Design Of An LED Constant Current Chip With High Power Factor And Low THD

LED, a new generation of lighting source, has been fully developed and widely applied in the people's life. But the harmonic current will cause pollution to power grid when the LED chip is working, which affects the normal function and the energy utilization of the LED chip. Thus,designing an LED chip with active power factor correction technology is of important significance.This thesis introduces the definition of harmonic and power factor and their mathematical expressions. The relationship between the total harmonic distortion and power factor is discussed. In addition, the basic ways of implementing of the APFC is expounded,and merits and demerits of CCM,DCM and CRM are analyzed as well. Finally, the constant on time controlled CRM flyback APFC circuit is researched.Based on the analysis mentioned above, an constant current LED chip with high power factor and low total harmonic distortion is designed in this thesis, which adopted the 0.8um 40 V process. APFC circuit controlled by constant conduction circuit can realize high power factor and low total harmonic distortion. Also,the circuit controlling the power MOSFET with a constant conduction time is builted-in the LED chip,which avoids the high cost and bulk volume caused by redundant discrete devices. The designed LED chip realizes the constant output of the current by controlling the product of the Vcs and(Toff / T) a fixed value. Because the operation mode of the LED chip is CRM, the power MOSFET opens in zero current state,which reduces the switching loss. Compared with the secondary feedback, the primary-side feedback adopted by the chip eliminates optocoupler and the other subsidiary devices, which reduces the volume and the cost of the system and enhances the reliability of the chip as a result.This thesis has simulated chip system circuit and each function module of LED chip via Hspice of the Candence company. These function modules include bandgap reference, internal power supply, voltage bias circuit, under voltage lock out, current reference, VCC clamping circuit, leading edge blanking, over current protection, high and low voltage compensation, zero cross detection circuit, dimming circuit, the biggest demagnetization time circuit, comp pin voltage clamping, constant current control circuit, constant conduction circuit for high power factor and low total harmonic distortion, over temperature protection and load compensation, etc. Meanwhile, the LED chip layout has been completed. Test results show that the power factor is greater than 0.9, the total harmonic distortion is less than 25%, the efficiency is higher than 87% while the linear regulation is within 5% and the load regulation is lower than 3%. The parameters of the LED chip are in accordance with the design requirements.