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A Study on High Efficiency Two-stage Battery Charging Systems over a Wide Range of Output Voltage

A Study on High Efficiency Two-stage Battery Charging Systems over a Wide Range of Output Voltage
Yeong-Jun Choi
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In recent years, batteries have played an important role as the demand for energy storage systems and electric transportation has increased. Therefore, the researches of the battery charging system including the front-end power factor correction (PFC) converters and back-end battery-charging DC-DC converters are also actively conducted. Similar to all modern power converters in which high efficiency and high power density are being actively studied, the researches of battery chargers are also conducted in this direction. In addition to these features, covering the wide range of voltages is also important for the battery charging system. The PFC converter, which supplies the current in phase with the grid voltage to the DC-DC converter for battery charging, has a relatively simple structure. Therefore, researches to increase the power conversion efficiency with additional circuit are mainly performed. In addition, current shaping research is actively pursued for the intrinsic purpose of a PFC converter that must satisfy harmonic regulation of input current. Back-end DC-DC converters for battery charging also have been actively researched to reduce the size of passive components and transformers to achieve high power density. The simple way to do this is to increase the switching frequency. However, this method has limitations because the switching loss increases as the switching frequency increases. Therefore, researches on a soft switching converter that can increase the switching frequency and improve the efficiency have been conducted recently. This thesis includes researches on high efficiency two-stage battery charging systems over a wide range of output voltage. For this purpose, the research of active partial switching method of PFC converter is conducted. This method improves efficiency without additional circuit. In addition, it is based on predictive current mode control that has robust current shaping capability. The active partial switching method proposed in this thesis consists of two parts: peak voltage part switching including switching stop angle controller and zero voltage part switching which adjusts the switching stop period according to the load condition. The peak voltage partial switching has a disadvantage that the stopping angle changes when disturbance occurs. Therefore, in this thesis, by using the controller the switching stop angle is maintained through adjusting the output voltage. In addition, when the zero voltage partial switching is applied, the input harmonic characteristic changes according to the load condition. Therefore, the input harmonic characteristic is analyzed and the guideline for selecting the switching stop period is presented. Moreover, this approach adopts two output voltage profiles, whether the peak voltage partial switching method is applied, thus satisfying the wide output voltage range of the system is possible. On the other hand, the research of DC-DC converter for battery charging was carried out based on the LLC resonant converter. The LLC resonant converter has a trade-off relationship between the output voltage range and the magnetizing inductance value. In order to overcome this problem, this thesis proposes a method that has the variable structure of the transformer. The proposed method generates two different voltage gains depending on the output voltage through the auxiliary winding operation. Also it includes an overshoot-less algorithm, so the stable charging process is guaranteed. The proposed methods are possible to be applied as a front-end converter and a back-end converter to the battery charging system respectively, for satisfying the wide output voltage range and improving the efficiency. To verify the validity of the proposed methods, various experiments have been conducted using prototypes. Through the experiments, the performance and the efficiency improvement of the proposed methods are verified.
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