Design Of Primary Side Regulated Flyback Converter Based On Mixed Mode Modulation

As the heart of all kinds of electronic products,power management chips played an important role in the Internet of Things,5G,automotive electronics and other major application fields,and the market has increasingly high requirements for power customization.The primary-side regulated flyback converter was widely applied in low-cost power supply with medium and low power due to its small size,natural input-output isolation and other features.The primary-side regulation technology avoided the use of environmentally sensitive optocouplers,its stability was improved,which was a hot spot in the research of flyback power in recent years.The basic principle of the primary-side regulated flyback converter was firstly introduced in this paper,including different operating modes and modulation methods,then the average switching model method was used to model and analyze the flyback converter,obtaining the system AC small signal model,the loop compensation scheme was determined and the related parameters design was completed by analyzing the system loop stability.The simulation results indicated that the system has good stability in the entire load application range of the system.And the system architecture design was analyzed in detail further.Then,one mixed mode switching circuit(PWM-PFM-PWM)was proposed in this paper to improve the efficiency of the system at light load or even in standby mode.PWM modulation mode was adopted(operating frequency was 100kHz)while operating at heavy load;PFM modulation mode adopted(operating frequency range was 25kHz-100kHz)to stabilize the output voltage by frequency reduction while operating at preset light load range;PWM modulation mode was adopted again(operating frequency was 25kHz)when the load was further lighter,reducing the on-time to the limit of light load.Different from the conventional method of judging the load based on the output voltage of the error amplifier to switch the modulation mode,the output current information was sampled directly in this paper,eliminating the deviation caused by the change of input voltage at the mode switching point,realizing the unity of mode switching at the full input voltage range.In addition,the load current detection circuit,the frequency-variable oscillator circuit,the error amplifier circuit,the PWM comparator circuit and the duty cycle limit circuit were designed,and the overall layout was drawn and verified.Finally,all sub-module circuits were simulated and analyzed based on the 0.18 μm BCD process,and the primary-side regulated flyback converter system was verified by pre-simulation and post-simulation.The input voltage of the system was 10-15 V,the output voltage was 15 V,and the maximum output power was 30 W.The simulation results indicated that the system could work at the range of full input voltage and full load,and make a correct judgment according to the actual load,then switch to the corresponding modulation mode.The output voltage error was within ±5%,and the voltage ripple rate was less than 0.7%,which met the design requirements.