1708 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 59, NO. 4, APRIL 2012 A Capacitor-Isolated LED Driver With Inherent Current Balance Capability Junming Zhang, Member, IEEE, Jianfeng Wang, and Xinke Wu, Member, IEEE Abstract—This paper presents a capacitor-isolated light- emitting diode (LED) driver with inherent current-balancing capability. Based on a series resonant converter, the resonant capacitor can be used both for safety isolation and the current balancing with the proposed two-output rectifier structure. Com- pared to the conventional current-sharing technique, the proposed LED driver circuit is simple and has low cost and high per- formance. Also, the capacitor-isolated structure is more efficient and compact compared to the conventional transformer isolation. The detailed theoretical analysis and design considerations of the proposed circuit are presented. The performance of the proposed circuit is validated by the experimental results from a 60-W prototype with two balanced outputs. Index Terms—Capacitor isolation, current balance, light- emitting diode (LED) driver, resonant converter. I. I NTRODUCTION I N RECENT YEARS, with the significant progress in high- brightness (HB) light-emitting diode (LED) technology, the HB LED is widely adopted in many applications, such as liquid crystal display (LCD) backlighting, automobiles, and street lighting. The efficacy (light output, or lumens per watt) of LED is now competitive with compact fluorescent lamps. Moreover, the lifetime of LED is quite long with very low maintenance requirement [1]. The application of LED for commercial and residential lighting looks to be an irreversible tendency in the near future. The maximum current rating of individual LED module is limited due to the thermal limitations of the package. Thus, it is necessary to put many LED modules in series or par- allel to achieve the required power level or luminance for certain applications, such as street lighting or large-size LCD backlighting application. The brightness of LED is directly related to its forward current. It is important to ensure that each LED has same brightness and thermal performance for reliable operation; thus, the current for each LED should be balanced [2]. The easiest way is to connect them all in series, and the output voltage of LED driver is very high if there are hundreds of LEDs in series [3]–[5]. Furthermore, the reliability of this configuration is poor if one LED is broken. Therefore, the parallel connection of LED strings with acceptable terminal voltage is preferred in real applications. However, individual Manuscript received September 30, 2010; revised January 13, 2011; accepted March 16, 2011. Date of publication April 5, 2011; date of current version November 1, 2011. The authors are with the College of Electrical Engineering, Zhejiang Univer- sity, Hangzhou 310027, China (e-mail: zhangjm@zju.edu.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TIE.2011.2138111 Fig. 1. Conventional current-balancing methods. (a) Active method. (b) Passive method. LED has slight tolerance in its forward voltage drop, and this small forward voltage drop difference can lead to a big current difference, which makes large difference of the light intensity and the lifetime [6], [18], [20]. Therefore, current balancing is necessary for parallel operation of LED strings [2]. Several methods have been proposed to achieve the current balancing among LED strings in the past couple years. Basi- cally, these methods can be divided into two categories: One is active method [6]–[16], and the other is passive method [17]– [20]. The active method uses active devices and related control circuit to form a current regulator connected in series with the LED strings. The current regulator can be linear or switch mode type, as shown in Fig. 1(a). The linear current regulator method usually has poor efficiency even with the adaptive voltage control, which makes it only suitable for low-power application [6]–[11]. The switching mode current regulator uses a dc/dc converter (usually a buck or boost converter) to regulate the current for each string with high efficiency compared to the linear-mode current regulator. Moreover, the digital control method can be implemented to decrease the complexity of the control circuit, but it still suffers from high cost and high component count [12]–[16]. To further simplify the circuit and reduce the cost, the passive current-balancing methods using passive components such as capacitors or coupled inductors were proposed [17]–[20]. In [17], a current-balancing method with tree-type coupled inductors was introduced. The circuit needs too many balance chokes, and the amplitude of the current cannot be regulated with a given bus voltage. In [19], by operating many dc/dc converters in discontinuous conduction mode and sharing 0278-0046/$26.00 © 2011 IEEE