| With the rapid development of the new energy power generation, aerospace,communications, etc. technologies, modern switching power supply get more and moreapplications. For the requirement of the power electronic device miniaturization makesswitching power supply must be developed in the direction of high power density. Highfrequency change makes the power density of switch power supply has been greatlyimproved, but this brings the high switching loss is the main obstacle to further improvethe efficiency of switch power supply. In order to overcome the shortcomings whichHigh frequency change brings to the switching power supply, all kinds of resonancecircuit was put forward. The LLC resonant circuit because of its good soft switchfeatures and wide gain control ability has been widely research and application.Paper detailed analysis and research the hot spot of LLC circuit, which researchersare focused on. The First Harmonic Model of LLC circuit was build and got its gainmathematical expressions. The influence of parameter to characteristic in LLC circuitwas analysis; establish the selection constraints of parameters. The LLC circuit needs alarge range of switch frequency, which is not conducive to the design of magneticdevices, so the latest put forward hybrid LLC circuit is used in the high efficient powerinverter circuit. It has two control patterns, variable frequency control and phase shiftcontrol. It makes the frequency of LLC circuit is reduced without change the LLCcircuit topology structure, so it has excellent control characteristic. Paper detailedanalysis the switching devices loss, conduction loss and shut off the loss. and establishthe relationship between them and the resonant current. Based on this, the optimalselection criteria of point from variable frequency control and phase shift control wasgiven, and the efficiency of hybrid LLC circuit was optimized.Because in low-voltage high-current output applications, rectifying loss willbecome the main resonance circuit loss, synchronous rectifier, using MOSFET replacediode, is applied to the high efficiency of power supply. The rectified current in hybridLLC circuit is sine on-off state, which makes synchronous rectifier control requirementsis very complex. According to this characteristic, the paper puts forward a kind ofsensorless synchronous rectifier digital control strategy, which shares voltage detectionwith resonance control. Compared to other synchronous rectifier control strategy, thisstrategy only need add the accordingly resonant control procedures in digital control chip, reduce the cost and volume, don’t need another devices, so it can reduce the costand volume. In order to get good adjusting speed and control precision, optimal gradienthysteresis comparison algorithm is applied to implement synchronous rectifier optimaladjustment. what’s more, when external conditions change such as the input and load,the synchronous rectifier control strategy can launched again by itself, and quick searchto a new optimal driving point.Papers designed the hybrid LLC circuit resonance control and synchronousrectifier control code generation model, used Simulink and Stateflow software to createcontrol model and automatically generate control program code. Finally, paper designedthe corresponding hardware circuit with the TMS320F2808which is made by TICompany. Using a experimental prototype with input voltage36-72V, output of3.3V/20A to verified the validity and feasibility of the proposed control strategy. |
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