Research On 9KHz-1GHz Band High Insertion Loss EMI Power Filter

Switching power supply, which is high efficient, small and light, is increasingly widely used in modern electronic equipments. However, as the system becoming more complex and higher integrated, there are more and more concerns about heavy noise and serious EMI that switching power supply provides as its high switching frequency. Nowadays, requirements of EMC standards, especially military standards and confidentiality standards on products' electromagnetic compatibility are becoming more and more severe. So it is very important to guarantee system's stability and pass relevant EMC certification by solving electromagnetic interference of switching power supply.Power filter is an effective means to solve the EMI problems. This paper mainly designs and develops one EMI power filter which is used in low radiating confidential cabinet. In accordance with the conducted emission limit of confidential standards and conducted emission level of equipment under test, this EMI power filter should meet the requirement of 80dB common-mode insertion loss in 9KHz-1GHz band.In order to achieve high insertion loss in such a wide band, this paper adopts multi-stage filter on different band cascade. Besides, circuit structure, filter parameter, core material, shielding and grounding are fully considered. The microcrystalline ferrite core is used to make inductor with large value to meet the requirement of the low frequency KHz band filter and two-stage Mn-Zn ferrite core inductors cascade can provide stable filter performance in MHz band. For the band from dozens of MHz to hundreds of MHz, Ni-Zn ferrite core inductor is added, which can make up for the Mn-Zn ferrite's decline of magnetic conductivity as frequency is increasing. Taking into account that lumped components filter's performance declines in high frequency because of the distribution parameter, the filter is designed in the form of isolation chamber, and capacitance cores are installed to increase the filter performance in high frequency. Capacitance core has little distribution parameter which can achieve good filter performance in hundreds of MHz up to GHz. The isolation chamber can shield high frequency noise against space coupling. All these measures, adding the good grounding and screw hole design in output port, guarantee that EMI power filter has high insertion loss in the broadband.In addition, according to filter insertion loss standard measurement methods, experimental measurement platform is structured. Using spectrum analyzer and vector network analyzer, the common-mode insertion loss of EMI power filter is measured. From the results it can be found that this filter can meet the design requirement very well.This paper also introduces the design of general power supply filter, including design process, design methodology, specific device choice and inductor design. Especially, the impacts on the performance of filter as impedance changing and distribution parameter being are simulated. These methods and simulations will provide recommendation and guidance to EMI filter design.