Design Of A Two-stage High Power Factor AC-DC Converter

With the rapid development of electronic technology and the increasing of the power of electronic equipment,the design of switching power supply is facing more and more challenges.Among them,it is particularly important to improve the power factor(PF),reduce the harmonic content,improve the power environment and reduce the waste of electrical energy.With the gradual enhancement of people’s awareness of power quality and power environment protection,active power factor correction technology will be more widely used.For the requirements of high PF and high output precision of adapters and chargers for portable electronic products,this paper studies two-stage PFC circuits,including PF control circuit with boost as the main topology and constant current and constant voltage circuit with flyback converter as the main topology.In this paper,a control method based on one-cycle control is proposed to improve PF.Compared with the traditional control method,the complex multiplier is omitted,and the input current is adjusted by controlling the duty cycle.The circuit is simplified,which makes the input voltage phase follow the output current phase.Then,the high PF is realized.The PF controller in this paper adopts fixed pulse width modulation mode(PWM).The circuit is constructed by the relationship between duty cycle and input current.The sampling value of the output voltage is integrated twice,then the conduction time is sampled by comparing the difference between the output voltage and the input voltage,and the duty cycle is obtained by logical control.The back stage constant current and constant voltage adopts the original feedback structure,and the constant voltage control adopts the PWM mode.The conduction time is regulated by the change of load to achieve constant voltage.The constant current output adopts pulse frequency modulation(PFM).Demagnetization time is detected by auxiliary winding,and frequency is adjusted by voltage controlled oscillator(VCO),which makes the product of demagnetization time and frequency remains unchanged and constant current is realized.According to the theoretical analysis,the paper gives the block diagram and circuit simulation design of the system.The chip designed in this paper adopts NEC 1um 5V/40 V HVCMOS technology to design the circuit.Then the layout of the circuit is drawn.Finally,the prototype is tested.The results show that the PF value of the system can reach more than 0.99 and the THD is less than 20% under different input voltages and loads.The output voltage precision of the latter stage is less than 1.6%,and the linear adjustment rate and load adjustment rate are less than 1.5%.The accuracy of constant current is 1.8%,the linear adjustment rate is less than 2%,the load adjustment rate is less than 2.8%,and the conversion efficiency is more than 78% under different conditions,which meets the design targets.It can be seen that the control chip proposed in this paper has a good application prospect in small and medium power adapters.