Study On Conducted Emi Modeling For Switching Mode Power Supply

The Switching Mode Power Supply (SMPS) is widely used in industry, military, medical and other fields. When either switching frequency or power density of SMPS is increasing, the Electromagnetic Compatibility (EMC) problems cannot be ignored. The Electromagnetic Interference (EMI) problems of SMPS are imminently under solution, especially, for the equipment with high power or large voltage. The Conducted Electromagnetic Interference (EMI) generated in SMPS can propagate along the line and couple into the power grid or other devices, which may break sensitive devices down. Therefore, conducted EMI modeling for SMPS have a good significance on mechanism analysis and suppression of conducted EMI.In this paper, a Boost converter is researched in the fields of conducted EMI measurement and modeling, and a simulation circuit for conducted EMI of the SMPS is built up.Firstly, we research the measurement technology of Conducted Electromagnetic Interference. The separation methods for Common Mode (CM) and Differential Mode (DM) interference are especially focused on. Two novel noise separation technologies are proposed, the first is a noise separation network for CM and DM interference based on Common Mode Choke (CMC). The core elements of the designed noise separation network are a RF transformer and two CMC, whose characteristic impedance equals 100 Ω. The noise separation network has a good simulation performance at the frequency range from 150 kHz-30 MHz. The second is a noise separation method based on current probe measurement and software computation, which is verified by measuring the EMI of a 150w/12V commercial switch power supply. The proposed noise separation method works at either the input power lines or the lines between two devices. On the other hand, we have measured the conducted EMI for the designed Boost SMPS. At the frequency range from 150 kHz-30 MHz, the measurement results show the conducted EMI spectra on phase line and natural line have a coincidence at peak value, which are beyond the B Class limits in CISPR 22. And the CM interference is approximately 10 dB more than the DM. A conclusion about the CM interference is in domain for the conducted EMI of the power lines is made.Secondly, according to the three elements of EMC, we build up the conducted EMI model for the Boost SMPS. Modeling objects of interference propagate paths including passive components, PCB broad and stray capacitance between the MOSFET drain and heat sink connecting to the ground. Modeling objects of the interference sources including the Fast Recovery Diode (FRD) MUR 860 and N type MOSFET IRFPE50. The simulation result of conducted EMI has a good coincidence with the measurement at peak value, which verifies the modeling method.Finally, a simplified model for CM interference calculation is proposed, which is obtained by the analysis on CM interference current propagation paths of the Boost converter. The calculation interference voltage of CM is compared with experiment result, a good coincidence at peak value at the frequency range from 150 kHz to 10 MHz verifies the simplified proposed model, which is practical and valuable for the CM interference prediction.