Research On Bidirectional CLLC Resonant Converter Based On GaN Device

Bidirectional CLLC resonant converter is a bidirectional DC/DC converter that realizes soft switching by resonant mode.In the bidirectional working mode,the zero-voltage turn-on of the inverter-side switch can be realized under all load conditions while the zero-current turnoff of the rectifying-side switch is realized within a certain load range,dramatically reducing the switching stress.The converter adopts frequency modulation control,which is easy to achieve high frequency,high efficiency and high power density.Similar forward and reverse voltage gain characteristics can be obtained by adding only one resonant capacitor compared with the traditional LLC resonant converter.The converter's bidirectional gain range can be greater than 1,so the voltage regulation capability is more flexible.However,the fourth-order resonant circuit makes theoretical analysis and parameter design more difficult.Moreover,since the realization of the soft switching of the bidirectional CLLC resonant converter is related to the parasitic parameters of the device and the increase in power density depends on the increase in frequency and the size of the device,achieving high frequency and high efficiency depends on the switching speed and loss of the device.However,the resonant type converters now mostly use conventional Si MOSFET devices,which have reached the limit in various performance indexes such as switching speed,parasitic parameters and efficiency.Compared with the traditional Si devices,the third generation wide bandgap semiconductor GaN device has the advantages of low conduction resistance,fast switching speed,small parasitic parameters,small size and good voltage and heat resistance,which is a breakthrough point to further improve the frequency,efficiency and power density of switching converters.However,due to the high requirements of the GaN device for the design of driver circuit and the large reverse voltage drop,the current research and application are relatively few.Therefore,this thesis focuses on the research of GaN device and its application in bidirectional CLLC resonant converter.Firstly,the principle and characteristics of GaN device are introduced,and the design of the driver circuit for GaN device is emphatically studied.Then the working principle of the bidirectional CLLC resonant converter is explained specifically.Furthermore,the forward and reverse working modes are analyzed in detail,and the mathematical modeling of the converter is completed based on the approach of fundamental mode approximation.The input impedance characteristics are also analyzed and the analytical solution of the resonant frequency is derived,which leads to the design rule of the bidirectional CLLC resonant converter.In addition,the advantages and disadvantages of GaN device in bidirectional CLLC resonant converter are analyzed,and the key issues in the design of bidirectional CLLC resonant converter based on GaN devices are explained as well.Finally,an experimental prototype with a rated power of 400 W was built.In the forward mode,the input voltage is 400 V and the output is constant current of 8A.In the reverse mode,the input voltage is 48~56V,and the output is constant voltage of 400 V.The highest efficiency of the prototype in forward mode is 97.02% while the highest efficiency in the reverse mode is 95.96%,and the power density is 21.41W/in3.