According to the International Energy Agency (IEA), 19% of the electricity consumed worldwide is used for lighting. Therefore, in recent years, countries around the world have been working to replace energy-efficient incandescent light sources with more energy-efficient solutions. With the continuous improvement of lumen output (LED) in lumen output and light efficiency, the average cost per lumen of light output is also decreasing, combined with the advantages of LED in high directivity, long life and low maintenance cost. LED lighting (also known as solid state lighting, or SSL) has become an extremely compelling alternative.
Energy efficiency specification requirements for solid state lighting
In order to promote energy conservation, government agencies or regulatory organizations around the world have developed different LED lighting specifications, mainly in terms of power factor (PF) requirements. For example, the European Union's International Electrotechnical Union (IEC) specifies the total harmonic distortion performance of lighting applications with power greater than 25W, which is also applicable to other international standards in some regions.
In addition, the US Department of Energy has developed and published an "Energy Star" standard for solid-state lighting. This voluntary standard includes a range of requirements for common residential and commercial lighting fixtures (such as recessed lights, cabinet lights, and desk lamps) covering minimum lumen output, overall efficacy, reliability targets, light color temperature, and a range of other critical system level requirements. . It is worth noting that this standard does not directly include the power efficiency requirements, but includes the power factor requirement, that is, regardless of the power level, the residential application requires a PF greater than 0.7, and the commercial application requires a PF greater than 0.9, while the integrated LED lighting The requirement is that the PF is greater than 0.7.
Of course, not all countries absolutely mandate improved power factor in lighting applications, but some applications may have this requirement. For example, utilities may strongly promote the commercial use of products with high power factor in utilities. In addition, when utilities own/maintain street lights, they can decide whether they want a high power factor (usually greater than 0.95+), depending on their wishes.
13W LED downlight design example
1) Establish maximum load design goals with reference to surrogate standards
Take the ENERGY STAR solid-state lighting standard as an example. This standard contains the overall requirements for determining the efficacy of the luminaire; in fact, this standard is a system-level standard that involves selected LEDs, field operating temperatures, optical components, and drives. Power conversion energy efficiency, etc. Lamp developers can therefore trade off LED choices, use of optical components, thermal management solutions, drive topologies, and designs to meet overall requirements. The following table lists the key system requirements for recessed lights in the Energy Star version 1.1 residential and commercial application solid state lighting specification version 1.1.
Table 1: Key Requirements for Recessed Lights in the ENERGY STAR Version 1.1 Residential and Commercial Solid State Lighting Specifications
The most common downlights are larger aperture type recessed lights. For residential and commercial applications, in addition to differences in power factor, designers have the flexibility to use neutral and warm white LEDs. From the minimum requirements in Table 1, it can be seen that the lowest output, 575 lumens, maximum input power The threshold is about 16.4W.
Since there is no directly applicable LED driver energy efficiency standard, consider the "Energy Star" version 2.0 external power supply (EPS) standard as a substitute standard. According to the EPS 2.0 standard, the minimum energy efficiency requirement for a standard power supply with a rated power between 1 and 49 W is 0.0626 × ln(Pno) + 0.622. Therefore, the minimum energy efficiency of a 12W rated power supply meeting this standard is 77.7%, and the 15W power supply is It is 79.1%. Since the LED luminaire standard is based on the energy efficiency of the input socket, it is necessary to convert the driver energy efficiency target into an effective LED load. In order to increase some design margin, we set the minimum target energy efficiency to 80%. As a result, the LED load is 16.4W × 80%, which is 13.1W.
In this way, we have determined the maximum load design goal. LED efficacy is subject to LED manufacturers as well as drive current and operating temperature. ON Semiconductor's GreenPoint reference design selects a constant current of 350mA to support most high-brightness LEDs on the market. Another factor to consider is that luminaire developers can choose a wide range of LEDs. The higher the efficiency of the selected LEDs. The fewer LEDs are required. Therefore, the GreenPoint reference design has a higher energy effect at 50% to 100% of rated load. As LEDs improve, the same basic power supply design can be easily modified to drive fewer LEDs, providing far greater than the minimum required luminaire efficacy.
2) Other design requirements
Once the basic design requirements are identified, other system factors related to the end application requirements need to be considered. For example, although there is no requirement in the standard, it is important to be compatible with existing line dimming solutions. Therefore, the design should be optimized for triac TRIAC wall dimmers. There are many challenges to TRIAC dimming, but one factor that designers may easily overlook is that the drive should be able to start and work with a low chopped AC input waveform. Moreover, the size of the drive power supply should match the junction light fixture junction box. Should also pay attention to a person's factor requirements. Although the LED actually illuminates in an instant, the driver is designed to have a specific start-up time. Regardless of the LED luminaire, this aspect should not be worse than CFL, or even better. Therefore, we can use CFL as a reference. In the "Energy Star" CFL bulb requirements, the maximum start-up time under rated conditions is 1 second, so we will set the LED driver's design time in terms of startup time to 0.5 seconds. Because this design is for residential or commercial applications, the specifications we set are more challenging. Table 2 summarizes the key design goals of this GreenPoint reference design.
Table 2: Key Design Objectives