Filter-free AC Direct LED Driver with Unity Power Factor and Low Input Current THD Using Binary Segmented Switched LED Strings and Linear Current Regulator Changbyung Park Dept. of Electrical Engineering KAIST Daejeon, Korea pcb@kaist.ac.kr Chun-Taek Rim Dept. of Nuclear and Quantum Engineering KAIST Daejeon, Korea ctrim@kaist.ac.kr Abstract— A compact LED driver with high power factor and low harmonic distortion is proposed for the AC direct LED lighting. For the compact implementation of LED lamp, all the bulky passive components such as inductors and electrolytic capacitors are eliminated. To meet power factor and total harmonic distortion regulations, the current flows through LED string is controlled by a linear current regulator. To minimize the power loss of the linear current regulator, the voltage drop of the whole LED string is adjusted by means of binary switching. The drain voltage of the linear current regulator is controlled by simple digital logics and comparators to limit the current within predefined range. The proposed LED driver was verified by simulations and a prototype experiment kit. I. INTRODUCTION To increase the luminance efficiency of lighting systems, traditional fluorescent lamps and incandescent lamps have been gradually replaced by LED type lamps. The replacement, however, is significantly impeded by the high cost and short lifetime of the LED strings and their drivers. The lifetime of LED itself is as much as 50,000 hours, but that of the LED driver is typically shorter than 10,000 hours. The major sources of short lifetime are the power switches and electrolytic capacitors used in the drivers. Conventional electrolytic capacitors have less than 5,000 hours of lifetime. This is why the LED drivers without electrolytic capacitors were proposed [1], [2]. Their operating switching frequency is quite high to eliminate the large size electrolytic capacitors [1]. This high switching frequency (200 kHz) deteriorates the life time of power switches in the LED driver. Using an inverted buck topology [2], a good power factor correction (PFC) and a low switching frequency is achieved; however, the inductor size is as much as hundreds of mH. Therefore, it is not appropriate for a compact size low cost LED driver. For the durability of LED drivers, passive type LED drivers [3], [4] were proposed, but this type of drivers do not have good voltage regulation capability, and usually adopt bulky passive components such as inductors and capacitors to meet total harmonic distortion (THD) and power factor (PF) standards. To eliminate all the passive components from an LED driver for low cost and high reliability, AC direct type LED drivers with switched LED string structure were proposed [5]- [7], where the number of turn-on LEDs is varied in accordance with the rectified input voltage to control its input current waveform. In these prior arts, the number of turn-on LED combination is typically not more than six due to the limited number of power switches, so they have large harmonics in the input current of the LED drivers. In this paper, the binary segmented switched LED string structure with a linear current regulator, called a digital LED lamp, is proposed. No passive component is used in the LED driver. The total number of turn-on LED combination is obtained by mixing multiple 2 k LED strings using k power switches. To meet THD specification, the LED current is controlled by a linear current regulator. Utilizing these techniques, power dissipation in the linear current regulator is significantly minimized by a simple controller consists of simple comparators and digital logics. II. OVERALL LED LAMP CONFIGURATION The proposed LED lamp is composed of a full bridge rectifier, a LED string with switches, a controller which contains comparators and digital logics, and a linear current regulator, as shown in Fig. 1 (a). The LED current i LED is controlled by the linear current regulator, that is a voltage controlled current source to follow v rec . The control voltage is a linearly divided and level shifted voltage of v rec . Just with the bridge rectifier and linear current regulator, the input PF This work was supported by the Korea Micro Energy Grid, granted from the Ministry of Knowledge Economy. 978-1-4673-4355-8/13/$31.00 ©2013 IEEE 870