Research On EMI Performance And Control Strategy Of Secondary Resonant Converter

Flyback converter is popularized in small and medium power applications because of its simple circuit and low cost.There are many disadvantages in the converter,such as high voltage and current stress,serious electromagnetic interference and so on,which affect its high frequency and high efficiency performance.It is necessary to accurately analyze and optimize its steady-state characteristics and EMI level.The secondary resonance technology is used in the flyback converter in this paper,and the EMI noise suppression technology and the control strategy are further studied.In view of the above problems,the secondary resonance technology is applied to suppress noise.Lower voltage stress and current stress are gained on account of the voltage of the resonant capacitor and a lager magnetic inductance.The voltage stresses of rectifier diode and resonant diode are clamped at the output voltage because of the position of resonant cavity,and the resonant diode can achieve ZCS for a suitable resonant period.The coupling path of the conduction interference is analyzed based on several parasitic parameters,such as earth capacitance of the switch and the rectifier diode,coupling capacitance of transformer.The equivalent model of conduction interference is derived.Formula for calculating common mode and differential mode voltage are presented as well,and optimization theory of conduction interference is gained.The theory show that,noise source is suppressed,and EMI level reduces.The Saber simulation and 60 W experiment results validate the theory is reasonable.Aiming at the problem of high voltage and current stress of flyback converter,secondary-side-controlled single end resonant converter is presented here.Although the secondary resonance technology gets lower stress,large current ripple emerges when input voltage varys.Replacing the secondary resonant diode with a switch,the advised topology is presented.Lower current stress and conduction loss are gained.The duty ratio of primary side switch is constant while the secondary one is adjusted to meet the output power constant.The working principle of the converter is analysed and voltage gain curve is shown.A feasible drive circuit scheme is given as well in consideration of driving problem of several switches.Finally,the simulation and experimental prototype verify the correctness of the theory.