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.
LINK WEB
http://www.jeet.or.kr/LTKPSWeb/pub/pubfpfile.aspx?ppseq=1943
