Design Of Primary-Side Controlled Flyback Converter With High Current Precision For AC-DC LED Driver

As the fourth-generation light source, LED has the advantages of high luminous efficiency, long lifetime, energy conservation and environmental protection, which is playing a more important role in general lighting applications. LED driver, which is the power supply of LED lamps, its performances directly affect the characteristics of LED lamps. Due to the small space, low cost and high reliability, LED driver based on flyback topology is widely used in low power household lighting application. Without the using of opto-couple, the primay-side controlled flyback for LED driver, which can further reduce the space and cost, is drawing much attention.By researching the state of the art of the primary-side controlled flyback AC-DC LED driver and the related commercial chips, the constant current control schemes are divided into five kinds, the control principles, adavatages and disadvatages, and the favorite application environments were analyzed to provide guidance for designers. The valley switching technology which can reduce the switching loss is also introduced. The switching turn-off delay time Td is the key parameter that affects the constant current precision, however, the delay time Td varies with the input voltage so the primary-side peak current is hard to be detected accurately. To improve the output current precison, an adaptive compensation method is proposed in this paper, the delay time Td and the input voltage are detected cycle by cycle and the voltage on the capacitor is regard as the primary current signal. This method is simple and can adapt to different peripheral structures.A primary-side controlled flyback AC-DC LED driver is designed based on output current estimation control scheme. The valley switching is realized to reduce the switching loss and the secondary diode on time is sampling accurately by hysteretic detection method. The adaptive compensation method is adopted to improve the primary-side current detection precision. The main control modules of the LED driver are designed and simulated, and the key sub-circuits in transistor level are designed based on CSMC 0.25 ?m BCD process. The modular simulation results show that the proposed compensation scheme can effectively improve the output current precision: the output current control error is less than 1%, the system power factor is greater than 0.9, and the efficiency is greater than 88% within the whole electric supply.To verify the proposed adaptive compensation method, a hardware test circuit is designed based on commercial chips,and the transformer, RCD clamp circuit as well as other system parameters are designed. The test results show that the output current control error is less than 1%, proving the feasibility and effectiveness of this method.