| Energy shortage and environmental pollution seriously restricts the future development of our country.Raising the proportion of electricity in terminal energy consumption is critical to deal with this problem.DC power has developed rapidly in the power generation,transmission,distribution and consumption.As the one of the most attractive topologies to realize the DC power conversion,resonant DC/DC converters have been widely used in datacenter power,electronics vehicles charger and battery energy storage system for the natural soft-switching characteristics,high power density and high switching frequency.However,the traditional resonant DC/DC converter cannot fulfill the increasing application requirements.The problems include the contradiction between high efficiency and wide voltage range,the difficulty of parameter optimization design and the uneven load distribution in parallel applications.Therefore,Aiming at improving the efficiency,voltage and operational reliability,this dissertation has developed the topology and modulation optimization,voltage gain modeling method and parallel current-balancing strategy of resonant DC/DC converter.This dissertation mainly includes the following research works.1.Aiming at the difficulty to optimize the efficiency and voltage range at the same time for traditional frequency modulated LLC resonant DC/DC converter,this dissertation proposes an uni-directional LLC resonant DC/DC converter with auxiliary switches.By adding the auxiliary boost unit,the resonant inductor store energy quickly and the total input energy increases.Thus,the peak voltage gain increased.Furthermore,by designing the magnetizing inductance with a larger value,the reactive current is depressed,and the peak operating efficiency of the converter is improved.The test results show that the proposed solution is suitable for data center power supply systems.Compared with traditional LLC converters,the efficiency under normal full load conditions increased about 1%.2.In order to meet the requirements of bidirectional and wide range operation,a series-resonant DC/DC converter with a symmetrical structure is proposed.With fixed switching frequency,the input energy during a single-period is controlled by changing the turn-OFF delay time of the bottom switches to adjust the voltage gain.Based on the law of conservation of energy,the operation process of the converter is analyzed and the voltage gain model is established.The soft switching constraints are deduced and the precise design principle of the converter is proposed.Test results show that the peak efficiency of the proposed converter reaches 97.9%,which is significantly better than the existing bidirectional CLLC resonant converter.This makes it suitable for electric vehicle charging and energy storage applications.3.A bidirectional three-level series-resonant DC/DC converter is proposed for high voltage applications.By introducing an active midpoint clamping half-bridge circuit,the voltage stresses of the secondary switches are reduced to half of the output voltage.The fixed-frequency phase-shift and PWM modulation strategies are proposed.By controlling the input energy during a single period,the voltage gain can be adjusted in both directions and a wide range.The voltage gain characteristics,soft switching constraints and the neutral point voltage balance mechanism are analyzed and provided.Test results show that the proposed converter has higher efficiency compared with the similar three-level CLLC and phase-shifting converters.Thus it is more suitable for high-voltage application.,4.Parameter inconsistency of the resonant DC/DC converter would lead to severe uneven load distribution in multiphase interleaved parallel system.This dissertation reveals the relationship between the terminal voltage peak of resonant capacitor and the input energy during a single period of each module.Base on the relationship,a novel characterization method is proposed to evaluate the uneven degree of load distribution.Then,a new current-balance strategy is designed to enhance the operational reliability.Simulation and experiment verify the effectiveness. |
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