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EETE JULAUG 2015

Power Supplies & Batteries LED power supplies versus standard ITE power supplies SBy Maggie Nadjmi ince LEDs are a solid state component and require electronic circuits to emit light, a driver that transforms AC to DC current will be utilized in the light fixture and this will drive the need to look for a power supply with sufficient power and output characteristics. Most light designers have purchased an off-the-shelf AC to DC power supply based on these requirements and, for initial testing, it works well. Light comes on and the fixture works. Next step in the process is to take the fixture for regulatory testing. At this point, the marketing collateral is getting ready and the sales people are anxious to take the new fixture to market. A call from the regulatory testing house with news of the product not passing the standards however, sends everyone looking for a solution. Add an EMI filter, one would suggest, put EMI cores on the input wires another person would say. Meanwhile, the cost of the fixture starts climbing and what seemed to be a nice-off-the shelf inexpensive power supply requires add-ons to perform. Life expectancy The longer life of LEDs of over 50,000 hours is very attractive to customers. However, the life of the LED driver also needs to be considered. Many think this is about mean time between failure (MTBF); however, this is not life expectancy. In most cases, MTBF is calculated based on component count and provides just one part of information on reliability and life expectancy. When selecting the optimum power supply for LED lighting, look for a power supply that is specifically designed for the demands of the LED and meets power and output voltage. Make sure it lists EN55015 for Electromagnetic Interference (EMI) compliance (see figure 1), has adequate life expectancy to match the life of the LEDs in the fixture and then move on to other requirements such as low leakage current for Canada, and meets IEC61000- 3-2 class C for the European market. In addition, make sure the power supply can withstand temperatures of over 50°C as the heat generated by LEDs can rise the thermal profile within the fixture. This will put thermal stress on components such as electrolytic capacitors in the driver and if it is not designed efficiently, could be detrimental to the long life expectancy of the LED fixture. Higher power efficiency will reduce the wasted heat generated by the supply. In addition, ensuring adequate thermal design, high temperature electrolytic capacitors rated at 105°C and ability to operate for more than 5,000 hours are the critical components in selecting an LED driver. This will ensure the minimum life expectancy of 50,000 hours for the LED fixture. Real world application A studio lighting application required a 200 watt power supply that would deliver 48V and required no moving air. The EMI compliance of the EN55015 to class B, meeting IEC61000-3-2 class C from almost dimmed-off to full illumination, were among the required specifications as well as long life expectancy. SL Power engineers designed a cold plate for this application to take advantage of all standard offerings of its LB240 AC/DC power supply – see figure 2. This process took less than two weeks. The fixture with the LB240 passed all regulatory testing without adding an EMI filter. The customer is pleased with the outcome and knows the reliability of his fixture is not compromised due to the power supply’s excellent thermal design and careful selection of electrolytic capacitors. Energy Star Residential and commercial lighting fixtures have been certified and have carried the Energy Star logo for a number of years, but since 2009 and updated in 2011, there are specific requirements added for LED sourced lighting products in the U.S. Not all LED light fixtures can apply for the Energy Star program. Entertainment lighting is excluded from the list of fixtures that displays the Energy Star Logo. Energy Star requirements include Lumens per Watt, which is a combination of a welldesigned optical source as well as a highly efficient driver. A minimum of 35,000 hours of life expectancy is also required to help ensure that the LED and driver are of the highest quality and work reliability during this period so the end customer does not have to worry about replacing the fixture year after year. The bulk of Energy Star requirements which are centered around drivers include: A minimum power factor of 0.7 for residential and 0.9 on commercial lighting products Transient protection circuitry to pass 2.5kV of input surge based on ANSI/IEEE C62.41 Operating temperature of -20°C or lower Less than one second delay from the time AC input voltage is applied until the Fig. 1a: Typical LED power supply for the EN55015 standard. Fig. 1b: ITE power supply for the EN55022 standard. light comes on and the no light standby wattage is less than half a watt. The standby wattage requirement can be increased to 1W if the LED lighting fixture has intelligence built into it. To meet a power factor of 0.9, the driver has to have power factor correction stage, which requires additional components Maggie Nadjmi is Product Manager at SL Power Electronics - www.slpower.com 36 Electronic Engineering Times Europe July-August 2015 www.electronics-eetimes.com


EETE JULAUG 2015
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