The Research And Design Of A Flyback AC-DC Controller With Constant Current And Constant Voltage Output

With the rapid development of integrated circuits,more and more portable electronic devices have entered the civilian market,so the market for power management chips that provide energy for these electronic devices is also constantly expanding.The primary side regulation flyback AC-DC converter is favored in portable electronic devices due to its low power consumption,fewer peripheral components required,and effective electrical isolation of input and output.Aiming at the performance requirements of chargers and adapters for portable devices,this paper designs a high-precision constant voltage constant current flyback AC-DC controller chip.The AC-DC controller chip designed in this paper uses the primary side regulation structure to omit the optocoupler device and TL431,which simplifies the volume and cost of the peripheral circuit.The chip adopts the PFM scheme and operates in DCM mode over the full load range,and has high precision constant current and constant voltage output function.Firstly,this paper analyzes the basic theory of flyback AC-DC and determines the overall framework and design specifications of the chip.Then,this paper elaborates on the factors affecting the accuracy of constant current and constant voltage,and finally adopts high-precision control method.After completing the circuit design and simulation of the sub-module such as the bandgap reference voltage module,the oscillator module,the demagnetization time detection module,the constant current control module,and the constant voltage control module,the whole system is simulated.Finally,the layout of some circuit sub-modules is designed.In constant current control,twice the demagnetization time is used as the switching period.The output current of this constant current control method is not affected by the primary inductance value,and the primary inductance compensation circuit is not needed,which can achieve accurate constant current control.In constant voltage control,the output voltage is accurately sampled at 2/3 of the demagnetization time,and a step-down wave is generated by the charge pump,which is compared with the output of the error amplifier to control the cut-off time of the system.This method can obtain large cut-off time and switching period,so that the system can also obtain stable output voltage under light load,expanding the load regulation range.And this chip designs a cable voltage drop compensation circuit.The constant current source array is controlled by switching period to generate compensation current proportional to the period.The compensating current is injected into the divider resistance to compensate the voltage drop on the charging cable in practical application,improving the constant voltage output precision.Finally,the peripheral topology applied to 12V/1A is built.Based on the Dongbu 0.18?m BCD process,the constant current and constant voltage function of the entire system was simulated using the Cadence spectre tool.The experimental results show that the chip can achieve the correct function conversion of constant current and constant voltage.The output current of constant current control mode is 1.3A,and the residual of 0.3A is set,which can be adjusted by the resistance of the original inductance current detection.The output current ripple is 25 mA.Under constant voltage control,the output voltage is 12 V,and the output voltage ripple is 20 mV.The constant voltage output line regulation is ±1.4%,the load regulation is ±1.5%,and the constant voltage output precision is ±2.5%.The system has good constant voltage output performance.The simulation results show that the control chip can achieve high precision constant voltage and current control,and meet the design requirements.