Isolated Resonant Converters With Wide Voltage-Gain Range

With the increasing demand of spacecrafts,electric vehicles and renewable data center,research and exploration on novel topologies of power converter have been promoted.This paper mainly focuses on the research of topology derivation of high-performance isolated resonant converters with wide voltage-gain range,and their control strategies and optimal design,to meet the growing requirement of high conversion efficiency,high power density,and high reliability of the power systems of spacecraft,electric vehicles and so on.The typical analysis and design methods of resonant converters have been summarized.Take the classical LLC resonant converter for example.Several analysis methods of resonant converters,such as FHA method,approximation method based on time domain,and numerical analysis method,have been illustrated and compared in detail.Generally speaking,the FHA method is a simple and applicable method,while its deviation of voltage gain is larger,especially when the rectifying voltage or current is discontinuous.With help of mathematical software,the state space and numerical analysis are most accurate,but more complex.With these comparisons,an empirical analysis procedure of resonant converters has been proposed.In summary,there are several constraints for the design of LLC resonant tank.Although there are different design methods,their optimization goals are same: find out the largest product of the ratio of magnetizing inductor to resonant inductor and quality factor of resonant tank while satisfying different combinations of constraints.The optimal design of LLC and LCLCL resonant converters for applications with wide outputvoltage range has been studied.Considering the charging strategy,a detailed optimization method has been proposed for LLC converter: Based on two limited operation points on the voltage-current plane of constant current and constant voltage,a larger ratio of magnetizing inductor to resonant inductor has been designed to decrease the conduction losses and increase the conversion efficiency.On the basis of LLC resonant tank,a notch filter has been added for the LCLCL resonant converter.Then two more factors during the analysis procedure: the ratio of notch inductor to resonant inductor and the ratio of notch capacitor to resonant capacitor.According to the analysis,they have more impact on the primary current,while little on the peak of voltage gains.The normalized notch frequency and the second series resonant frequency are more sensitive to the latter than the former.Therefore,the resonant capacitors should be well-designed before the resonant inductors.The peak of voltage gain is dependent on the ratio of magnetizing inductor to resonant inductor.When the requirement of maximum voltage gain has been satisfied,the primary current decreases with the increase of product of the ratio of magnetizing inductor to resonant inductor and quality factor of resonant tank.Then a detailed optimal design has been proposed for LCLCL converter with wide output-voltage range.A family of modified LLC converter with two split resonant branches has been proposed for wide input-voltage range applications.With the split and combination of transformers and resonant branches,the proposed converter can achieve the smooth transition between three operation modes: high gain mode,medium gain mode,and low gain mode.Then for every operation mode,the required voltage-gain range of the resonant tank can be narrowed greatly.And the optimized resonant parameters can reduce the primary circulating current,increasing the overall efficiency.When the parameters of two branches are same,the voltage gain of medium gain mode is in the middle of high gain mode and low gain mode.In order to improve the required voltage-gain range of LLC resonant tank and be more applicable,the resonant parameters of two resonant branches are reconfigured from multi sliced branches.After optimization,the voltage gain of the medium gain mode can be shifted between two other modes freely.While the parameters of two branches are not always same,all of the impedance ratios of corresponding resonant elements are the ratio of two transformers’ turn-ratio.As both of two transformers deliver power throughout,smaller cores have been used for the proposed LLC converter.A family of modified LLC converter with integrated step-up unit has been proposed for the applications with hold-up time requirement.Only during the hold-up time,the unit works to pump the energy from the enery-storage capacitor to the resonant inductor or series resonant tank.Then the voltage-gain range of the resonant converter has been expanded,and the requirement of hold-up time can be satisfied.In normal operation,the proposed converter works around the resonant frequency.When the integrated step-up unit works,the voltage gain is mainly determined by the duty cycle or phase-shift angle of auxiliary switches,and the quality factor of LLC resonant tank,while it is independent of magnetizing inductance.Then a larger magnetizing inductance can be designed,resulting in low magnetizing current and primary current and then higher efficiency in normal operation.Prototypes are built with both the simulation and experimental results in detail to verify the analysis mentioned above,respectively.