Common-Mode Noise Modeling And Reduction For MHz GaN Based Multiple-Output DC-DC Converters

With the improvement of system integration,the technique of isolated multiple-output dc-dc converters have been widely used in commercial,industrial and military applications.In order to power supply for different loads requiring legacy voltages in the electronic circuit,a multiple-output dc-dc converter is typical used to convert high bus voltage to multiple low voltages of ±12 V,5 V,3.3 V,etc.To ensure the entire system functions properly in its electromagnetic environment,electromagnetic interference(EMI)filters are usually adopted to ensure that these converters meet the EMI standards.In order to improve the power density of the whole system,it is important to achieve low EMI emissions to shrink the size of EMI filters.Differential-mode(DM)EMI can be effectively suppressed by the DM filters.However,compared to DM EMI,common-mode(CM)EMI has more complicated noise propagation path.Hence,it is more difficult to be identified.Besides,owing to the limitation of the leakage current,the CM filters usually consist of small capacitances and large CM inductances that are bulky,which reduces the power density seriously.It is essential to characterize the CM emission of the multiple-output converters to seek for effective and economic suppression technique.The multiple-winding transformer is a key part of isolated multiple-output dc-dc converters.It usually has the primary,auxiliary,and several secondary windings on the same core,which is adopted to provide galvanic isolation and a variety of lower voltage levels.It is a serious challenge to identify the equivalent lumped capacitances of multiple-winding planar transformers for CM noise analysis to optimize the EMI filters.In this thesis,the couple-turn with its one-capacitor model is proposed to quantify the total CM noise current via the high frequency transformer.With the modeled total CM noise current,the equivalent lumped interwinding capacitance model of the transformer can be derived conveniently and accurately.A complete CM noise path model is built for a gallium nitride(GaN)based active clamp forward with self-driven synchronous rectification(SR)multiple-output converter.Then,a CM noise cancellation capacitance is applied and optimized to generate a reverse extraction current to cancel the CM noise current.Compared with other methods,the proposed modeling technique can build the equivalent lumped interwinding capacitance model of the multiple-winding planar transformer in the printed circuit board(PCB)layout stage before it is fabricated physically.The impacts of transformer leakage inductance and GaN device on CM noise are also analyzed and provided in this thesis.The proposed techniques are verified by the experimental results with a 1-MHz GaN prototype with 100 V input,5 V /6 A and ±12 V/ 0.83 A outputs.The CM noise spectrum is well predicted.The CM noise is reduced by around 15 dB(a reduction of 12.6%)below 9 MHz.