On-line Measurement Implementation Of EMI Source Impedance Of Switching Power Supply

Among various ways to suppress electromagnetic interference, EMI filter design is an effective method for reducing the conduction interference in circuits. The development of the method for measuring the noise source impedance has greatly improved the efficiency of EMI filter design. However, the current noise source impedance measurement is separated from EMI filter design, making it impossible for the two processes to integrate with each other. This kind of off-line design fails to enable the filter to track the actual noise sources and to make corresponding changes, and at last, leading to the failure of the filter.In this paper, a dual-current-probe approach based on the frequency characteristics tester is introduced, to facilitate on-line measurement of noise source impedance. This paper analyzes and verifies the importance of the noise source impedance measurement on EMI filter design, and also provides details in introducing the application of dual-current-probe approach in noise source impedance measurement. A current probe is designed and made on the basis of the analysis of the test principles. It also provides the way to improve the testing methods of short-circuit impedance, and the experience of managing to measure the noise source impedance of the equipment under test(EUT) by following the test procedures. To make on-line measurement possible, a digital frequency test platform is designed and built on the basis of the technology of direct digital synthesizer(DDS) and FPGA, whose detailed design principles, design scheme and design specification can be found in this paper. A PCB and a simple frequency characteristic tester are also successfully made. The output of the frequency sweep signal source and the spectrum test function of the equipment are also tested and verified through experiments.An on-line measurement of the CM and DM noise source impedance in a micro PV inverter is carried out with the dual-current-probe approach, based on the frequency characteristics tester, used in this paper. The validity of this measurement method is verified after it is compared with normal source impedance test methods.