A High Efficiency LLC Resonant Converter-based Li-ion Battery Charger with Adaptive Turn Ratio Variable Scheme Yeong-Jun Choi*, Hyeong-Gu Han*, See-Young Choi*, Sang-Il Kim* and Rae-Young Kim†
Abstract – This paper proposes an LLC resonant converter based battery charger which utilizes an adaptive turn ratio scheme to achieve a wide output voltage range and high efficiency. The high frequency transformer of the LLC converter of the proposed strategy has an adaptively changed turn ratio through the auxiliary control circuit. As a result, an optimized converter design with high magnetizing inductance is possible, while minimizing conduction and turn-off losses and providing a regulated voltage gain to properly charge the lithium ion battery. For a step-by-step explanation, operational principle and optimal design considerations of the proposed converter are illustrated in detail. Finally, the effectiveness of the proposed strategy is verified through various experimental results and efficiency analysis based on prototype 300W Li-ion battery charger and battery pack. Keywords: LLC resonant converter, Battery charger, High efficiency, Adaptive turn ratio, Wide output voltage 1. Introduction Recently, various green transportations such as xEVs, E-bike, and E-scooter using batteries as a main power source, have expeditiously developed and penetrated the commercial marketplace. Generally, lithium-ion batteries are primarily used in such applications due to diverse reasons such as high energy densities, no memory effects and low self-discharge rates [1]. Meanwhile, the importance of the Li-ion battery charging system has also been increased to ensure safe and powerful use of the battery. Most charging systems employ a constant current-constant voltage (CC-CV) charging profile as displayed in Fig. 1 to prevent overcurrent and overcharge of the battery where the dashed line stands for a battery voltage Vbatt and the solid line stands for the battery charging current Ibatt [2]. As can be seen from the figure, the terminal voltage of the battery is widely varied from the cut-off voltage to the maximum charging voltage during the entire charging process; hence, Li-ion battery chargers should cover a wide output voltage range. LLC resonant converter is a promising high-efficiency battery charger candidate possessing several advantages [3-5]. The converter inherently achieves zero voltage switching (ZVS) of the primary side switches and soft commutation capability of semiconductor devices over the whole operational range. Therefore, the turn-on loss is small and the reverse recovery loss of the diode is reduced, so that a high switching frequency operation is possible and the power density of the entire system can be increased
Additionally the magnetic elements of the resonant tank can be integrated into a single transformer core, which isadvantageous in terms of cost.
However, the LLC resonant converter also possesses ahandicap as a battery charger: There is a trade-off in thedesign methodology to achieving high efficiency and meeting wide output voltage variations to cover wide variations in battery terminal voltage. To achieve a wide output voltage range, the LLC resonant converter need tooperate at a frequency lower than the resonant frequency.
However, such operation decreases efficiency due to the presence of a large circulating current. Moreover, a small magnetizing inductance of the resonant tank is usually required; therefore, efficiency became worse due to an excessive turn-off loss and magnetizing current. Hence, the optimal design of an LLC resonant is realized to be adelicate process [6].
To overcome this problem, several studies have been performed [7-15]. One solution is to develop a design method for resonant tanks to meet a given battery voltage and efficiency specification. [7-12]. However, these studiesonly reveal the inherent trade-off relationship limitation that cannot naturally be overcome. In the literature, a strategy of using auxiliary windings was proposed to realize a wide output voltage [13]. However, since this method was proposed for hold-up compensation, so there is no focus on maintaining the ZVS condition, and it is
difficult to apply it to a battery charger. Some other researches discussed solutions for high power applications such as 3.3kW or 6.6kW [14, 15]. The main concept of these studies is to integrate the many advantages of two or more DC-DC converters with topology modification.
However, it is impossible to apply results of theseresearches directly for low power applications below 500W.
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