Research On Wide Output Voltage Technology In 3kW Full-bridge LLC Resonant Converter

Full-bridge LLC resonant converter is widely used in high power applications due to its high efficiency,small electrical stress of devices and ability to achieve soft switch in wide load range.However,traditional FHA method used in design of this converter brings deviation of voltage gain.The method also brings wide operation frequency range which leads to difficulties in design of magnetic components and more losses of the converter.Besides,it is difficult for LLC resonant converters to output an ultra-low voltage.These problems bring limits to the use of LLC resonant converters in some applications like battery chargers of electric vehicles.Based on time domain modes analysis,an analytic model is developed in the thesis which provides the relationship among output voltage range,operation frequency range,resonant capacitance and resonant inductance.Using the model,wide output voltage range,operation frequency range and resonant parameters can be selected accurately and intuitively.The method can improve the design accuracy and optimize the operation frequency range under certain wide output voltage range.Based on the model,the relationship among power loss of key circuits,current operation frequency and resonant parameters under certain operation frequency range can also be given.The design of resonant parameters can be further optimized by taking both of operation frequency range a d loss of circuits into consideration using the a alytic relationship.Lastly,a single pulse burst mode is proposed which can make the control of ultra-low voltage output very simple.Using the control strategies based on the single pulse burst mode,the converter can output an ultra-low voltage with low ripple and short start-up time.A prototype of full-bridge LLC resonant converter is designed and made to test and verify the solutions proposed above.According to the test results,the converter has an operation frequency range between 115kHz to 195kHz when wide output voltage range achieves 126.7V to 256.1 V.Among the wide output voltage range,the peak efficiency is 95.3%.The output voltage ripple is about 5.8V when 20V/2A is output and the start-up time is about 1ms.These results meet the design requirements in the thesis.