Conduction EMI Analysis And Optimization Design Of Primary Feedback Flyback Converter

The original side flyback switching power supply has a simple structure,compact circuit and low cost.It is widely used in the small power situation of 5~150W,and occupies a large market in the power adapter and off-line battery charger.But the switch characteristic determines the EMI(Electromagnetic Interference)problem more prominent,it needs to analyze the electromagnetic interference and optimize the design,which is one of the key research points of the current power supply.Aiming at the EMI(Electromagnetic Interference)of the original side feedback flyback converter,this paper firstly analyzes the basic working principle of the power supply topology by taking the main topology,the reason of the EMI noise,the efficiency analysis and the method of reducing the EMI technology as the breakthrough point.Secondly,combining with the circuit principle,the two reasons that the switching modulation waveform and the structure design of high-frequency transformer affect the conduction EMI are obtained concretely.Then the frequency jitter of switching power supply is controlled by triangular wave modulation to improve the EMI at the switching frequency.The mechanism of the second part of EMI problem is studied by analyzing the parasitic parameter model of high frequency transformer,and the frequency jitter circuit is ensured to be realized.Finally,the conduction EMI principle and the optimization design method of the original feedback flyback converter are verified by the verification of the prototype and the actual test.According to the board-level test results and verification analysis,the switching frequency jitter amplitude of the original side-feedback flyback converter prototype is about 5% at 132 KHz switching frequency,and the conducted EMI of the optimized system is about 10 dB lower than that before optimization.The conducted EMI of the transformer with different winding modes is reduced about 20 dB,and can meet the requirements of EN55022 transmission emission limit of class B information technology equipment.On the basis of optimizing the transmission EMI peak value,the efficiency,ripple and stability indicators have not significantly deteriorated,to achieve the desired research objectives.