Research On High Efficiency LLC Resonant Converter

LLC resonant converter can achieve ZVS of primary switches and soft recovery of secondary rectifiers, is suitable for working at high frequency, and reaches high efficiency, thus is widely applied in communication power supplies, LED drivers and other DC-DC converters.Aiming at the requirements of high efficiency and tight adjustment of380V/24V DC-DC brick communication power modules, the project researches the efficiency and control optimization design method of LLC topology used in brick type DC-DC modules.LLC research results are reviewed:DC gain calculation, small signal characteristics, dynamic analysis, and LLC design method based on first harmonic approximation (FHA).The paper investigates the relationship between the resonant parameters and LLC DC gain, interprets the impact of various resonant parameters on the efficiency and working frequency range of LLC more intuitively, and then put forward the optimization design method of LLC based on resonant parameters. Based on in-depth analysis in LLC mode, deduces the full range ZVS conditions of LLC, and presents the LLC optimization design process. Combined with the brick power modules’ requirements of high efficiency and high power density, several LLC rectifier structures are compared, PCS windings with a center tap synchronous rectifier is selected, and the winding structure of the transformer and the through hole scheme are determined by analyzing the winding loss.Small signal analysis is necessary to feedback design, LLC small signal state equation is obtained by modeling LLC with extended describing function, frequency characteristic curve is drawn with Mathcad and verified by simulation. Based on the simulation, the small signal of control to output analysis is explained, the small signal model of the optocoupler is analyzed, one zero two pole compensation network is designed, the transfer function of feedback network is built. The loop gain of the simulation agrees well with that of the experiment.A prototype is built based on L6599, and circuit parameters are given. Analyzes and calculates loss breakdown, then gives dynamic waveform and efficiency curve.LLC analysis design method based on resonant parameters reveals the relationship between the resonant parameters and the efficiency, frequency regulation range, simplifies the design of LLC, it has some significance on the analysis and design of LLC.