Design Of Li-ion Battery Charger Based On Constant-current/constant-voltage Strategy

Li-ion battery is being used widely because of its unique advantages, such as high energy density, long life, high operation voltage, pollution-free and absence of memory effects. However, it puts forward relatively stringent requirements on the charger because its comparative fragility. In thesis, a Li-ion charger was designed. Its output voltage is 42 V and output current is 4A.By studying Li-ion battery chemistry, the charge and discharge characteristics and charging method of Li-ion battery, the overall scheme was determined.The Li-ion battery charger uses constant-current/constant-voltage mode in thesis which includes two stages: First, the battery is charged by large constant current to allow the fast charging. Second, the constant voltage charging is adopted to ensure that the battery can be charge to its full capacity.The preceding stage of the charger is PFC converter. By comparing several types of PFC topology and combining the application environment, the PFC converter was determined to be boost converter with discontinuous current boundary mode(DCBM). It has some advantages such as simple control strategy, high power factor and high efficiency. It is suitable for small and medium size power supply products. Then, the paper deduces and designs the principle and main circuit parameters of the PFC with DCBM. Finally, the design method of the PFC with DCBM based on L6561 chip was introduced in detail.The backward stage of the charger is DC/DC converter. By comparing several DC/DC converters and combining the application environment, the dual-switch forward DC/DC converter was chosen to be the main circuit of the charger. Due to its low stress switching voltage, resistance to bridge legs intrinsic direct ability strong and high reliability, it has been used widely. Then, the basic principle of dual-switch forward DC/DC converter was analyzed in detail. Based on the fixed frequency PWM controller SG3525, the design method of the controlling circuit of constant-current and constant-voltage is introuduced in detail. By the voltage and current regulation loop, the function of the auto switch from constant current stage to the constant voltage stage is realized. At last, the selection criteria and design method of the main circuit is introduced in detail.A prototype of the Li-ion battery charger is developed, the experiment results verify the feasibility of the scheme. The charger has some advantages such as high control accuracy, high efficiency, small bulk and weight and high reliability.