Design Of Flyback Constant Current And Constant Voltage ACDC Converter

In recent years,the application of switching power supply technology has been further developed,which plays a very important role in LED,household appliances,car charger and some factory production equipment.With the rapid development of technology,the demand for charging equipment will be higher.Low power consumption and isolation switch charger are the first choice for small and medium power devices,such as power supply,light-emitting diode,etc.In this paper,a high efficiency flyback constant current constant voltage AC / DC converter is designed.The adjustable frequency mode is used to reduce the loss under low load and improve the high load efficiency.The valley bottom conduction is used to further improve the transmission efficiency.The main work and innovation of this paper include:1.The system design of flyback AC-DC converter is completed,and a design method combining secondary side feedback and primary side feedback is proposed.The secondary side optocoupler feedback is used to realize constant voltage output,and the primary side feedback auxiliary winding is used to collect current to realize constant current output,which effectively realizes the judgment of output load and increases the accuracy of the chip.By selecting the appropriate turn ratio of transformer and adding RCD protection circuit,the output ripple of the circuit is smaller and the loss of diode is lower in practical application.2.The chip circuit of flyback AC-DC controller with high efficiency is designed.The combination of pulse width modulation(PWM)and pulse frequency modulation(PFM)makes the converter work efficiently under both low load and high load conditions.The valley bottom conduction mode is adopted to reduce the output current peak,reduce the switch loss,and suppress the electromagnetic interference noise.In order to ensure the stability of the chip,the leading edge blanking technology is adopted to reduce the instability and logic error caused by small disturbance.The soft start module is added to the circuit to make the current over-current point gradually increase with the working cycle.The starting speed of the converter is relatively stable,which reduces the impact of power grid and increases the service life of the converter.3.Various types of protection circuits are designed,such as over temperature protection,under voltage protection,over-current protection,to ensure the safety of the chip in different special situations.The design adopts 0.18μm 5V/40V/700 V BCD process,the chip size is1035μm×755μm.Cadence and HSPICE are used to simulate the circuit.The input voltage is85 V ～ 265 V,and the output frequency is between 25 KHz and 130 KHz.The output current of constant current control is 1A and the output voltage of constant voltage control is 12 V.The output accuracy of constant current and constant voltage is ± 5%and ± 2.5%,respectively.The standby power consumption is less than 75 m W and the maximum output power is 60 W.It has good output performance and the results meet the design requirements.