Research On Digital Control Full-bridge LLC Resonant Converter

High-voltage DC (HVDC) aeronautical power system has the advantages such as lower weightand easiness to realize uninterruptible power supply, which has been regarded as the future trend ofthe power system in aeroplane. This dissertation is devoted to theoretical study and engineeringpractice on DC/DC converter of secondary power supply in HVDC power system.The Full-Bridge LLC resonant converter is very suitable for the DC/DC stage in HVDC powersystem, because it has the advantages of simple configuration, zero-voltage-switching (ZVS) for theswitches and zero-current switching (ZCS) for the rectifier diodes in a wide load range. The controlstrategies for full-bridge LLC resonant converter normally include variable-frequency control andfixed-frequency control, which have advantage and disadvantage of their own. In order to combinethe advantages of both the two control strategies, a hybrid control is adopted in this thesis. Theoperation principle of the converter under the two control strategies are discussed in the thesis, thecharacteristics of the full-bridge LLC resonant converter under variable-frequency control andfixed-frequency control are analyzed by fundamental harmonic approach and time domain analysisrespectively, based on this, the switching point selection principle of the converter is given. When theswitching frequency is equal to the resonant frequency, the efficiency of the converter is optimal.Considering the deviation of the parameters, a resonance frequency detection method is proposed inthe thesis, to ensure that the converter work at the resonant frequency at rated input voltage.The traditional analog control circuit is complex and has various components, witch greatlylimits the improvement of power density and power cost reduction, so a digital control program isgiven in the thesis. The control circuit design of full-bridge LLC converter is based on DSP, includingdrive circuit, sampling and modulating circuit, resonant frequency detection circuit and so on. Theflow chart of digital program is also given in the thesis.Finally a prototype of270±10%V input,28V/1kW output was favricated and tested in the lab,and the experimental results were provided to verify the effectiveness of the proposed converter.