Analysis of low power LED general lighting applications

    In recent years, lighting has become an important area that countries around the world are targeting to promote energy conservation and environmental protection. According to statistics, about 20% of the world's electricity is used for lighting each year, and about 40% of this electricity is used for inefficient incandescent lighting. With the continuous improvement of LED in almost all aspects such as light output performance and cost, LED general lighting has become a very attractive alternative solution to traditional lighting such as incandescent lamps.

　　Typical LED general lighting applications include replacement of light bulbs and fluorescent tubes, recessed lights, street lights and parking lights, work lights, landscape lighting, billboard text circuits, building lighting, etc. The power of LED street lights is relatively high, generally between 50W and 300W; the power of LED building and regional lighting applications is generally between 40W and 125W, which belongs to the medium power range; those below 30W can be collectively referred to as low-power LED lighting applications, including specific directional lighting, such as cabinet lighting, recessed lights, spotlights PAR20/30/38 light replacement, table lamps, etc., as well as omnidirectional lighting, such as accent lighting, home appliances, general lighting A-type lamp replacement, decorative lamps and ceiling fan lamps, etc. This article will focus on low-power LED general lighting applications with a power of less than 30W, and the corresponding high-efficiency solutions of ON Semiconductor.

　　Factors to consider when selecting a driver for low-power LED lighting applications

　　The main function of LED drivers is to limit current within the range of operating conditions, regardless of how the input and output conditions change. Its application design faces many restrictions, such as high energy efficiency, high cost performance, wide environmental conditions, high reliability, flexibility, compliance with standards such as electromagnetic interference and harmonic content, adaptability to existing applications, and the ability to work with traditional control methods.

　　It is not easy to choose the right driver for low-power LED applications, and different factors need to be considered. For example, the commercial and residential markets have different requirements for LED lamps in terms of operating temperature, usage time, performance, and industry standards such as "Energy Star". In addition, bulb replacement applications also have unique challenges, such as thermal limitations of LED power supplies and drivers, size restrictions, and compatible dimming technologies.

　　As for the standards applicable to LED general lighting, the main ones are the power factor correction standards required by the US "Energy Star" and the European Union's International Electrotechnical Commission's limit standards on total harmonic distortion. Among them, the "Energy Star" V1 version of the lighting standard requires LED lighting fixtures to have PFC, which is applicable to specific products such as recessed lights, cabinet lights and table lamps, but has nothing to do with the power level. This standard requires the power factor (PF) for residential applications to be higher than 0.7, and for commercial applications to be higher than 0.9. This standard is a voluntary standard and will be implemented in September 2011. The "Energy Star" version 1.1 integrated LED bulb standard has been in effect since August 2010, requiring the power factor of bulbs with input power higher than 5W to be higher than 0.7.

　　As mentioned above, many factors must be considered when selecting a suitable driver for low-power LED lighting applications, among which industry standards such as power factor are particularly important. Next, we will take ON Semiconductor's related products as an example to explore how to provide high power factor in low-power lighting applications.

　　Solutions to improve energy efficiency of low-power LED residential lighting applications

　　For example, residential lighting applications such as table lamps and cabinet lamps generally have a power range of 3W to 8W. Such low-power applications are best suited to isolated flyback topologies. However, traditional offline flyback power converters use a full-wave rectifier bridge and large capacitors in front of the switching regulator. This configuration has a low power utilization or PF of the input line waveform, which is only in the range of 0.5 to 0.6.

　　This is where PFC comes in. For example, an active PFC such as NCP1607B can be used before the flyback converter to provide a PF higher than 0.98, but this increases the number of components and complexity, and the most suitable power is much higher than the application requirements. There are many passive PFC solutions that can improve PF, but they usually use more additional components, increase cost and board space, and reduce reliability.