Research On An Improved Electrolytic Capacitor-less Led Driver With High-efficiency

With the advantages of high efficiency,long lifetime,energy saving and environmental protection,light emitting diode(LED)is widely used in many fields,such as indoor lighting,automotive lighting,biomedical equipment and smart street lamps.In order to match the advantages of LED light source,LED driver should have the characteristics of high power factor,long lifetime,high reliability and high efficiency.With the AC power supply,in order to meet the requirements of power factor,LED driver needs to perform power factor correction(PFC).Electrolytic capacitor is usually used to buffer the imbalance between input alternating AC power and output constant DC power.Compared with LED light source,the electrolytic capacitor has shorter lifetime and is difficult to match the lifetime of LED sources.Currently,active power decoupling technology has been widely used in electrolytic capacitor-less LED driver.Based on this idea,the ripple cancellation circuit(RCC)for buffering unbalanced power is divided into series and parallel structure.In the series RCC,the output capacitor voltage of RCC is controlled as a twice-line-frequency waveform with the same amplitude and the opposite phase to storage capacitor ripple voltage.Then the series connection will cancel the ripple voltage.Compared with the parallel RCC,the series RCC only deals with a small part of ripple power,and most DC power will supply the LED load directly,so that the conversion efficiency is higher.However,the series RCC still needs additional auxiliary storage capacitor at input side.Previous researches have proposed that the AC input after rectification can directly supply power to a DC-DC RCC,which can save the storage capacitor at RCC input.However,with the existence of DC bias,the instantaneous input power may be lower than the DC bias power near the zero-crossing point of AC input voltage,which is not large enough to support the RCC capacitor voltage.Then,ripple cancellation can not be totally completed and the output voltage of LED driver is out of control near the zero-crossing point of AC input voltage.In order to solve the above problems,this paper proposes an improved and efficient electrolytic capacitor-less LED driver.The flyback PFC converter is used here to provide two outputs,where one directly supplies power to the LED load,and the other is cascaded with a DC-AC RCC.The flyback PFC converter works in discontinuous current mode(DCM)to realize power factor correction automatically.The DC-AC RCC based on a full-bridge inverter has no DC bias and the input power near the AC zero-crossing point can still support the required voltage on the RCC capacitor for ripple cancellation.Hence,this topology can effectively reduce the uncontrollable time of output voltage.In addition,the output current control loop of DCM flyback PFC converter and the output voltage control loop of DC-AC RCC are designed respectively.By designing the appropriate cross-over frequency and compensator parameters,the DC-AC RCC can generate a stable AC ripple voltage,and avoid the voltage drop across the zero-crossing point of AC input voltage.Finally,a 17 W LED driver prototype has been built in the laboratory to verify the above analysis.Experimental results agree well with the theoretical analysis and prove the proposed LED driver with good performances of electrolytic capacitor-less,wide AC input voltage range,high power factor and high efficiency.