POWER
Customor standard Many factors need to be considered for any discussion of customversus standard solutions and a detailed discussion would require an article of its own. The number of standard (off-the-shelf) products continues to increase and provide solutions for a wide-range of lighting applications. Customsolutions are still required in those applications with unique requirements, including form- factor constraints or special performance or functionality. Customsolutionsmay also be considered in high volume applications where functions and parameters can be optimized tominimize cost.
Figure 3: A typical "bath tub" curve
depending on application – some applications are relatively benign, others extremely harsh; some applicationsmay be predictable, others highly unpredictable. You can expect a good supplier partner to ask questions in that regard to better understand the application and howit affects the details of thewarranty. Suppliers who don’t ask these questions and don’t understand these issuesmay not be in a position to support the offeredwarranties.
Reliability and useful life The penetration of LED lighting is driven primarily by two factors. The first is the energy efficiency of the lighting. We have already touched on this when discussing power levels and noting the downward trend in power level as the efficiency of system components improve.
The other primary driver is the suitability of LEDs to reliably
performover a long period of time. But LED reliability and long life are not sufficient by themselves. The other components in the systemmust also havematching reliability and life characteristics. For example, if the optical systems discolor over time, light output is reduced and performance will disappoint. Similarly, the power supply/drivermust continue to performas specified for the life of the lighting fixture. To further abuse an overused cliché “the chain is only as strong as the weakest link”.
It should be noted here that reliability and life are not the same.
Reliability is ameasure of ongoing failures rates attributable to randomcomponent failures, batch problems and/ormanufacturing issues. This is reflected on the flat part of thewell-known ‘bath- tub’ curve (see Figure 3). The ideal situation of no failureswould be indicated by this part of the curve lying on the x-axis. Product life, on the other hand, is a function of predictablewear-out mechanisms. In the case of LED drivers, this is usually driven by electrolyte loss in certain capacitors. Good design practices can ensure that this does not incurwithin the specified life time of the LED driver. On the ‘bath-tub’ curve this is reflected by the rising part of the curve indicating the end of useful life.
This is the area where it is important to evaluate your power
supplier’s capabilities. Everything in the design andmanufacturing processmust be geared towards high reliability and long life. It does not work to start with a poor design and then screen or test for reliability. Itmust start with how the power supply designer chooses components, how these components are derated, how the design is qualified and proven and ultimately how they product ismanufactured. High quality suppliers will be readily able to address these topics. Their goal is constant improvement of the performance of power supply reliability as characterized by the ‘bath-tub’ curve.
Warranties The ability of a supplier to offer and support awarranty depends on their ability to control failure rates and design for an adequate useful life. But thesewarrantiesmay also vary significantly
46 | September 2010
www.electronics-sourcing.com
Summary Although the choices of power supply aremany in LED lighting applications, they can be narrowed quickly throughmatching the power supply or driver specifications and functionality to the application. Multiple suppliers can be narrowed further by critical evaluation of their design andmanufacturing processes. Thiswill ensure that the power supply not onlyworks as required butwill also provide a reliable solution for the life of the lighting fixture.
Conor Quinn is director of technical marketing for Carlsbad, Calif.-based Emerson Network Power
www.emerson.com
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