Modelling And Analysis Of Conducted Emi In A Small And Medium-Sized Power Of Quasi-Resonant Flyback Converter

Flyback converter is an important topology of switching power supply. It is widely used in small and medium-sized power consumer electronics. Quasi resonant flyback is a special kind of flyback structure, has the advantages of low switch loss and high energy conversion efficiency. However, its high-frequency switch actions will cause serious conducted electromagnetic interference problem. Therefore, its conducted electromagnetic interference has been paid great attention.Based on a small and medium-sized power quasi resonant flyback converter, conducted electromagnetic interference analysis model has been proposed in this thesis, which can be used to predict the conducted electromagnetic interference level quickly. The model includes the noise source and conduction path model and simplified model of the receiver. Firstly, the principle and key voltage waveforms of quasi resonant flyback are analyzed, the switching action of MOS is determined as the main noise source. Secondly, equivalent circuits of noise paths under different working conditions are proposed based on the analysis of common mode and differential mode noise transmission path. The expression of noise voltage is derived from the equivalent circuits. Then the full conducted electromagnetic interference analysis model is obtained with the construction of simplified receiver model. Based on this model, the impacts of devices and parasitic parameters on the peak conducted electromagnetic interference of the system have been studied with the help of MATLAB. Coupling capacitance of the noise source to gound is found to be the main factor which affects the conducted electromagnetic interference of the system. According to the results of the analysis, optimization of a 40w quasi resonant flyback prototype has been conducted to improve its conducted electromagnetic interference characteristics. Finally the accuracy of the analysis model and the optimization method has been verified by the test.Simulated and measured results show that the error of conducted electromagnetic interference curve at peak frequencys can be controlled in 5dB. Test results also show that conducted electromagnetic interference of the optimized system is about 10 dB lower, meeting with the class B ITE EN55022 conducted emission limit requirements. This reflects that the proposed model has played an effective guiding role in the optimization design.