State Trajectory Control Strategy And Modular Cascade Technology Of High Voltage LCC Resonant Converter

The LCC resonant converter has the advantages of utilizing the transformer's leakage inductance and parasitic capacitance,wide input and output ranges,and full-range soft switching.The high-voltage DC power supply adopting this topology has been widely used in X-ray machines,electrostatic precipitators,particle accelerators,et al.However,the existing control strategies and topologies cannot meet the industry's demand for lower output ripple and faster transient speed of high-voltage pulses.In order to improve the steady-state ripple and transient response speed of the high-voltage DC power supply,this paper has conducted in-depth research on several key technologies of the LCC resonant converter.The state trajectory control strategy can accurately control the state of the resonant tank of the resonant converter and achieve a fast transient response.However,the premise of applying the state trajectory control strategy is to conduct state trajectory modeling and analysis of the converter.For LCC resonant converter,the traditional state trajectory analysis is represented by the three-dimensional state curve,which is not intuitive enough and the mathematical calculation is too complex.In this paper,a simplified two-dimensional state trajectory analysis method is proposed to simplify the three-dimensional state space into the two-dimensional state plane.Then,the equivalent circuit of each mode of LCC resonant converter is analyzed,and the state trajectory of the converter in the two-dimensional state plane is deduced.For the LCC resonant converter working in the ZVS region,the steady-state state trajectories of the converter are derived by combing the state trajectories of each mode,which provides a theoretical basis for the state trajectory control strategy.Faster transient response is always the unremitting pursuit of high-voltage DC power supply.However,in the start-up process of the LCC resonant converter,it is difficult to consider the startup speed,current and voltage overshoot in the resonant tank under the traditional PI control strategy,which limits the further improvement of the start-up speed.In this paper,the start-up strategy based on state trajectory control is proposed,which achieves the optimization of electrical stress and start-up speed.By setting the maximum and minimum value of the resonant current during the start-up process,the state of the resonant tank can be accurately controlled with the planning of the state trajectory,which eliminates the current and voltage overshoot in the resonant tank during the start-up process.Finally,the experimental results show that under the same start-up time,the proposed control strategy has no current and voltage overshoot,while the traditional start-up strategy has 43% resonant current overshoot and 56% resonant capacitor voltage overshoot.In order to cover wide input and output range of high-voltage DC power supply,the existing LCC resonant converter design mainly considers the full load situation,which leads to high switching frequency and significantly reduced efficiency under a light load.Burst mode can be used to optimize the light load of the converter.However,due to a large number of resonance elements in the LCC resonant converter,unnecessary resonance is easy to occur in burst mode,which induces low efficiency and transformer saturation.In this paper,a burst mode control strategy based on state trajectory is proposed.Through the precise planning of the state trajectory of the resonate tank,the risk of unnecessary resonance and transformer saturation is eliminated,and the light load efficiency of the converter is improved.In addition,when the load changes between light and heavy loads,the converter switches between Burst mode and normal steadystate mode.A switching strategy based on state trajectory control is proposed to achieve fast switching while avoiding overshoot.Finally,experimental results verify the proposed Burst mode control strategy based on state trajectory and the switching strategy between Burst mode and normal steady-state mode.Compared with the single-module LCC converter,the multi-module LCC converters with input parallel output series(IPOS)have the advantages of modularity,small electrical stress of components,small output ripple,high output voltage and high power.However,the deviation of the resonant tank parameters between the modules will cause the unbalanced output voltage of different modules.Aiming at this problem,a large-signal model of the multi-module LCC cascaded converter is proposed in this paper.By using this model,the influence of resonance parameter deviation on the output voltage of each module is evaluated,which provides a theoretical basis for the tolerance analysis of resonance parameters.Based on the analysis of the output voltage imbalance,this paper proposes a dual-loop control strategy to adjust the frequency and duty cycle to achieve the tracking of the total output voltage and the balance of the output voltage between the modules.Furthermore,based on the large-signal model,the small-signal model of the converter is derived,and the design of the control loop is analyzed.Finally,the experimental results show that the proposed analysis method can accurately calculate the voltage imbalance caused by the resonance parameter deviation,and the proposed control strategy has achieved a good voltage sharing.In summary,to meet the demand for lower steady-state ripple and faster transient response of high-voltage DC power supply,this paper proposes a simplified state trajectory analysis method of the LCC resonant converter.By applying it to the start-up process and light load optimization,the start-up process is sped up and the light load efficiency is improved,which provides a new method for the transient control of the LCC resonant converter.Besides,the research on the output voltage sharing of the IPOS multi-module LCC resonant converter also provides a theoretical basis for the practical application.