Study On Characteristics Of LLC Resonant Converter Switching Losses

Increasing of the power switch frequency can significantly reduce the volume of capacitor,inductor and transformer, and consequently reduce the cost of electronic components and improve the power density of the converter. However, the high frequency causes low efficiency because of high switching losses. Meanwhile, the rectifier diode will have serious reverse-recovery problem,bringing about not only serious EMI and making PCB design complexly, also further reducing the efficiency of the converter. Owing to the hold-up time and switching losses, the traditional PWM converter cannot meet the requirements for high frequency and efficiency. In this context, LLC resonant converter came into being.LLC resonant converter can achieve high efficiency, high power density, low EMI noise,wide range of input voltage through the use of soft-switching of both power switch and rectifier diodes and integrated magnetics. So it has been widely used for small and medium power level electronic products. LLC resonant converter and phase-shifted full-bridge converter now have been widely used at different power levels. The scholars at home and abroad have conducted numerous studies on the two converters, but few studies on the comparative analysis of their switching losses. Due to the PFM woke mode and many key parameters, there still exists issues of control difficultly, parameter selection hardly and resonant current overload protection and so on.To solve these problems, the following work has been carried out:Firstly, the LLC soft switching distributed power system design is given and then the principle of EMI filter and APFC circuit are introduced and APFC circuit based UC3854 SABER simulation model is also given. The hold-up time capacitor calculation process is analyzed. The analysis of the half-bridge LLC steady-state modeling is talked about in detail and also the impact of key parameters on voltage gain through the curve draw by ORIGIN.Secondly, based on the establishment of equivalent circuit model of LLC resonant converter,the extended describing function method is used for small signal modeling and the voltage gain is obtained, confirming the correctness of FHA modeling method.Thirdly, the establishment of LLC resonant converter switching losses mathematical model is given, and with the requirements of the design, the selection process of key parameters of the converter based on MATHCAD is talked about in detail, and so the power switch and diode.Accordingly, the switching losses of power switch and diode is calculated in the end. The comparative analysis of switching losses between LLC resonant converter and PWM ZVS phase-shifted full-bridge converter is executed based on SABER simulation with the same input,output and switching frequency.Finally, the comparative analysis between the old version and the improved version of the half-bridge LLC resonant converter is executed based on SABER simulation, confirming the current limiting function of the improved version in overload conditions. Then conduct a small signal time-domain simulation for the improved version through SABER, and the Bode plot of the input frequency to the output voltage is obtained, through which the design of closed-loop compensation controller is completed. The simulation experiment shows that the LLC resonant converter can ensure stable output under different load conditions with the compensation controller.