An Efficient Driver for Dimmable LED Lighting

An efficient driver circuit is proposed for light-emitting-diode (LED) lamps with dimming feature. The driver consists of a flyback converter in series with the dc-link. By processing partial power of the driver circuit for current regulation, the loss produced by power conversion can be diminished. The dimming feature is accomplished by means of current amplitude modulation (AM) or double pulse-width modulation (DPWM). The detailed circuit operation stages and analysis are provided. A laboratory circuit is designed for a 45 W LED lamp. Experimental results demonstrate that a high efficiency can be achieved, even at a dimmed lamp power.The light-emitting-diode (LED) characterized by mercury-free, high efficiency and long life cycle, is expected to be the new generation of lighting source . With great improvement in power capacity and reduction in cost, the LED has been becoming more and more popular in many applications. The increased popularity of LEDs has challenged electronic engineers to come up with efficient and cost-effective solutions on the design of the driver circuits.Up to now, one of the most commonly used high-brightness white LED is rated at 1 Watt, which is driven at 350 mA, even though manufacturers are constantly working on driving LEDs at a higher current.To provide sufficient light output, an LED lamp fixture always involves large arrays of individual LEDs stacked in series. LEDs are current-driven devices. A white LED's luminous intensity and chromaticity are proportional to the forward current. For this reason, a driver circuit is designed essentially to drive LEDs at a required constant current . Conventionally, driving a string of high-brightness LEDs at an accurate dc current typically resorts to a linear regulator or a more complicated switching regulator with sophisticated control, especially when dimming function is included. These solutions, however, are of either poor efficiency or higher cost. To improve the overall efficiency of LED lighting, an efficient driver is proposed in this paper. The main concept of the efficient driver is explained by Fig. 1. The operating characteristic of an LED. Theoretically, only a trivial current is generated on an LED when the forward voltage is lower than its cut-in voltage. Beyond the threshold, a small variation in the forward voltage results in a significant change in the LED current. the proposed LED driver circuit, in which a string of LEDs is driven by two sources, one for providing the cut-in voltage and the other for regulating the forward current. In practice, the cut-in voltage is much higher than that for current regulation. In such a configuration, the dc voltage source for cutting in LED linear operation supplies the most part of LED power directly, while only a small amount of power has to be processed for current regulation, leading to a high overall efficiency.