# A High Power Factor LED Driver with Intrinsic Current Balancing Capability
## 1 Introduction
When the load is two anti-parallel LED strings, the currents flowing through the LED strings are automatically equal without adding any components for current balancing [19; 20]. Another research topic related to LED drivers is PFC; the power factor and THDi of lighting equipment must comply with standards such as IEC 61000-3-2 Class D and IEEE 519. Therefore, LED drivers must use an additional ac/dc, which serves as a power-factor correction (PFC) circuit to satisfy these standards. It results in a two-stage circuit topology. The first stage is a PFC circuit to shape the input current into a sine wave and the second stage is a dc/dc converter to regulate the LED voltage. Although performing well, the two-stage approach requires more circuit components. Aiming to reduce the number of components and to improve energy conversion efficiency, many single-stage LED drivers which integrate the PFC circuit and the dc/dc converter have been proposed [21; 22; 23; 24; 25]. In references [21; 22], the PFC circuits are boost converters, which are operated at either boundary-conduction mode (BCM) or discontinuous-current mode (DCM). Operating the boost converter at BCM or DCM can achieve a high power factor with easier control. However, when a boost converter operates at BCM or DCM, the dc-bus voltage should be at least twice the peak of the ac input to achieve a nearly unity power factor. In contrast, buck-boost or flyback converters are also commonly used as PFC circuits. However, they have no such limitation on the dc-link voltage. A high power factor can be achieved by operating them at a fixed switching frequency and a fixed duty cycle over the line frequency cycle [23; 24; 25]. In reference [23; 24], a buck-boost converter was used as the PFC circuit, while in reference [25] a flyback converter was used. Typically, the LED voltage is much lower than the magnitude of the input voltage. The flyback converter uses a coupled inductor as an energy storage component and the winding turns ratio can be selected according to the output voltage specification. When the primary-to-secondary turns ratio is high, the voltage rating of the dc/dc converter can be effectively reduced. In this paper, a new single-stage LED driver derived by integrating a flyback-typed PFC circuit and a Class-D series resonant converter was proposed. One of the two active switches of the resonant converter was shared with the flyback converter. The flyback converter served as the PFC circuit while the Class-D series resonant converter served as the dc/dc converter. A differential-mode transformer was added to the resonant converter [26]. With this 1:1 turns ratio transformer, the automatic current balancing capability can be extended to four LED strings without resorting to any complex control circuitry. This paper is organized as follows. Section 2 describes the circuit topology of the LED driver and the circuit operation in different operating modes. Detailed circuit analyses, an illustrative prototype, and the experimental results of LED driver are provided in Sections 3 and 4, respectively. Finally, some conclusions are given in Section 5.