Abstract：Usually, to achieve effective insertion loss of EMI filter, the characteristic of noise source impedance in frequency domain should be considered. As known, voltage insertion loss method is the simplest way to test noise source impedance. In this paper, the assumptions used in the conventional voltage insertion loss method were analyzed at first. Then it was found that some of the assumptions did not so strictly hold mathematically that the approximate calculation might lead to some significant errors, and that some calculated results were even contradictory to the assumptions, which made the results invalid. Thereafter, still following the conventional test plan, some correction on the calculation method of the noise source impedance was made, the accurate analytical expression of source impedance was derived and the maximum and minimum values of the amplitude of the source impedance were worked out. Moreover, the deduced results were verified mathematically. Finally, a test on DC/DC converter used on electric vehicles was carried out, in which measurement and calculation was made and the validity and applicability of the proposed method was verified. By the analysis of the test and design case, it is shown that noise source impedance can be calculated more accurately using the improved voltage insertion loss method, which avoided EMI filter overdesign, facilitated filter design and device choice, and aided optimization of filter volume and weight.
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