Research On Characteristic Analysis And Strategy Control Of Three-level LLC Resonant Converter

LLC resonant converter has the advantage of higher operation frequency,lower switching loss,higher power density,smaller volume,higher efficiency.So it is received extensive attention and research in some applications with high voltage and high power,such as DC-DC supplies and electrical vehicles.Compared with the traditional resonant converter,Three-level(TL)converter is suitable for high voltage input situation because voltage stress of each switch in TL,converter is half of input voltage.The primary switches can work at the state of zero voltage switching(ZVS),and Secondary diodes can achieve zero current switching(ZCS),so it has high conversion efficiency.This paper analyzes the traditional frequency control strategy,and describes the work process and equivalent circuit of variable frequency(VF)modulation.The relationship between the output voltage gain and the switching frequency is analyzed by the fundamental analysis method,and it points out the shortcoming of VF modulation that the voltage range is small.In order to achieve a wide range of output voltage and to overcome the shortcoming of VF modulation,this paper introduces phase-shift(PS)modulation to TL HB LLC converter.It analyzes the working process,voltage adjustment range and soft switching condition of PS modulation.Besides,it deduces the demarcation point at working state of soft switching,thus proposes a hybrid modulation strategy of TL HB LLC converter.According to the soft switching state,this strategy can achieve soft-switching during the whole control process and wide range of output voltage from switching PS mode and VF mode.Small signal analysis is a necessary part of closed-loop control.In PS mode,the LLC resonant converter is modeled by the state space averaging method because the switching frequency is kept constant,the steady-state model and the small-signal model are deduced and the controller is designed.The traditional state space method is not suitable for the VF mode,so the equivalent circuit method is used to model the LLC resonant converter and the controller is designed in this paper.The two control strategies are verified by closed-loop simulation.Finally,the hardware circuit of the three-level half-bridge LLC resonant converter is designed,including the driving circuit,the protection circuit and the sampling and conditioning circuit.By adopting digital control scheme and using TMS320F28335 as the control chip,this paper validates two control strategies by software programming.The experiment can verify the correctness of theoretical analysis results,the feasibility and effectiveness of the proposed modulation strategy.