The Design And Research Of Single-stage PFC Flyback LED Driver

With energy source crisis and environmental contamination are deteriorating continuously, LED(light-emitting diode), a solid illuminant, has become a new kind of green illuminating products due to its distinct advantages such as high efficiency, long lifetime, environmental friendliness, which guidelines the future developing trend of illuminants. It can be foreseen that 21st century must be a new age brightened by solid illuminants represented by LEDs.But LED is a low-voltage-driven electroluminescent cold light source, and has a negative temperature coefficient, so a special driven source is needed to provide power for LEDs. Currently, the LED driver power supplies are suffered by several problems such as inconsistent quality, low efficiency, low reliability, unacceptable EMC (Electro Magnetic Compatibility) performance. For the long run, high efficiency, high reliability, small size and precise control are the inevitable requirement of LEDs driver in the future, so research and development of high-power LEDs drivers, compatible with civil electrical system, hold a strong theoretical and practical significance.Working principle of high-power LED driver is analyzed in this paper. Active two-stage PFC (Power Factor Correction) and single-stage PFC are compared. PFC converters can be classified into two types: two-stage and single-stage. Two-stage PFC converters consist of a PFC stage and a dc/dc stage. They have been widely applied in adaptors for Laptops and silver box. Single-stage PFC converters integrate the PFC stage and the DC/DC stage, leading to simple topology and low cost. They are suitable for low-power applications. Unfortunately, no effective method has been proposed to significantly reduce the storage capacitance in a PFC converter such that a long lifetime of a converter can be achieved. That single-stage PFC is more suitable for LEDs driver power is concluded based on the consideration of secure isolation, low cost, high power coefficient. Then single-stage PFC flyback switching power source is examined further, some improvement is adopted to make it more effective.In accordance with the requirements of high-frequency switching power supply,three single-stage high factor correction LEDs driver power supplies are designed and implemented, one with an output power of 30W, the other two with an output power of 50W of which synchronous rectifier technique is used.The principal controlling chip (L6561) in this single-stage power source is working in Transition Mode, i.e. on the boundary between continuous and discontinuous inductor current mode, therefore at a frequency depending on both input voltage and output current, achieving power factor correction as well as DC/DC step-down and electrical isolation between high and low voltage, which provides a constant current for LED, ensuring the stability of LED luminance. By analyzing the special structure of single-stage PFC power supply, a constant pressure limiting feedback loop design is achieved to meet the special requirements of constant LED current. Thermal issues of power supply are taken into account to sustain excellent heat dissipation and improve the reliability.Energy preservation of primary inductance and secure voltage-decreasing function of flyback transformer must be integrated in single-stage PFC flyback switch power source transformer, so it necessitates especially elaborate design. After working principle of single-stage PFC flyback switch power source transformer analysed, a transformer with high transmission efficiency, low EMI, and low temperature rise, that is suitable for single-stage PFC flyback switch power source, is designed and implemented according to the fundamentals of transformer design.TD clamp circuits, RCD clamp circuits, and TRCD clamp circuits are researched and compared, and TRCD clamp circuits are chosen to improve the efficiency, reduce the electromagnetic interference of this system, and enhance reliability of clamp circuit. It is a real challenge to comply with EMC norms regarding the THD of line current, especially in universal mains applications. In order to reduce EMI of the power source, sustain good EMC performance, the PCB design of power supply is elaborately optimized. After a detailed analysis of the single-stage PFC principle of electromagnetic interference, EMI filters, suitable for single-stage PFC, is devised to reduce the EMC conductive interference of power supply to meet the GB17743 standard.Power dissipation of MOSFET, transformer, clamp circuit and output rectifying circuit in this power source are dissected in depth, CoolMOS switching transistor, improved transformer design, clamp circuit design and output rectifying circuit design are used to reduce their power dissipation, respectively. Synchronous rectifier control technology is adopted to elevate the efficiency of power supply, reaching 88%, and maximum of 91%. The temperature rise of the power supply has been reduced efficiently to only 30℃, enhancing the reliability notablely.