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Advantages of LED power supply

Posted: 01 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:LEDs? EMI filter? MTBF? Electromagnetic Interference? EMI?

Since LEDs are a solid state component and require electronic circuits to emit light, a driver that transforms AC to DC current will be utilised 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, 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 50C as 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 105C 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. 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 well-designed 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 centred 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 -20C or lower
???Less than one second delay from the time AC input voltage is applied until the 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 and complexity of design that can be addressed with the electronics designer who has a solid technical background in power conversion and magnetic design. Inexpensive drivers can claim to meet the power factor requirements at full power, but as loading is dropped through dimming of the light emitted by the LED, these drivers will have a much lower power factor than 0.9. This will disqualify the fixture from meeting the Energy Star requirements. Adding metal oxide varistors (MOVs) is the simplest way to meet transient protection. Operating at -20C temperature can be a challenge for the power designer and not all commercially available power supplies can comply. Careful selection of a driver can go a long way in obtaining Energy Star certification and avoiding costly delays in time to market as well as the assurance of offering quality LED lighting fixtures for this rapidly changing market.

Illumination control
The ability to control the illumination based on area occupancy is a growing field in the LED lighting market. Cost reductions can be realised by lighting only the path or the area that is being occupied in an office building or parking structure. Since LEDs are electronic devices, limiting the current can achieve the desired outcome in dimming the light and saving energy and costs. In some applications, both wired and wireless control systems are taking advantage of existing infrastructure developed for networking schemes. Moreover, LED drivers are built to have embedded IP addressing in order to interface with an electronic console to accept commands and change the light intensity based on occupancy sensing and other parameters. They can also report dynamic information, such as ambient temperature, back to the controlling console. This is an opportunity for the manufacturer of the LED drivers to influence lighting design in the early stages by providing flexibility in sensing and reporting critical information via wired or wireless antennas and receivers. With new technology offered, this can also be done via an AC line. The communication protocol is varied by application-specific LED fixtures. Area lighting uses Digital Addressable Lighting Interface (DALI) commands where entertainment lighting DMX-512 is widely employed. Remote Device Monitoring or RDM is the next step-up for wireless control in architectural lighting. No matter what the protocol is, an intelligent driver will be the future for powering LED light fixtures.

About the author
Maggie Nadjmi holds an electrical engineering degree and is a product manager for SL Power Electronics and has served more than 25 years in the power supply industry and is currently working on new products for LED applications with special emphasis on architectural lighting.

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