Research on Noise Soure Impedance Extraction Method Based on Inserting Passive Two-Port Network

Insertion loss (IL) of EMI filter is closely related to noise source impedance and load impedance. Therefore, in order to make the EMI filter design achieve the expected IL and achieve good noise suppression effect, it is necessary to consider the impact of noise source impedance. The voltage insertion loss method can only calculate the maximum and minimum values of the amplitude of the noise source impedance, and cannot solve the phase of the noise source impedance. To solve the above problems, this paper proposes a noise source impedance extraction method based on inserting passive two-port network without adding experimental equipment. Combined with experimental measurement and theoretical calculation, the accurate values of the amplitude and phase of the noise source impedance are obtained. First, insert three different passive two-port networks between the equipment under test (EUT) and the linear impedance stabilization network (LISN). Secondly, the equivalent circuit of noise source impedance extraction is established, and then the binary quadratic equations about the real part and imaginary part of the noise source impedance are established according to the equivalent circuit. Finally, through the programming of the mathematical software platform, the binary quadratic equations corresponding to each frequency point are solved automatically and circularly, and the effective frequency points measured experimentally in the conducted EMI test frequency band (9kHz～30MHz) are selected, so that the real and imaginary parts of the noise source impedance can be obtained, and then the accurate calculation of the amplitude and phase of the noise source impedance can be realized. The experimental results show that, no matter the common mode (CM) noise source impedance or the differential mode (DM) noise source impedance, most of the amplitudes of the noise source impedance calculated by the proposed method fall between the maximum and minimum values of the amplitudes of the noise source impedance calculated by the voltage insertion loss method in the conducted EMI test frequency band (9kHz~30MHz), which further improves the calculation accuracy of the amplitude of noise source impedance. In addition, the proposed method can also obtain the exact phase of the noise source impedance, which is not available in the voltage insertion loss method. Finally, the noise current predicted by the proposed noise source impedance extraction method is compared with the noise current actually measured when the LED driver is working. The experimental results show that the two are in good agreement, which further verifies the correctness and effectiveness of the proposed noise source impedance extraction method. The following conclusions can be drawn from the experimental analysis: ① Compared with the voltage insertion loss method, the amplitude of noise source impedance calculated by the proposed method has higher accuracy, and the phase of noise source impedance can also be obtained. ② At the same time, the noise source impedance calculated based on the proposed method can accurately predict the noise current of LISN after inserting EMI filter, which provides a theoretical basis for designing high-performance EMI filter. ③ The proposed method has the advantages of low cost, simple operation, strong practicability and universal applicability. © 2023 Chinese Machine Press. All rights reserved.