Study On Isolated Resonant DC/DC Converter Based On GaN

The developmental tendencies of power converters are high efficiency,high power density,high reliability and low cost.Power semiconductor devices based on Gallium Nitride(GaN)materials have the merits of smaller gate/output charge,smaller on-resistance and no reverse recovery loss compared with state-of-the-art silicon MOSFET.Thus,it can operate at tens of megahertz or even higher frequency,greatly improve the operating frequency of power converters.With the increasing of switching frequency,the volume of passive components would be reduced dramatically,and the power density of power converters would be improved greatly.Therefore,very high frequency(VHF)power converters have become a new research field of modern power electronics technologies over the last few years.In this thesis,a 10 MHz isolated resonant DC/DC converter based on GaN HEMT is proposed and designed in detail.The power stage of the isolated resonant DC/DC converter is composed of Class DE resonant inverter topology,Class E resonant rectifier topology and air-core planar transformer,in addition,the operating principle of the Class DE resonant inverter and the Class E resonant rectifier are analyzed carefully in this thesis.During dead time of driving signals,the shunt capacitor of the power transistors are charged or discharged and its drain-source voltage will reduce to zero when the driving signal of power switches is applied.That is to say that the power semiconductor devices can achieve ZVS turn-on by calculating dead time and duty cycle of driving signal reasonably.Based on aforementioned theory analysis,the component values of inverter are attained.For the purpose that the input equivalent resistance of rectifier stage matches with the output resistance of inverter,the resonant component values of rectifier topology are designed by sweeping method.PCB planar transformer have the merits of lower volume and weight,faster heat dissipation,better parameter consistency.Air-core planar transformer is used in this thesis for electrical isolation between primary side and secondary side of power converter.The geometric model of air-core planar transformer is built up in HFSS and the geometric parameters of PCB winding including winding copper width,inner radius of winding,distance between primary and secondary side winding,turns,are scanned separately.Then,the relationship curves between the geometric parameters and the performance parameters of planar transformer,such as winding inductance,quality factor and couplingcoefficient etc,are obtained.Finally,combining with the design indices,the geometric parameters of the PCB winding can be determined by Finite Element Method(FEM).Hysteresis ON/OFF control is used to achieve closed-loop regulation for output voltage of DC/DC power converter.Low-frequency ON/OFF control signal generated by hysteresis comparator could control the ON or OFF of DC/DC power converter.When converter is in the state of ON,the power converter operates in the optimum state,that is to say,the power semiconductor devices of inverter topology can achieve ZVS turn-on,and the output voltage of DC/DC converter rises.When converter is in the state of OFF,the output voltage decreases accordingly.Output voltage of DC/DC power converter fluctuate between the upper and lower threshold voltage of comparator through this control method,thus the stability of the output voltage can be achieved.Finally,a 10 MHz isolated resonant DC/DC converter prototype with 20 V input,5V/1A output is built up.The experimental results show that the eGaN HEMTs can achieve ZVS turn-on.The coupling coefficient of air-core planar transformer reach up to 93.5% at the frequency of 10 MHz.The output voltage ripple of the DC/DC converter also meets the relevant design index.