Analysis And Design Of Three-level LLC Resonant Converter Opreated At DCX Considering Parasitic Capacitance

Bus converter is usually adopted in power conversion systems to achieve electrical isolation and voltage matching.LLC resonant converter can realize soft-switching for the power devices and thus achieve high efficiency.In this thesis,LLC resonant converter is selected as the topology of bus converter,and the operation and optimized design is discussed when considering the parasitic capacitance.For high input voltage applications,the three-level LLC resonant converter is chosen in this thesis for reudcing the voltage stress of the power switches.Thus,appropriate available power devices could be easily obtained.The operating principle of the three-level LLC resonant converter is analyzed,and the voltage gain of the converter is derived with the fundamental approximation method.By intentionally setting the switching frequency slightly lower than the resonant frequency,the voltage gain of the converter is nearly constant and is almost independant of the load,exhibiting an excellent characteristic of dc transformer(DCX).Meanwhile,the power switches can realize zero-voltage-switching(ZVS),and the secondary rectifier diodes can realize zero-current-switching(ZCS).So,the three-level LLC resonant converter operated at constant switching frequency is very suitable for implementing the bus converter.In practical circuit,there are parasitic capacitances,including the transformer winding capacitance and the rectifier diode junction capacitance,which will result in significant increase of the output voltage at extremely light load.In this thesis,the operating principle of the three-level LLC resonant converter at light load is analyzed when considering the parasitic capacitance.In this case,the expression of the voltage gain with the fundamental approximation method is not accurate any longer.So,the time domain analysis method is used here,which solves the time domain expressions of each mode of the converter at light load,and the expression of the voltage gain at extremely light load considering the parasitic capacitance is derived.The reason why the output voltage significantly increases is explained.This is because the parasitic capacitance will cause the resonant inductor current to be larger than the magnetize inductor current when the transformer primary voltage changes its direction,and the surplus current will be transferred to the load when the transformer primary voltage commutation is completed,which makes the output voltage increase significantly when the load is very light.Finally,a prototype of three-level DCX-LLC resonant converter with 640 V input voltage,40 V output voltage,160 W power and 120 kHz switching frequency is fabricated and test in the lab,and the design of the main circuit parameters and control circuit is given.Meanwhile,the transformer windings is optimized for minimizing the winding capacitance.The experimental results are provided,and it is shown that the prototype of the converter has good characteristics of DCX and the increase of output voltage at extremely light load is very small,and the power devices realize soft-switching and thus a high effciency is achieved.