Research On High Power Factor AC/DC LED Drivers Without Electrolytic Capacitors

Thanks to the advantages of high efficiency, good optical performance, long lifetime, andenvironmentally friendly，high-brightness light-emitting diode is a promising new generation of greenlighting source. High quality of the LED driver is a key part of LED lighting systems, which impactson the overall performance of LED lighting systems directly. In order to meet the power factorlighting standards, AC/DC LED driver needs PFC function. In the two stage LED driver with PFC, italways needs to choose much larger electrolytic capacitor to balance the difference between the inputand output power. But the lifetime of electrolytic capacitor (5000h/105℃) is much shorter than theLED’s lifetime (100000hours). Therefore, it is an important research direction in the market toachieve a long lifetime LED driver.In order to reduce the storage capacitance in the two stage LED driver, thus the long lifetimefilm capacitor can be adopted to replace the electrolytic capacitor, this paper proposes a method todrive LEDs by the pulsating current at twice line frequency, while the size and cost of the capacitor issame. This paper derives the relationship between the storage capacitance and the bus ripple voltage,when the LED is driven by the bias of the sine wave current and PWM current. Compared with theconstant DC current driving, while keeping the value of bus ripple voltage same and the peak-averageratio of1.33, the storage capacitance of the proposed method can be reduced to52.7%. At the sametime, the input power factor is0.99. With the analysis of the voltage changes around the storagecapacitor in the PWM dimming, it is feasible to select the device in100%luminous flux. The controlmethods of the proposed current are also presented in this thesis.When the input current contains3rdharmonic current, the pulsating input power can be reduced.Through the analysis of output voltage ripple of the PFC compensator on the power factor and busvoltage ripple, mathematical model is derived, which demonstrates the capacitance can be reduced bycontrolling output voltage ripple of the compensator. Combined with the proposed method of drivingLEDs by the PWM current, the storage capacitance can be reduced more with high power factor.Based on the analysis, this paper completes a50W LED driver without electrolytic capacitor byPWM pulsating current driven. The storage capacitance is3.3μF. Experimental results verify thefeasibility of the proposed methods.