Abstract:

Aiming at the problems of high switching loss and large du/dt leads to poor EMI of flyback circuit in quasi resonant mode,a valley bottom conduction control method for slope detection is proposed,which effectively improves the detection accuracy of valley bottom voltage,reduces the on voltage of switch tube and reduces the turn on loss.The du/dt of the switch is reduced and the EMI performance of the circuit is improved.Flyback circuit and the principle and working process of the proposed control method are analyzed in detail,the influence of the proposed control method on the EMI characteristics of the circuit is analyzed,and the key parameter design is carried out.Finally,the efficiency curve and EMI characteristic curve of the prototype at power 20~40 W,input DC 430 V and output constant current DC 1.05 A are given.Compared with the widely used demagnetization detection method,the effectiveness of the proposed method is illustrated.

Key words: quasi resonant flyback circuit, primary side feedback, slope detection valley bottom conduction, electromagnetic interference(EMI), switching loss