Excerpted from an article originally published in Power Electronics Technology, Feb 2009
Solid State Lighting technologies, along with the Energy Star program, have challenged designers and integrators. Understanding of the requirements as well as knowledge of the related technologies is paramount in delivering successful products to the marketplace.
In January 2009, a New Energy Star Requirement for solid-state lighting (SSL) luminaires has been introduced that has implications for LED power supply designers and may also provide insight into future Energy Star requirements. Most people are familiar with the Energy Star logo but may not recognize its impact. The program, which is coordinated jointly by the U.S. Department of Energy (DOE) and Environmental Protection Energy (EPA), proposes Energy Star requirements for a host of goods, from appliances to roofing products.
The Energy Star document is divided into Category A and Category B. Category A describes the applications covered and their associated limits. The interior installation applications listed in Category A include undercabinet kitchen and shelf lighting, portable desk task lights and recessed downlights. Outdoor applications include wall-mounted porch lights, step lights and pathway lights. Category A additions are already under consideration, with roadway lighting perhaps being the most notable. Category B is included in the text of the criteria; however, it contains no product relevance at this time. Category B eventually will encompass all types of SSL and provide future performance goals that will allow the Energy Star document to keep pace with technology advances.
The DOE’s Energy Star requirements for SSL compliance took effect on Sept. 30, 2008. The criteria are intended for general illumination luminaires (not signage or decorative lighting) in both commercial and residential applications installed indoors or out. Fig. 1 shows an SSL porch light.
| FIGURE 1
Impact of Past Lighting Initatives
To understand the Energy Star goals for SSL, it is helpful to understand the impact of past lighting initiatives. In particular, the goals of the program were shaped by the introduction and evolution of compact florescent lamps (CFLs). Although common today, CFLs were a new technology when introduced in the 1970s. One of the primary issues when the product was introduced into the market was that no benchmark for comparison evaluation existed. The lack of a common standard, in addition to the fact that the technology itself was still unproven, led to consumer distrust. The distrust of CFLs remained long after the products were proven to be a vast energy savings compared to traditional incandescent bulbs.
The Energy Star SSL program was specifically designed to include the complete luminaire. Individual performance claims of light-emitting diode (LED) bulb manufacturers and/or driver manufacturers are of little value, because the luminaire is evaluated as a complete system. This also eliminates potential integration issues that may negatively impact performance. Test methodologies and benchmarks are included to assure customers that the performance comparison is based on common testing. The DOE is determined to see these mistakes do not occur again, and has worked to develop the criteria along with all the industry stakeholders, including LED makers, power supply companies and integrators.
The realm of SSL is a dramatic technological shift from past lighting solutions such as incandescents and fluorescents. LEDs used in SSL applications require a current drive from a power source. The LEDs conduct, rather than radiate, the majority of the heat generated. When used properly, LEDs last significantly longer than previous lighting solutions.
Within the Energy Star criteria, key attributes of the power supply requirements are summarized in Table 1.
The Energy Star program requires a minimum 3-year warranty on the luminaires. Because the LEDs themselves used within the SSL luminaire are significantly longer in life, pressure will be on the power manufacturer to provide high-quality products backed by a robust warranty. Today, many driver products carry a 3-year warranty. In order to take full advantage of the lifetime benefit of SSLs, power designers must continue to push the technology forward. Design practices such as selecting reliable components and minimizing stress will assist in achieving this goal. Higher efficiency units will also increase lifetime by reducing temperature rise caused by dissipation.
For power supply designers, each of the requirements has design implications. The diagram in Fig. 2 shows a typical switch-mode power supply overlaid with the Energy Star SSL requirements.
| FIGURE 2
Fig. 2. Typical power supply block diagram, with Energy Star considerations that
include standby power, EMI, PFC, transient response and cold start-up.
Many of the requirements are typical of other power supply applications. To meet the transient requirements, the input must be protected against multiple strikes of 100-kHz ring wave at 2.5-kV level. Design practice and input impedance play an important role in satisfying the transient, with the biggest impact achieved by using a transient voltage suppressor across the input line. The device must be sized appropriately but will prevent the strike from having an undesirable effect on the power supply driver/luminaire.
One of the most significant implications of compliance is the requirement for PFC without regard to minimum power levels. The result is the need to address power factor, which thus can have a significant impact on material cost. This is particularly noticeable in lower power where the added material is a significant percentage of the overall cost. The reduced power factor limit for residential applications provides the opportunity for passive PFC methods. However, passive implementations typically use inductors and capacitors to smooth the current draw, which has the drawback of increasing size and weight. For commercial designs, the power factor limit will require active PFC. One approach is to employ a single-stage PFC that can be facilitated through commercially available controllers (Fig. 3).
| FIGURE 3
Fig 3. SSL power supply designs must address power factor, which can impact material cost. One approach is
to use a single-stage PFC that can be facilitated through commercially available controllers.
The single-stage approach eliminates the traditional independent boost and isolation stages by integrating the boost/buck stage.
For applications requiring low power consumption, several alternative approaches are available. Due primarily to previous Energy Star programs, including external adapters, there are numerous options for low-power standby controllers. These ICs often operate by cycle skipping, sleep modes or selective feature shutdowns when load is not present. This allows the power supply to draw very low current.
Switch-mode power supplies operate at frequencies that are orders of magnitude greater than the 120 Hz referenced. Typical switching frequencies for offline power supplies range between 35 kHz and 200 kHz. However, it will be important to consider any 120 Hz from the input rectification that could find its way to the output and cause flicker issues. This issue is most prevalent when improving EMI performance by using a capacitor across the isolation transformer.
Temperature, EMI and reliability also must be considered throughout the design. Although -20°C for outdoor applications is not extreme, component selection is critical to ensure proper low-end operation. Furthermore, care must be taken if aluminum electrolytic capacitors are used in the design. Aluminum electrolytic capacitors typically lose significant energy storage capability at lower temperatures. Sections of the circuit typically impacted include the startup/bias, where a reduction in energy storage may prevent the Vcc from climbing high enough or holding enough charge until the power supply can start and enter steady-state operation. For the same reason, regulation and output noise also can be influenced where electrolytics are used for filtering.
Energy Star criteria for SSL luminaires are in place and affecting all involved in designing the power to drive these applications. Also referred to as a “ratcheting” standard, it features steps that increase the limits for acceptance. The first of these will occur in September 2009. Therefore, it is important to join the energy-saving initiative now to be prepared for tomorrow.
Energy Star programs are being introduced in greater numbers of applications that impact power design. The standard discussed here illustrates a model for the program. Common threads through the Energy Star program are the target of widespread applications and progressive plans to raise the bar in future years. Of note in this standard is the allowance for family submittals aimed at easing the burden on manufacturers. Also important is the decision to evaluate the luminaire as opposed to the individual parts.
by P. Resca and D. Murray