Analysis And Optimization Of High Frequency Parasitic Parameters Based On Half Bridge LLC Resonant Converter

The half-bridge LLC resonant converter is widely used in systems sensitive to power supply volume and efficiency due to the characteristic of high efficiency,simple structure and high power density.However,the high frequency parasitics cause lots of problems such as soft-switching failure,large gain error,resonant current malformation and high voltage oscillation.It is difficult for the half-bridge LLC resonant converter to achieve stability and high efficiency and the parasitics limit the further improvement of the operating frequency and power density of such converters.In view of these problems,the following work is completed:(1)An S-domain equivalent model considering the parasitic capacitance of the secondary side is established.Based on this model,it can be analyzed how the parasitic capacitance causes resonant current malformation and soft-switching failure.The results show that the parasitic capacitance resonates with the resonant inductor of the primary side,resulting in the resonant current malformation.The larger the capacitance is,the more serious the current malformation is.The parasitic capacitance causes the softswitching to fail because it shares the charging current of the primary side,which increases the switching loss of the system.(2)A FHA model considering the parasitic inductance of the secondary side is established.Based on this model,it can be analyzed how the parasitic inductance causes rectifier voltage oscillation and system gain error.The results show that the parasitic inductance resonates with the junction capacitance of the rectifier,resulting in the rectifier voltage oscillation.The larger the inductance is,the larger the oscillation amplitude is.The parasitic inductance causes the voltage gain to deviate from the traditional curve near the resonant frequency,resulting in a large system gain error,which increases the rectifier losses of the system.(3)A double pulse test model considering the GaN HEMT gate drive parasitic is established.Based on this model,it can be analyzed how the parasitic parameters cause high voltage oscillation and gate drive glitch.The results show that the drain-source capacitance of the GaN HEMT resonates with the inductance of the power loop,resulting in the high-voltage oscillation.The larger the capacitance is,the larger the oscillation amplitude is.The instantaneous current generated by the dV/dt coupled to the gate-drain capacitance charges the gate-source capacitance to generate gate drive glitch,which increases the drive losses of the switch.According to the analysis results,this paper improves the traditional design by optimizing PCB layout and gate drive loop and selecting the device reasonably.Finally,the effectiveness of the program was verified on a 1MHz,12V/25 A half-bridge LLC resonant converter prototype.The test results show that the rectifier voltage oscillation is reduced from 13.8V to 1.87 V,the gate drive conduction and turn-off glitch are reduced from 800 mV,2V to 100 mV and 200 mV respectively,the high voltage oscillation is reduced from 31 V to 6V,the output ripple is reduced from 500 mV to 250 mV.Current malformation,soft-switching failure and system gain error have been eliminated.In addition,the peak efficiency is increased from 94.5% to 96.8%.The experimental results meet the design specifications.