A Study on High Efficiency Technology for Charging of Electric Vehicle -Author Jin-Hak Kim Department of Electronic and Electrical Engineering Graduate School

A Study on High Efficiency Technology for Charging of Electric Vehicle 
Author:Jin-Hak Kim 
Department of Electronic and Electrical Engineering Graduate School Keimyung University (Supervised by Professor Jun-Ho Kim) 

ABSTRACT
 As environmental pollution intensifies, government regulations and eco-friendly policies are being strengthened to solve these problems. In the automobile market, research on electric vehicles, which are environmentally friendly automobiles, is becoming active. For the development of electric vehicles, it is essential to build charging infrastructure and develop charging technology for competitive prices. Therefore, in this paper, presents a technology for high efficiency of electric car rapid charger. The conventional rapid charger for electric vehicle, three 20kW AC-DC converters are arranged in parallel, and each 20kW module is composed of one PFC(Power Factor Corrector) and DC-DC converter. Since DC-DC converter directly charges the battery, the efficiency of the DC-DC converter greatly affects the overall fast charger efficiency. Therefore, it is very important to select a DC-DC converter topology suitable for high efficiency and to design an optimum for high-efficiency rapid charger development. The conventional DC-DC converter topologies for electric vehicle rapid chargers have used LLC resonant converters or phase-shift full-bridge converters. Both converters have a fatal disadvantage that can be difficult to achieve with high efficiency and miniaturization in battery charger applications. Therefore, in this paper, we propose a new LLC-Buck DC-DC converter for a rapid charger that can overcome the existing disadvantages. The proposed converter is designed as frequency fixed type at the resonance point by separating the insulation function and the charging function. The buck converter, which is a structure without circulating current, is connected in series. The proposed converter has the following advantages. 1. The switching frequency is fixed to the resonant frequency so that LLC resonant converter operates at the highest efficiency point. Then, ZVS and ZCS are achieved even if the load fluctuates, and the circulating current is minimized. 2. Based on the ZVS and ZCS achievement, a small scale transformer was designed to allow a high frequency operation to be achieved. 3. By using two transformers, it is possible to design more optimally in terms of size than one high-capacity transformer. 4. The series connection of the secondary rectifiers lowered the internal voltage of the secondary rectifiers, reducing costs and losses. These four advantages, it possible to charge the battery with high efficiency throughout the battery charge period, and the efficiency fluctuation is minimized. And high power density can be achieved. As a result of the fabrication and testing of the proposed converter, it was confirmed that the battery is always charged at a high efficiency of 97% in the entire load range. This is more than 1% higher efficiency than conventional frequency-variable converters. As a result of applying the proposed converter to the rapid charger, the high efficiency of 95.242% of the total efficiency was achieved through the high efficiency of the DC-DC converter. Also confirmed that it is competitive in terms of volume through comparison with other companies products. Therefore, the proposed LLC-Buck DC-DC converter is expected to be widely used in electric car chargers requiring high efficiency and small scale.
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