Modeling And Active Suppression Of Electromagnetic Radiation In A Flyback Converter

With the gradual increase of switching frequency,the electromagnetic interference(EMI)caused by high-speed transient voltage(d V/dt)and transient current(d I/dt)becomes more and more serious when switching power supply is working.These EMI energy will conduct between the wires in the low frequency band,and then pollute the power grid and affect the normal work of the surrounding electrical equipment.In the high frequency band,it will radiate into the free space in the form of electromagnetic wave,which not only affects the service life of the equipment itself,but also causes interference to the working electrical equipment around,and even causes harm to human body and ecological environment.Therefore,switching power supply products need to pass EMC safety certification before they can enter the market.Flyback converter is the most widely used isolated power electronic converter topology in the current small and medium power switching power supply market.Effective radiated interference prediction and suppression methods can avoid the risk of EMI problems to the greatest extent at the beginning of switching power supply product design,greatly reduce the period of product EMC rectification,and accelerate the product update iteration.In this paper,the electromagnetic simulation and prediction technology of radiated interference in flyback converter and the active suppression method of radiated interference based on fractional order chaotic spread spectrum are studied.Firstly,the high-frequency equivalent model of the components is established,and the loop analysis of the flyback converter studied in this paper is carried out,and the loop simulation is carried out in the SIMetrix/SIMPLIS circuit simulation software.Furthermore,using a simplified two capacitor model of high frequency transformer,the transfer function of common mode noise path is established,and the common mode noise path of flyback converter is analyzed.Matlab is used to simulate the common mode interference of flyback converter.The numerical results are consistent with the simulation results of saber software,which verifies the correctness of the established common mode noise model and proves that the common mode noise comes from V_ds.The interference of V_ds is the direct source of common mode noise.Secondly,based on the antenna theory,it is analyzed that the radiation of the vertical antenna is greater than that of the horizontal antenna,and the theoretical basis that the radiation of the components on the board can not be ignored is clarified.The pins of MOSFET are equivalent to radiation antenna,and the finite element model is established in HFSS.The S-parameters and electromagnetic field distribution of the package structure are obtained by simulation.Furthermore,the finite element models of H-shaped and quincunx shaped heat sinks are established,and the radiation effect of MOSFET antenna with heat sink is considered.The electric field,magnetic field distribution and antenna gain are analyzed,and the conclusion that the radiation generated by quincunx shaped heat sink is higher than that of H-shaped heat sink is obtained.A MOSFET driving circuit is built,and the experiment is carried out by near-field probe.The correctness of the simulation results is verified.Thirdly,the basic principle of field circuit coupling simulation of flyback converter circuit board with ANSYS SIwave,Circuit designer and HFSS software is described.The S-parameters and electromagnetic field distribution of MOSFET structure are obtained by HFSS simulation.Combined with the high frequency equivalent model of transformer obtained by theoretical calculation and experimental test,the board level radiation of a 5W flyback converter is simulated by ANSYS SIwave,Circuit designer and HFSS software,and the 3m far-field prediction curve and near-field electromagnetic field distribution are obtained.Compared with the 3m far-field test curve,the simulation error of the two transformer high-frequency models is investigated,and the radiation intensity of MOSFET and transformer secondary side diode is the highest.Finally,through adamain algorithm,the fractional Lorenz Chaos is simulated on MATLAB,and the phase diagram and chaotic sequence of fractional Lorenz Chaos are obtained.QR decomposition algorithm is used to analyze the Lyapunov exponent spectrum of fractional Lorenz system and the spectrum and variance of fractional Lorenz chaotic sequences of integer order,2.7 order,2.4 order and 1.8 order when the order changes.The theoretical conclusion that the spread spectrum effect of 1.8 order Lorenz chaotic sequence is the best is obtained.The fractional order chaotic PWM is realized in STM32 single chip microcomputer,which acts on flyback converter.Through the radiation test of the near-field probe and the efficiency test of the whole machine,it is verified that the fractional Lorenz can actively suppress the radiation interference of the flyback converter while ensuring the efficiency of the whole machine.