Three-Phase Three-Level LLC Resonant Converter For High Voltage And High Power Applications

For medium-to-high power applications, the power switches in traditional converter topologiessuch as forward, half-bridge and full-bridge converter may suffer high voltage and current stress,which limits the power level. In order to reduce the voltage and current stress, three-phase three-leveldc-dc converter was proposed which limits the voltage stress to half of input voltage and lower theRMS current significantly.LLC resonant converters are good candidates for medium-to-high power applications because theswitches can realize zero-voltage-switching(ZVS) and the rectifier diodes can achievezero-current-switching(ZCS) in a wide load range. Variable-frequency control and fixed-frequencycontrol strategies can be applied for LLC resonant converters.In order to combine the advantages of three-phase three-level converter and LLC resonantconverter together, a three-phase three-level LLC resonant converter was proposed in this thesis. Themain advantages of this converter are listed as follows: The converter has a simple topology with onlysix power switches. The voltage stress on power switches is only half of input voltage, and the voltagestress on rectifier diodes is output voltage, so that the converter is applicable for high input/outputvoltage applications. The output current ripple is much smaller compared with single phase converter,and the output filter capacitance is reduced. It can realize ZVS of the power switches and ZCS of therectifier diodes in a wide load and input voltage range.This thesis analyzes the operation principle of the converter under variable-frequency controlstrategy. The characteristics of three-phase three-level LLC resonant converter undervariable-frequency control were analyzed by Fundamental Harmonic Approach. The characteristics ofthe converter in different operation regions were compared, and the ideal operation area wasdetermined. The parameters of the converter were designed in detail. Also, the operation principleunder fixed-frequency control strategy was analyzed.Finally, a2kW prototype was fabricated and tested in the lab, and experimental results wereprovided to verify the effectiveness of the proposed converter.