Research And Design Of A Buck Based LED Controller With Active Power Factor Correction

The rapid expansion of the LED lighting market keeps driving the development of the power management IC thereof and demands more of its performances:smaller size, lower cost, higher precision of output current and higher power factor.Considering the market demands, this thesis mainly focuses on improving the power factor (PF) of LED driver and the precision of its output current. Through refining the system architecture and control methods, a buck power factor correction (PFC) based LED controller with high precision output current is proposed and implemented.The proposed controller adopts the floating ground architecture in which the controller and the power stage operate with isolated grounds, solving issues of traditional Buck constant current converter such as poor stability, low output current precision, and the output current being easily influenced by the inductance as well as the input and output voltages. Instead of voltage mode, peak-current mode or average-current mode control methods, the controller adopts constant-on-time control to simplify loop compensation and the chip size can thus be reduced. The buck converter works under critical conduction mode (CRM), thus input current can follow input voltage accurately and high PF and low THD are guaranteed; besides, with the power MOS transistor switching on under zero current (ZCS), the switching losses are reduced and the system efficiency can be improved. Considering the reliability and integrity of the proposed system, circuit techniques like cycle-by-cycle current limiting and open load protection are also discussed.The proposed buck LED controller has the capacity to drive a 16W LED lighting system with an input voltage range of 85Vac-265Vac. The chip was designed and taped out with CSMC 1μm 40V CMOS process. The chip area is 834μm×896μm. Testing results show that the chip operates normally under 85Vac-265Vac input and 35V-75V output. The output current is 220mA and its error is within±3%. The input current can follow input voltage accurately, and the corresponding PF is as high as 0.98 and THD is lower than 20%. The above testing results show that the designed chip meets the expected specifications and thus verify the proposed architecture and control methods.