Research On Optimized Control Strategy Of Hybrid Clamped Half-Bridge Three-Level LLC Resonant Converter

Half-bridge three-level LLC resonant converter has the advantages of high efficiency,high power density,electrical isolation and the voltage stress of the main switches is only half of the input voltage.The hybrid clamped half-bridge three-level LLC resonant converter is widely used because of its voltage self-balancing characteristics of the supporting capacitors.However,the applications of wide input voltage range set higher demands of closed-loop control strategy for its output voltage.In addition,the voltage of flying capacitor is zero before startup,which affects the safe operation of the converter,and during startup,the resonant current peak is high,which is easy to damage the switching devices.To solve the above problems,this paper focuses on the voltage closed-loop control strategy,flying capacitor precharging control strategy and soft start control strategy of the hybrid clamped half-bridge three-level LLC resonant converter.The topology and characteristics of the hybrid clamped half-bridge three-level LLC resonant converter are analyzed,and the main waveforms,commutation process and soft switching realization process of the converter in each working stage under frequency modulation and phase-shifting control are studied.Based on the Fundamental Harmonic Approximation(FHA)analysis method,the gain characteristics of hybrid clamped half-bridge three-level LLC resonant converter are studied,and the simplified equivalent circuit of the converter is obtained,which is solved by phasor method.The expressions of voltage gain under frequency modulation and phase-shifting control are derived.The Zero Current Switching(ZCS)of rectifier diodes under frequency modulation and phase-shifting control is analyzed and the advantages of the two control methods are analyzed.The limitations of the two control methods are discussed for applications with wide input voltage range.A hybrid control strategy is proposed,in which frequency modulation is used when the voltage gain is greater than or equal to 1,and phase-shifting is used when the voltage gain is less than 1,so as to reduce the switching frequency variation range and achieve wide range input voltage regulation and good soft switching performance.The lowest switching frequency,minimum total energy storage of inductors and minimum dead time to realize Zero Voltage Switching(ZVS)for main switches are derived.Based on the support capacitors precharging circuit,a flying capacitor precharging control strategy without additional auxiliary hardware is studied.By controlling the main switches and the switches of the support capacitors precharging circuit,the flying capacitor precharging can be realized to ensure the normal operation of the converter.In addition,the traditional phase-shifting soft start control strategy is optimized and a "two stages" phase-shifting soft start control strategy is studied.By controlling the duty ratio to increase from 0 to 1 in two stages with different slope with time,the rate of voltage gain is reduced in the initial stage of startup,suppressing the resonant current shock during startup.Aiming to achieve wide range input voltage regulation as well as ZVS of the main switches and ZCS of rectifier diodes under the whole input voltage range with hybrid control strategy,the main parameters of hybrid clamped half-bridge three-level LLC resonant converter are designed,and the converter loss is analyzed theoretically.The simulation model and prototype are built and the studied control strateges are verified by simulations and experiments.89 figures,2 tables,61 references.