Research On Electrolytic Capacitor-less AC-DC LED Driver Based On Active Power Decoupling Scheme

Semiconductor light-emitting diodes(LEDs)have the advantages of long life,high efficiency,and no environmental pollution,and thus they have been widely used in indoor lighting,automotive lighting,display,intelligent lighting and other fields.LEDs are typically current-driven devices,and the LED drivers should provide the required constant current.In the AC power supply situation,in order to reduce harmonic interference to the AC power grid,LED drivers should achieve a high input power factor,by using a power factor correction(PFC)circuit.Also,since there is inevitable power difference between input alternating power and output constant power,the electrolytic capacitors with large capacity are usually used as energy balance.However,the lifetime of electrolytic capacitors are usualy much lower than that of LED light sources.Therefore,the active power decoupling method is used in this paperto achieve a balance of input and output power,while avoiding the use of electrolytic capacitors and achieving high power factor.The active power decoupling method is essentially to use active switching devices and energy storage elements to the circuit to transfer the low-frequency pulsating power from the input AC side to the energy storage element in the non-output end.Because the average voltage or ripple voltage of the non-output port is not limited by the load characteristics,its output capacitance value can be reduced,thereby avoiding the use of electrolytic capacitors.In addition,different active power decoupling circuits can avoid the secondary flow of energy and improve efficiency.Therefore,based on this principle,starting from the Boost converter in the discontinuous current mode(DCM)with automatic PF function,an improved Boost PFC converter is proposed in this paper,which includes two output ports.One port is used for transferring active power to the load directly.When the input power is larger than the output power,the other port transfers the low-frequency pulsating power to the intermediate energy storage capacitor.When the input power is less than the output power,the intermediate energy storage passes through the Buck-Boost converter power decoupling circuit to transfer energy to the load.In this way,the high power factor at the AC input side is achieved,and the output power of the LED is kept constant without electrolytic capacitors.The doublefrequency ripple power can only flow once through the power decoupling circuit,which guarantees efficiency enhancement.This paper analyzed the derivation process of the AC-DC LED drive circuit without electrolytic capacitors in detail.The power flow path of the decoupling input power is also given,proving high conversion efficiency from the perspective of topology.To control this circuit,a carrier stack control strategy is also proposed in this paper to achieve the constant current output of the LED driver,that is,two different DC voltage sources can be stacked on the same carrier to form two stacked carriers.Therefore,this control method has only one feedback loop to achieve constant output current,without the need for multipliers,logic gate circuits and other devices.It is simple and fast responsing.Finally,a 40 W experimental prototype was built in the laboratory,and the parameter design of the main circuit and control circuit was also given.Experimental results verify the proposed LED drive power topology and control strategy well.