| Switching mode power supply is a distributed system,which is widely used because of its high reliability,low loss,easy maintenance and so on.The distributed system consists of three stages,the first stage is power factor correction(PFC),the second stage is DC-DC converter,and the third stage is DC-AC converter with guiding function.The second stage DC-DC converter in the switching mode power supply is the core part of the whole power supply,which can adjust the output voltage.In order to reduce the volume and weight of the power supply,the high frequency,high efficiency and high power density of the DC-DC converter has become a development trend.Because the switching device of traditional converter works in hard switching state,the increase of switching frequency leads to the increase of switching loss,which reduces the efficiency of converter and is unfavorable to the high frequency of switch.LLC resonant converter can make periodic voltage of the resonant circuit to zero by the resonance element,realize zero voltage switching,reduce the loss of switch tube,and improve the efficiency of the converter.LLC resonant converter is quite suitable for high voltage and wide input situations.LLC resonant full-bridge converter has been widely used in the electrical field because of its high frequency,high efficiency,high power density and so on.The variation of load and input voltage leads to output voltage fluctuation and LLC resonant full-bridge converter to adjust the output voltage automatically according to the actual output by frequency conversion or phase shift to adjust the output voltage,which plays the role of stabilizing the output voltage.This thesis depends on many characteristics of LLC resonant full-bridge converter that using the converter to adjust the output voltage.A reasonable and effective digital control scheme and selects DSP+CPLD as the control unit is exploited.The hybrid control strategy combines phase shift and frequency modulation is adopted,which can smoothly switch and transition in two modes.The converter adjusts the output voltage according to the instructional requirement,which selectively works in frequency modulation mode or phase shift mode and realizes the arbitrary output voltage in the whole range.Finally,the working of design and debugging is completed on a LLC full-bridge resonant converter of principle prototype with the input of 620(1±2.5%)V DC,and the output of 400V/2694 W DC.The simulation results and experimental results verify the correctness of the theoretical analysis,the rationality of the design,and the effectiveness of the control scheme. |
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