| Nowadays, with the serious environmental problem and energy crisis, Electric Vehicle(EV) is given much attention for its environmentally friendly energy saving characters. The power resouce of EV is generated from the power battery carried on the vehicle. As a result, the safety, reliability and efficiency of the charging techology are three of the key points of EV. On-board EV charger is the most popular charging method, the research topics of the on board EV charger are high power density, high power efficiency and intelligent charging.With the respect of the research topics, the overall system composition of on-board EV charger is presented. Based on the design index, half bridge LLC resonant converter is selected. This paper gives a detailed discussion of the precise model of the LLC converter by considering the leakage inductance. The equivalent circuit based on fundamental harmonic analysis is put forward and the influence on resonant parameter and equivalent turn ratio are also discussed under this model. In addition, this paper discussed the impact of imbalanced leakage inductance which is usual in center tapped transformer. The impacts of the converter include the change in voltage gain, the differences on resonant frequency and the soft switching character.In order to minimize the imbalance of the leakage inductance, this paper derived an analytical model of the leakage inductance. The cause of the imbalanced leakage inductance which is due to the different coil distribution is revealed by calculating the leakage inductance within the bobbin. As a result, this paper proposed a symmetric winding method which could realize good balance performance, the practicability of this winding method is verified by simulation and experimental results. In order to improve power density, this paper proposed an integrated LLC transformer. The transformer could adjust the leakage inductance in order to meet the design requirement of the resonant tank. The adjust character is verified by simulation and experimental results. In addition, this paper also proposed an asymmetric control method to compensate the imbalance secondary current casued by imbalance leakage inductance.In order to achieve high efficiency and high intelligent charging, this paper selects digital signal processor(DSP) to realize digital control. Based on DSP controller, the resonant converter could operate in either constant voltage or constant current or constant power stage. This paper also discussed the peripheral circuits design such as drive circuit, sample circuit and protect circuit. In addition, the flow chart of the software is also presented.A 2000 W on-board EV charger is invented with a 180~260Vac input and 54~72Vdc output. The detail design principle is presented. By using the integrated transformer, the optimized system has decreased the weight of magnetic elements by 30%. Due to the removal of the resonant inductor, the overall productive efficiency can be increase. The on-board EV charger using the proposed integrated transformer could realize a 94.4% overall efficiency. The imbalance of the leakage inductance could be less than 2%, the value of the inductance could be adjusted in a certain range. In addition, the asymmetri control method has been verified by experimental results. Finally, the whole charging process is demonstrated by the charging curve, the theoretical power loss analysis of the EV charger is agreed with the measured power loss. The experimental results of the EV charger prove the effectiveness of the proposed idea and design. The 2000 W EV charger has passed the project accenptance. And it has been made on mass production which makes good economical benefits. |
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