Research On LLC Resonant Switching Converter

LLC resonant switching converters can realize the soft-switching technology: primary side switch can zero-voltage-switching ZVS on, secondary winding side rectifier can zero current switching ZCS off, the voltage stress of switch and rectifier are low, switching loss is low, without rectifier reverse-recovery loss, high operation frequency, wide range of input voltage, high efficiency, high power density, integrated magnetic technology can easily involved. With these advantages, the LLC resonant switching converters have become more and more popular.However, there are some disadvantages, the design process of resonant components parameter of LLC resonant switching converters is complex, verbose the formulas, vague physical concepts, poor intuitiveness etc. At the same time, the current of converter in resonant tank is large under conventional control strategy of PFM when without load, it will bring the converter serious loss, special working conditions such as short-circuit protection are difficult to solve, the cost of the control circuit are high, the dynamic response is slow etc. The design flow of resonant components parameter of LLC resonant switching converters will be given with the help of engineering application software MathCAD in the thesis, the design of the resonant components parameter will become faster, intuitive, visual, it will easy to understand and accept by people. Based on the design of conventional control strategy of PFM, a novel coalition PFM&PWM control strategy is proposed. PFM and PWM are adopted in one converter, and realize two control modes"seamless switch". Under this new strategy, the converters can not only keep many excellent performances but also cope with the special working conditions easily, lower the cost of the control circuit.Two prototypes of LLC resonant switching converters have been designed, one is low-voltage high current output buck-LLC converter, it adopt the conventional control strategy of PFM; another one is high-voltage output boost-LLC converter, it adopt the novel coalition PFM&PWM control strategy. The main working waveform of the two LLC resonant switching converters have been observed in detail, the experiment data have been handled and analyzed, the comparative analysis have been done also, the preciseness and the practicability of resonant components parameter design flow of LLC resonant switching converters are verified, the merit of the LLC resonant switching converter under control of the novel coalition PFM&PWM strategy has been Highlighted.