Research On LLC Resonant Converter

High frequency, high power density and high efficiency is the development trend in DC/DC converter applications. Traditional hard-switched converters restrict the development of DC/DC converter. Phase-shift Full Bridge PWM ZVS DC/DC converter has been widely used owing to its ZVS condition of main switches. But it still has some disadvantages, for example: lagging-arm switches is hard to achieve ZVS in light load conditions; rectifier diode have unavoidable recovery problems and they not only cause great secondary loss, but also increase the voltage stress of the rectifier diodes; converter can achieve high efficiency in low input DC voltage conditions but low efficiency in high input DC voltage conditions, this kind of efficiency characteristic restricts its applications on high input DC voltage occasions and high-quality converters which have hold-up time requirements.Fortunately, LLC series resonant Half Bridge DC/DC converter can solve these problems successfully. Considering from realization, resonant converter has the advantage of higher operation frequency, lower switching loss, wider range of input voltage, higher efficiency, lighter weight, smaller volume, smaller EMI, lower switching stress to PWM converter. The prominent merits of LLC resonant converter can be specified as follows, realization of full load range of ZVS for primary switches and ZCS for secondary diodes, easy integration of resonant inductor and transformer, wide range operation. It is focused on due to the above advantages.LLC resonant converter is developed on the base of components resonant converter by adding a parallel inductance, so its DC gain characteristic is better to series resonant or parallel converter. But on the other hand, it has some difficulties in design as: (1) more complex to control; (2) difficult to modulate the parameters; (3) in application of large power, the current in resonant circuit is so large that the magnetic part is easy to saturate and voltage control topology is not suitable, but it is difficult to achieve PFM using current control topology.The thesis includes some parts as following:1. Compared resonant converter with traditional PWM converter, classified resonant converter, and studied three traditional resonant converter, then summed up the profits of LLC resonant converter.2. Thesis discussed the principle and the operation modes of LLC resonant converter in each frequency range, and drew its detailed working waves.3. The mathematics model of the converter is obtained based on the fundamental element simplification method. And develop the method of modulating the parameter based on the analysis.4. In order to design controller, small-signal model of the converter must be given. In this paper, the small-signal model of LLC series resonant DC/DC converters is deduced using Extended Describing Function Method. Also, stability of the converter is analysed and controller is designed to meet the requirement of dynamic process.5. Compared voltage mode topology and current mode topology in the LLC converter, and design a PFM circuit using current mode topology to achieving single cycle control.6. Simulation results prove the correctness of theory, The experimental results prove the availability using current mode topology in high power LLC converter.