Suppressing Methods Of Common-mode Noise In Half-bridge LLC Resonant Converter

With the rapid development of information technology,the off-line AC/DC power supply is widely adopted in territories such as communication,computer server and data center power architecture.The off-line AC/DC power supply usually consists of three parts: the PFC converter,the isolated DC/DC converter and the EMI filter,in which most of the DC/DC converters are using LLC resonant topology.Although LLC resonant topology has good soft-switching characteristic and can achieve zero-voltageswitching(ZVS)for the primary switching devices and achieve zero-current-switching(ZCS)for the secondary switching devices,the large inter-winding capacitance in the transformer conducts severe common-mode(CM)noise and the EMI filter still occupies about one fifth of the total volume.In order to improve the power density of the power supply and reduce the volume of EMI filter,it is significant to reduce the CM noise in LLC resonant converter.First,for the half-bridge LLC resonant converter using hybrid transformer structure and constructing with spiral litz-wire winding and PCB winding,this dissertation focus on optimizing the transformer structure to reduce the CM noise,and several suppressing methods are tried.First,the CM equivalent model of the LLC resonant converter is built to analyze the dv/dt distribution along the winding,and the transformer windings are rearranged following the principle to achieve the smallest CM noise.Taking advantage of such a good winding arrangement,an improved shielding method is proposed to reduce the CM noise which conducts between the primary and secondary winding.Furthermore,wrapping the transformer with a shielding case and inserting a shielding layer between the windings and the core are adopted to reduce the CM noise conducting between the winding and the core.Finally,an experimental prototype is built to verify the CM noise reduction effect.In order to improve the power density,the switching frequency of the converter is usually pushed to MHz and the transformer is using printed circuit board(PCB)as windings.But the inter-winding capacitance is larger in the planar PCB transformer,and the CM noise is more severe,which not only drives the converter operating in the wrong condition,but also interfaces even damages the equipment around.In conclusion,it is also urgent to solve the large CM noise in MHz LLC resonant converter.For the MHz half-bridge LLC resonant converter using planar transformer and constructing with all PCB windings,the primary winding and secondary winding are usually fully interleaved to reduce the loss caused by the eddy current,so inserting shielding layers is not suitable anymore because it makes the transformer structure more complex and reduces the PCB utilization.So the dissertation reduces the CM noise by optimizing the topology and the control strategy.First,the Fourier Transform is applied to locate the key factor which influences the CM noise,and the result is that the ?v on the winding determines the CM noise.On this basis,a static point construction method is proposed to reduce the CM noise.In this method,the primary side is separated into several half-bridge topologies,so the ?v on the winding can be reduced.Besides,a series inputs and parallel outputs(ISOP)LLC resonant converter is built using the static point construction method,and several optimizing interleaving control strategies are adopted to achieve the cancellation effect and further reduce the CM noise.Finally,an experimental prototype is built to verify the CM noise reduction effect using the optimized ISOP topology and interleaving control strategies.