Study On Optimization Of LLC Resonant Converter Under Light Load Condition

The LLC resonant converter topology can achieve zero voltage switch of MOSFET and zero current switch of the secondary rectifier diode based on its topological feature. With the advantages of higher efficiency, higher power density, smaller EMI and the use of magnetic integration technology, the LLC resonant converter topology can effectuate the soft switch of MOSFET under all load conditions. The maximum efficiency of LLC resonant converter can reach 97% in the application of kilowatt level. Therefore, the LLC resonant converter has an extensive popularity among engineers in the application of medium-power DC-DC converter under light load condition.In the application of industrial power supply, load status of power inverter will switch among various states such as full load, overload, half load, light load and no-load. As for the fuel cell standby power of communication stations, it sets a higher requirement for the wide ranging load variations of power converter due to the acute voltage fluctuation of its input voltage and a wide range of load. Traditional LLC resonant converters have problems like gain error under light load condition. Focusing on employing dummy load, the traditional solutions result in additional loss and increased cost.This paper focuses on the efficiency and gain error of LLC resonant converter under light load states.First, it indicates that the traditional steady-state analysis model is not applicable to that under all load conditions from the perspective of the topology theory. For the influence of the parasitic capacitances in circuit should be considered. The existence of the parasitic capacitances changes the working process of the resonance circuit under light load condition. Taking the parasitic capacitance into consideration, this paper establishes the steady-state model of LLC resonant converter, and presents an analysis of the working process of the converter under light load condition as well as the voltage gain of resonant tank based on fundamental wave method. Meanwhile, the loss under light load condition is modeled in order to improve converter efficiency.Then, the influence of the LLC resonant parameters on the gain and efficiency of the converter is analyzed in this paper. Methods to avoid the gain error and optimize efficiency under light load condition are presented.Finally, a simulation model is structured in SABER to prove the validity of the methods. After that a full-bridge LLC resonant converter with output power of 5KW is designed and an experimental prototype is built to verify these analyses.