High frequency modeling for CM noise coupling path of transformer based on transmission line theory

This paper reveals the common-mode (CM) noise coupling path impedance of the transformer for the whole conducted frequency range (150kHz - 30MHz) based on transmission line theory. Lumped circuit model is commonly used to describe the CM noise coupling mechanism through the transformer, but it´s only valid below several mega-hertz due to the influence of distribution parasitic parameters. The transmission line theory provides a good solution for transformer high frequency CM noise coupling path modeling as well as the low frequency. Compare with lumped circuit method, transmission line theory can describe the distribution of parasitic parameters along the winding, reveal the high frequency resonant phenomenon in depth and calculate it accurately. Based on the proposed model, CM noise spectrum is calculated and analyzed, which is validated by measured results.