Research On LLC Resonant Converter And Its Current Sharing Control

In recent years,trade frictions and regional instability have made the independent localization of equipment more and more important.In this thesis,a digital isolated DC/DC power supply is developed based on the full-bridge LLC topology commonly used in switching power supplies,with the goal of replacing domestic components.It is designed for soft switching characteristics over a wide input voltage range and full load range.To obtain higher power levels for application,the current sharing control of parallel modules is studied.Firstly,the working principle of the full-bridge LLC converter is analyzed,and uses the first harmonic approximation to model the converter to derive the expression of the resonant tank voltage gain.Under rated load,the high-precision timer-based frequency conversion control and digital controlled synchronous rectification strategy are used to obtain efficient and stable operation of the circuit.The hybrid control strategy of frequency reduction and the variable duty ratio is adopted to solve the problem of large start-up current impact of the converter.In view of the output voltage distortion phenomenon of the converter at light load,the light load gain characteristic diagram is drawn to analyze the influence of the parasitic capacitance of the transformer and the rectifier on the resonance gain at light load,and the output voltage stability is obtained by the intermittent mode control of hysteresis voltage comparison.Secondly,to solve the problem of uneven current sharing in the parallel operation of multi-module LLC converters,the relationship between the system current sharing error and the resonance parameter error in the case of natural current sharing is drawn under the condition of considering the manufacturing error of the circuit resonant element parameters.Aiming at the influence of resonance parameters on parallel current sharing,a multi-module parallel control strategy is designed,which preferentially adopts master-slave current sharing,and can also be transferred to droop control current sharing.Then,the resonance parameters of the LLC converter are optimized for soft switching and high efficiency.For the 2k W full-bridge LLC converter,nationalization selection was accomplished with resonant capacitor,switches,drive circuit and other devices.The losses of LLC converter are analyzed,the losses of each part of the converter are calculated,and the loss distribution diagram under rated operating conditions is given.Finally,the hardware circuit construction and software design of the 2k W fullbridge LLC converter are completed,and the frequency control,synchronous rectification,soft-start,light-load and current sharing control strategies are experimentally verified on the experimental prototype.The experimental results show the correctness of the principle analysis,the rationality of the prototype design and the feasibility of the control strategy.