The Design Of Low Ripple Electrolytic Capacitor-Less AC-DC Led Driving Circuit

LED as a new type of green lighting, is popular with its high luminous efficiency, low-carbon health, safety, energy conservation and environmental protection which promote LED industries becoming the focus of development. Because LED is a non-resistive load, it needs power factor correction technology and limiting harmonic current.As the light-emitting diode drive power uses power factor correction technology, so input current follows input voltage which is a sinusoidal variation that leads to pulse at twice line frequency of output power.So as to make sure of constant output power,so an electrolytic capacitor with short lifetime limits is needed.If the capacitor is too small,output power will be pulsating and the ripple of output current at twice line frequency will be too much.Achieving power factor, avoiding to use electrolytic capacitor and obtaining low current ripple are needed,which demand higher requirements for the design of LED driver circuit.Upon the analysis of BCM primary feedback flyback PFC drive circuit,the adaptive fixed on-time generating circuit is designed in this paper.which could achieve constant current with changing peak input voltage and load.On-time is fixed,When peak input voltage and load are both fixed,so power factor is achieved.So,the multiplier is avoided and the volume and complexity of the circuit are reduced.For the output current at twice the power line frequency,ripple rejection circuit is designed to replace the energy storage capacitor with low lifetime.At the same time.low ripple of output current is achieved.And then, model of the low ripple electrolytic capacitor-less AC-DC LED driver circuit is proposed in this paper.The feasibility of the improved circuit is verified in Simplis software. Based on the principle analysis of the improved circuit and designs of the key sub-block circuits, the whole circuit simulation results are done by Cadence tool. Finally, the chip is implemented using HHNEC 0.5μm 5V/40V HVCMOS. The prototype circuit is tested when AC input voltage is within the range 90V-264V and output power at 12-18W. The test results indicate that the entire system power factor is more than 0.9, the max value is up to 0.9961 and the total harmonic distortion is less than 20%, and the minimum is 6.29%. With the use of ripple rejection circuit,the ripple of output current is less than 4.37% and the conversion efficiency is more than 81.07%.The fliker which is leaded by current ripple at twice line frequency is catching a lot of attention,rejection of the ripple caused by using power factor correction,especially one-stage-PFC, on LED driver circuit has become a hot spot.But the research will last long and there are lots of problems:how to avoid decreasing conversion efficiency,how to achieve rejecting current ripple without electrolytic capacitor and ensuring power factor stay the same.In this thesis,ripple rejection circuit makes sure of low current,electrolytic capacitor-less and small influence of the other aspects,which lays a good foundation of designing of fliker-free PFC LED driver circuit on integration,miniaturization and transition with independent intellectual property rights int the future.