Research On Characteristics Of LLC Resonant Converter And Its Performance Improvement

As the development of power devices and theories of powe electronics, Highfrequency is one of the main trends of power electronics converters. The improvement offrequency can remarkably decrease the volume of magnetic components so the powerdensity of converters will be improved. However, the subsequent result is that switch lossalso increase. To solve this problem, soft switch technology has received attention a lot.Resonant topology is such a class of circuit that use periodic current and voltage ofresonant inductors and capacitors to turn on or turn off the switch each switch cycle. In thecondition of zero-voltage or zero-current, the switching process loss can be reduced to zero.First, this paper introduced the fundamental characteristics of converters in resonant family.Aiming at a wide range output voltage and load adaptability application, LLC topology isselected as the main circuit of the prototype in this paper——charging devices for ElectricVehicle batteries. Then LLC’s research status and attention were summarized in detail andthe problems that this paper focus on were proposed.In order to better understand and describe the complicated operation modes andcharacteristics of LLC converters, this paper decomposed them into several fundamentalmodes,witch was called P, N and O. Using resonant frequency as a basic boundary, thework status in different frequency and load conditions and its changing trends wereanalyzed in detail. To further analyze and calculate LLC circuit, accurate models of thesefundamental modest should be built first. And then this paper came up with somemathematical solution methods for the combined modes.With a strong nonlinearity, the LLC resonant DC/DC system is difficult to design acontroller based on traditional linear control methods. This paper study the small signalcharacteristics of LLC converters based on SABER simulation. For a full-digital controlsytem under high switching frequency, problems like frequency-regulation precision,voltage-regulation precision and digital delay shows up. In order to achieve a betterstability and dynamic performance, this paper applied higher order digital control strategyand brought in more feedback variables. Simulation results validated the effectiveness ofthe proposed method.Finally, this paper had a discussion on LLC’s efficiency issues and gave an accuratecalculation and analysis for normal operation point. Considering the necessity of ZVS forLLC operation, this paper calculated out the minimum dead time to achieve ZVS, using theconclusion from chapter2and discussed the frequency range as a supplement. Finally, Simulation and experiment results validated the accuracy of the calculation.