable for LCC compensation topology.Generally,the fully tuned SS compensated WPT system has the characteristic of constant current in receiver coil.Since the battery charging requires constant current(CC)and constant voltage(CV)charging mode,it is necessary to design a WPT system with wide output voltage range.In practical,the LCC-S compensation topology is widely adopted in wireless battery charging application because of its constant primary coil current and immunity of the load variation.In this paper,a hybrid control strategy based on phase shifting and switch-controlled capacitor(SCC)is proposed.The critical soft switching condition of primary inverter in LCCS compensated WPT system are analyzed by the time domain model.The phase-shift angle and SCC control angle are linked by a polynomial function.Only one PI feedback control loop is required to realize the regulation of two control variables and therby the control circuit can be simplified significantly.In this paper,the design procedure of circuit parameters and control functions are provided in detail.Experimental results show that the proposed hybrid control strategy can minimize reactive power and improve system efficiency on the premise of ZVS.In addition to the achieve high efficiency,the misalignment tolerance capability is also crucial.Due to the influence of metal conductor on the magnetic field distribution,the selfinductance of primary coil will change when misalignments occur,which makes the system deviate from the resonant condition.To tune the receiver circuit of the IPT system dynamically,an inductor is parallelly connected with a diode full-bridge rectifier cascaded with a buck converter serving as a tuning circuit.By adjusting the duty cycle of the buck converter,the equivalent reactacne of primary coil can be regulated.However,the circuit is quite complex and numerous devices are required,which increases the cost and volume of the system.In this paper,digital controlled SCC is proposed to adjust the equivalent compensation capacitance of primary coil,and the driving signal of SCC can be directly generated by DSP,without additional synchronous signal generation circuits.The experimental results show that the proposed digital controlled SCC can compensate the self inductance variation of primary coil in real-time,and make the WPT system operate with unity power factor and high efficiency.On the other hand,the coil misalignments result in the fluctuation of the output power.In order to ensure stable power transmission,hybrid compensation network is used to provide two power transmission channels.The variation trend of the transmission power of these two channels is opposite.When one channel increases(decreases),the other channel decreases(increases)accordingly.As a result,the total output power can maintain relatively stable in a certain misalignment range.However,a large number of passive components is required,which increases the cost and volume of the system.This paper presents a SS compensated WPT system based on coaxial measuring coil to realize primary side control.The primary side coaxial detection coil can detect the change of output current in real time to ensure the stable output when coil misalignments occur.Therefore,the wireless communication module is not needed.Experimental results show that the proposed coaxial detection coil can accurately estimate the variation of the output current when coil misalignments occur.The stability and anti-interference capability of the WPT system can be effectively improve.