More The Research And Design Of The Flyback Converter Modulation Mode

Since the nineteen sixties, switching power supply has experienced three stages, namely, the development and application of high voltage MOSFET, the improvement of switching frequency,the intelligent application of the whole integration technology. Based on the switching power supply has the advantages of small volume, light weight, high efficiency, low cost,it has been widely applied in medium and small power range. Compared with linear power supply, the circuit of switching power supply is complex.Besides, it has high electromagnetic interference, big noise and is difficult to design.But switching power supply has the advantages of power direction of the mainstream.Now with the research of control and modulation technology of switching power supply and development of new power device, its disadvantages are gradually being reduced and its scope of application become more and more bigger.This paper design a control IC for flyback AC/DC whose input is85-265VAC, output power is30W and output voltage is12VDC.It’s applied to the power adapter, battery charger, digital photo frame, set-top boxes and other.Current mode and PWM mode are adopted.BURST MODE and PFM mode are used to implement in full load range of high efficiency.Oscillator produces variable frequency range of22KHZ65KHZ with load changes, avoiding the switching frequency goes into the audio range.A600V power MOSFET is external integrated.TL431and optocoupler element are used for isolating the input and output circuit and sampling the output voltage.Current feed-forward way is used to regulate power tube current.This chip also has the function of frequency jitter, over current protection, overload protection, slope compensation,over current line protection, power supply voltage clamp and other.On the basis of theoretical knowledge the sub circuit modules are designed, including the bandgap reference, linear regulator LDO, frequency jitter oscillator and mode control module.The circuit is simulationed by HSPICE.The external circuit is analyzed,mainly inductive selection.At the end the chip combined with peripheral circuit is verified by simulation and the function of the chip can be basically achieved.