Research On Control Strategy Of LCC Resonant DC/DC Converter

LCC resonant converter has soft-switching characteristics and good ability of voltage regulation, it can be used in high voltage DC power supply, distributed DC power supply, power electronic transformer, fast charging of electric vehicle and large capacity energy storage system in grid and other fields. The soft switching state of the resonant converter is influenced by the control mode. In the practical application, the resonant converter needs to maintain the soft switching state in the case of variable input voltage or load. Traditional fixed frequency control mode can only maintain soft switch when the change of working condition is less. In this paper, the fixed frequency asymmetric control applied to the LCC resonant DC/DC converter is studied, it can expand the soft-switching range, improve the efficiency when the working condition changesFirstly, in order to study the effect of asymmetrical control on the realization of soft switching, the steady state model of LCC resonant DC/DC converter is established by the fundamental analysis method in this paper. The characteristics of voltage gain and input impedance was analyzed. Based on the steady-state model, the output voltage of the inverter bridge under the asymmetric control mode is decomposed by Fourier decomposit. And the condition of switching tube to achieve ZVS is obtained. Set achieving ZVS as the goal, the optimal control model is obtained by graphical analysis. The analysis found that when normalized output power is between 0-0.25, single side asymmetric voltage mode is the optimal control mode; when normalized output power is between 0.25-1, asymmetric voltage mode is the optimal control mode.Secondly, the effect of snubber capacitor on resonant DC/DC converter under asymmetric control mode is studied in this paper. Snubber capacitor can reduce the turn off loss, but it will affect the range of ZVS. The effect of snubber capacitor on ZVS and turn off loss is analyzed under different quality factor and resonant capacitance ratio, and the calculation formula of the turn off loss is given. In addition, the conduction loss of the switching tube and the anti parallel diode are analyzed under different control modes.Lastly, through the MATLAB simulation, it is proved the asymmetrical voltage control has a larger ZVS range, and has a higher efficiency at low output power. The experimental platform of LCC resonant DC/DC converter based on DSP+FPGA is designed. By comparing the experimental results, it is proved the asymmetric voltage control mode has a wider range of ZVS range.