| Inductive power transfer(IPT)technology transfers power from the source to the load without mechanical contact,which avoids the potential risk of electric shock.Therefore,both low-power applications such as mobile phones and wireless sensor networks and high-power applications such as fast chargers for electrical vehicle(EV),high-speed trains and marine applications,are gaining more and more attentions.Modulazed IPT systems can work under different number of modules,which increases the expansibility and realiability of IPT systems.Besides,this technology decrease the cost of exploration and production,and it is good for the industrialization development.However,the impedances among multiple modules are coupling in the current parallel topologies.When the number of working modules varies,the impedances of remaining modules change,which means that the stresses of components in the remaining modules may exceed the rated value.Besides,the coils with multiple windings connected in parallel is suitable for modulazed IPT systems.However,the current distribution of multiple windings is uneven,which decreases the reliability of IPT systems as well.Thus,how to increase the power capacity and achieve high-efficiency and high-reliability operation of modulazed IPT systems are the major issues to be addressed in this paper.(1)The impedance coupling among multiple modules is the main reason that IPT systems cannot switch modules as needed.Aiming at this problem,the impedance-coupling characteristics of the traditional parallel topology are analyzed,and the existent shortcomings are clearly pointed out.Then a principle of topology construction featured with impedance decoupling is proposed and the impedance-decoupling characteristic among multiple modules are studied.Besides,the influences of parameters derivation to current unbalance among modules are analyzed.Finally,the impedance-coupling characteristic of the traditional parallel topology and the impedance-decoupling characteristic of the proposed topology are verified by experiments.After adopting the proposed method,the current imbalance factor is only 1.6%compared with 70.4% adopting the traditional method.(2)The structure of multiple litz wires connected in parallel is a feasibale method of highpower coils in modulazed IPT systems.However,the slight paraleter derivation among bundles will lead to the severe current unbalance problem.To solve this problem,the current-unbalance characteristics among multiple litz wires with the traditional compensation method are analyzed in this paper,which reveals the severity of current unbalance even if only a little parameter inequality occurs.Besides,a current-sharing compensation method is proposed.By connecting a specially designed capacitor in each litz wire to increase the impedance of litz-wire branch,the current balance among multiple litz wires can be achieved without changing the resonant state of IPT system or increasing the volume and cost of IPT systems.Finally,the currentunbalance characteristic of the traditional coil compensation topology and the current-balance characteristic of the proposed method are verified by experiments.Under the rated power condition,with the traditional coil compensation method,the current ratio of two windings in primary and secondary coils are 1.60 and 3.11 respectively.After adoting the proposed method,the current ratio of two windings in primary and secondary coils are 1.01 and 1.03 respectively,which means the currents are equal in two winding connected in parallel.(3)When the power level of IPT systems is low,the single module operation is a feasible choice.The multiple resonant component network inductor-capacitor-capacitor-series(LCC-S)topology that is commonly used due to the constant current characteristic in primary coil.The familiar variable frequency phase shift(VFPS)control strategy cannot be used directly due to nonnegligible harmonics,which results in detection difficulties or imprecise calculation of turnoff current.Thus,to solve this problem,the harmonic-considered universal time-domain model of LCC-S topology is proposed for the first time,and the closed-form expression of MOSFET current is also derived,which can be used to obtain the key turn-OFF current accurately and is suitable for both variable width control and variable frequency control.With the calculated turn-off current,the optimal VFPS control strategy applied on LCC-S IPT system is proposed,with which the Zero Voltage Switching(ZVS)operation can be realized within wide power range.In addition,the frequency deviation(between the switching frequency and resonant frequency)is minimum,which ensures high efficiency of the inverter and the resonant tank.This strategy is universal in LCC-S IPT system.Finally,the validity of the proposed method is verified by experiments.Compared with PS control and the VFPS control based on fundamental-harmonic-approximation method,the systems realizes ZVS operation under wide power range with efficiency increase of 2.6%-6.9% and 0.8%-8.8% respectively.(4)When the power level of IPT systems is high,the multiple modules operation is a feasible choice.However,the current control schemes cannot realize the efficiency match,ZVS operation and unchanged resonant state at the same time.To address this problem,the parallel topology proposed in this paper,which has the advantage of modules switching,is taken as the basic topology.Then the efficiency match condition of the impedance-decouple parallel topology is analyzed and the relation between efficiency and the number of modules is discussed.Based on this relation,an efficiency-match control by switching modules is proposed.This method can achieve high-efficiency operation without complex variable pulse width control and variable frequency control,and the ZVS operation remains unchanged with wide power range.In addition,the process of module switching is designed to guaretee the smooth switching of modules.Finally,the validity of the proposed method is verified by experiments.When the power range is [1 k W,12 k W],the corresponding efficiency range is [92.43%,95.36%]. |
