Research On A Non-isolated Buck Active PFC LED Driver Circuit

LED lighting has many advantages compared to traditional lighting sources, such as high efficiency, long lifetime, high luminance, pollution-free, changeable and so on. With the birth of white LED, as well as the improvement of materials and LED chip research and development, LED lighting will gradually replace traditional lighting. An AC-DC constant current was needed to driver LED, for it worked under low DC voltage condition. As core component, the LED driver chip must have the quality of low power consumption, high efficiency, high current precision and reliability. In addition, since the AC-DC switching power can easily cause harmonic pollution to the power grid, power factor correction technology was needed in application.This article focused on the design of a non-isolated buck high power factor AC-DC LED driver circuit, which applies to 85 ~ 265 Vac input voltage. Using two feedback resistors, along with a single winding structure, zero-crossing time of the output inductor current can be directly monitored, which greatly reduced the system size and cost; With an on-chip active power factor correction module, the PF value was improved, and the volume of the external circuit was reduced; Meanwhile, the selection of inductor current critical conduction mode reduced the power loss and improved efficiency; The full cycle inductor current sampling structure and internal output line voltage compensation algorithm ensured excellent line regulation, as a result, the accuracy of output current was within ± 5% under full voltage range. Furthermore, a series of internal protection circuits were designed to ensure the reliability of the chip, such as: LED short/open circuit protection, CS resistor open circuit protection, VCC under voltage protection, intelligent temperature and so on.When finishing circuit design, the chip overall simulation was presented. Simulation results showed that the drive circuit met preset targets. Finally, after taping out and packaging, DC parameters test and engine test were verified, whose results were all met the requirements. Under different loads and input voltage conditions, the accuracy of output current was within ± 5%. In addition to 265 Vac, PF values were above 0.9. What’s more, based on actual application environment, the PF value of 265 Vac can be improved by adjusting the external circuit parameters appropriate.