Research Of Conducted EMI Characteristics Of Transformer In Power Converter

The EMC (electro-magnetic compatibility) issue is an important part in the power converter. The isolated power converter transformer has a very important influence on its electromagnetic compatibility characteristics. The paper research on the transformer, the EMI characteristics of the transformer is mainly studied, and the electrical & magnetic comprehensive model of power converter transformer is established, it is not only of theoretical significance, but with good usability.The paper summarizes the research and development status of transformer EMI characteristics, indicates that the capacitive distribution parameters of transformer is one of the key factors of EMI noise clearly. The effective capacitance of the common-mode ports of transformer a certain extent reflects the suppression of transformer to the common-mode noise. The article introduces the calculation and measurement methods of effective capacitance of the common-mode ports and analyzes the effect of effective capacitance of the common-mode port with different structure of the transformer, the different option of Connection terminals, and the different positions of the auxiliary winding, so realizes the impact of these factors on the transformer EMI characteristics clearly.Then the electric energy storage characteristics of distribution capacitance of transformer and the suppression characteristics on common mode EMI noise is analyzed. Then a new transformer model with three capacitors is built which can describe both the behaviors of electric energy storage and CM EMI noise transfer. The new model is also verified for the transformers with multiple-layer windings and multiple windings structure arrangements.A further study on EMI characteristics of the transformer is carried on, and we indicate that EMI noise characteristic parameters of the transformer in the high frequency range is not purely capacitive in nature. There will be a big deviation compare to the actual situation if only use the capacitive distribution parameters to characterize the conducted EMI characteristics of the transformer. We propose an electrical & magnetic comprehensive model based on three-capacitance model which considered the electric potential distribution of transformer windings and the effect of magnetic fields. The model uses the inductance decoupling by mutual inductance to characterize the inductive state of transformer insertion loss. In order to make the model better suited to the transformer with different turn ratio, the article proposed an electrical &magnetic improved model and an electrical & magnetic comprehensive unified model which can be applied to the actual simulation.Finally, a fly-back power converter is researched, the paper analyzes the common mode conducted noise coupling path and differential-mode conducted noise coupling path, and points out that the distribution parameters of the transformer can only affect the common mode noise of the converter, while the differential mode noise is hardly been affected. The electrical & magnetic comprehensive unified model of the prototype transformer is established. To verify the accuracy of the model, we compare the insertion loss curves of the model and the measured insertion loss curves. And the conduction EMI model based on the electrical & magnetic comprehensive unified model of the prototype transformer is created. The common mode noise spectrum obtained by simulation and actual measurement is compared to verify the validity and usefulness of the model.