A Three-coil Wireless Battery Charger With Inherent CC-to-CV Transition Capability

With the development of science and technology,a safe and convenient power supply method has attracted more attentions.Inductive power transfer(IPT)technology utilities the principle of electromagnetic induction to transfer energy from transmitting end to the receiving end within a certain distance via high-frequency alternating electromagnetic field,which is efficient and reliable.Compared with traditional wired power transmission,IPT technology effectively reduces interface wear caused by frequent plug-in,and eliminates the use of cumbersome wires.Therefore,IPT technology with advantages of flexibility,convenience,and applicability has found many applications such as consumer electronics,electric vehicle charging,and implantable devices,and so on.IPT systems mostly use storage batteries to support portable loads.With high power density and long service life,Lithium-ion batteries are widely used as energy storage batteries and have a typical charging profile consisting of the first constant current(CC)charging and the subsequent constant voltage(C V)charging.Therefore,it is necessary for IPT systems to efficiently provide the first CC and then CV output required by the battery.A two-stage structure is mostly researched in order to achieve the above goals.The front-end IPT converter transfers power efficiently,and the cascaded converter regulates the required CC or CV ouput.But the two-stage structure is relatively complicated.Some studies have adopted a single-stage battery charging system.It is reported that a single basic compensation topology can not achieve input zero phase angle(ZPA)and load-independent CC and C V outputs simultaneously.Therefore,there are researches using hybrid topologies or higher-order networks to fulfill these objectives.For example,the hybrid toplogy can be achieved by combining the topology with input ZPA and load-independent CC output and the other topology with input ZPA and load-independent CV output.The higher-order topology can switch the operating frequency by control to realize the CC and CV transition as well as input ZPA.The all above schemes greatly rely on precise battery voltage detection,wireless communication and control circuits,and their errors will affect the transmission performances.To address the above issue s,this paper aims to eliminate the complex control and communication and proposes an IPT system with the inherent CC-to-CV transition capability by adding an auxiliary coil,a compensation capacitor,and a clamp bridge rectifier on the primary side.The secondary coil is connected to the devices directly.The three-coil battery charging system uses the common Series-Series(SS)compensation network to first realize the input ZPA and CC output characteristics.When the battery voltage increases to the C V limit,the primary auxiliary coil and its clamp ing circuit work and clamp the output voltage to the CV limit of the battery.At this time,the current of the primary series compensation network is also clamped to a fixed value.This structure always works in the vicinity of the natural resonance frequency point of the series compensation in both CC mode and C V mode,which facilitates the soft switching realization of inverter switches and near zero reactive power in the circuit.In addition,the IPT system can perform automatic open-circuit protection function to avoid over-current failure,when the battery and secondary coil are moved away suddenly.The paper systematically analyzes the operational principle of the charging system for CC-to-CV automatic transition.The parameter design process is also given in detail.Finally,a 48V/2.5A battery charger prototype is built in lab and the experimental results verify the theoretical analysis wel.