Design Of Digital Constant Current Controller For Primary-Side Regulation Flyback Converter

The constant current driving performance of LED driver, which is the most important part of LED light, has the direct bearing on the property, safety and longevity of LED light. Flyback converter is applied in small power lighting driver extensively because of its simple structure, small volume and low cost. In recent years, with the development of process conditions, digital control techniques give expression to their advantages in power management area, including programmability, stronger noise immunity, better portability and so on.A digital controller for primary-side regulation flyback constant current converter is designed in this thesis. The proposed digital controller samples the current variation of topology circuit through sampling resistor Rs, and quantified by ADC module. The primary side peak current reference threshold compare with the sampling primary current value in digital controller, in order to control the primary-side peak current to be same in every switching cycle. The comparator module compares the auxiliary side voltage with zero voltage to get a high/low level signal. The demagnetization time is obtained by sampling elapsed time of the first high level through sampling state machine. Meanwhile, digital controller adjusts the ratio between demagnetization time and switching period to be constant by bidirectional counter, and then realize constant output current. In order to boost the accuracy of the output current of the system, this thesis presents demagnetization-time compensation technique to minish the affect by the imprecise demagnetization time sample. The lookup table is used to acquire the compensation amount under different load and then improve the load regulation. In the process of acquire power switch on-time will be produce △t delay, actually the primary peak current will get larger. The primary current change information is acquired through primary-side peak current compensation technique, which is used to calculate compensation amount under At delay, in order to obtain more according with the actual situation primary side peak current reference threshold and enhance line regulation.The system simulation is implemented by the co-simulation between Matlab/simulink and Modelsim. Lastly, digital controlled constant current flyback converter verification platform is constructed based on FPGA to verify the designed digital control technique. The experiment results show that the proposed digital controller can maximize the operating frequency to 86 KHz and the work efficiency to 84%, the line regulation and load regulation are all within 1% under the output current 2A, input AC voltage 110V～220V and output voltage load 2V～7V.