Efficient Resonant Converter For On-Borad Chargers Of Electric Vehicles

With the depletion of fossil fuel reserves and the increase in greenhouse gas emissions, the research and development of plug-in hybrid electric vehicles(PHEVs) and electric vehicles(EVs) have been carried intensively. In today’s PHEVs and EVs, an on-board charger is installed to charge the high power lithium-ion battery pack through the utility power, which has been regarded as one of the key components to overcome the range anxiety. This thesis aims to improve the efficiency and power density of DC/DC resonant converter for medium to high power level EV battery chargers.According to a comprehensive topological survey of the currently available charging solutions carried out in Chapter 1, the end-stage resonant converter is the key to improve the overall efficiency of the battery chargers. In Chapter 2, the LLC resonant converter is investigated since its high efficiency and ability to achieve high power density, which fit the demand of EV conductive charger applications. However, unlike traditional resistive load applications, the characteristic of a battery load is nonlinear and highly related to the charging profiles. In this case, the characteristics of the charging profiles are analyzed. The features of an LLC converter are explored based on the fundamental harmonic approximation(FHA). The design considerations are studied thoroughly. The worst-case conditions for primary side zero voltage switching(ZVS) operations are analytically identified when a constant maximum power charging profile is implemented. Then the worst-case operating point is used as the design targeted point to ensure primary ZVS operation globally. To avoid the inaccuracy of fundamental harmonic approximation approach in the below-resonance region, the design constraints are derived based on the waveforms of a specific operation mode. A step-by-step design methodology is proposed and validated through experiments on a 3.3k W prototype.Taking a step forward to avoid the inaccuracy of the FHA, the time-domain model of LLC converter is investigated for an optimal design in Chapter 3. The efficiency oriented design considerations are discussed based on the operation mode analysis of the LLC converter considering the characteristics of charging profiles. The mode boundaries and distribution are obtained from the precise time domain model. Base on the charging profile, the whole charging process is projected to the operation-modes-distribution domain as operating trajectories. The operation modes featuring both-side soft-switching capabilities are identified to design the operating trace of the charging process. Then the design constrains for achieving soft-switching with the load varying from zero up to the maximum are discussed. A charging trajectory design methodology is proposed and validated through experiments on a 6.6k W prototype.Furthermore, the inductive power transfer(IPT) system, which is actually a special category of the resonant converter, has been regarded as a preferable solution since an efficient wireless charger can significantly improve the ease of charging for the end users. To achieve high power and high efficiency, a double-sided LCC compensation network and its tuning method for wireless charger are proposed in Chapter 4. With the proposed method, the resonant frequency of the compensated coils is independent of the coupling coefficient and the load condition. The IPT system can work at a constant frequency, which eases the controller design. Nearly unit power factors can be achieved for both the primary side and the secondary side converters in the whole range of coupling and load conditions. Thus high efficiency for the overall IPT system is easily achieved. A parameter tuning method is also proposed and analyzed to achieve ZVS operation other than zero phase angle(ZPA) switching for the MOSFET-based inverter. Simulation and experimental results from an 8k W prototype verify the analysis and validity of the proposed compensation topology and the tuning method.To improve the compactness of the double-sided LCC compensated IPT system, a novel magnetic coupler structure with a compensation-integrated feature is proposed in Chapter 5. The inductors of the LCC compensation networks are designed as planar-type and attached to the power-transferring main coils. Extra space and magnetic cores for the compensated inductors outside of the coupler are saved. The cost is that extra couplings between the compensated coils(inductors) and the main coils are induced. To validate the feasibility, the proposed coupler is modeled and investigated by the 3D finite-element analysis(FEA) tool. This is followed by the circuit modeling and characteristic analysis of the proposed IPT topology based on the FHA method. A rated power of 5.6k W prototype has been built and tested.At last, the design of a primary side controlled 7k W wireless charging system for electric vehicles is presented in Chapter 6. A systematic design procedure and practical design considerations are discussed for a high efficiency wireless charger. The fundamental design considerations of the bipolar magnetic coupler are introduced. The double-sided LCC compensation circuit is chosen based on its characteristic analysis. The front-stage AC-DC converter consists of a four-phase interleaved boost-type power factor correction(PFC) and four-phase interleaved buck converter for the primary side control. Simulation and experimental results are presented for a prototype with up to 7k W output power for EV charger. The efficiency of the front end PFC stage and the IPT stage at rated power is 97.5% and 96% respectively. A peak efficiency of 93% of the whole charging system from the grid to the battery load is achieved at rated operation condition. Besides, the system efficiency can be maintained above 90% in the load range of 50%-100% rated output power, which satisfies the demand on the high efficiency of EV chargers.