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LED Lighting operational temperature. Thus, unless the datasheet clearly states lifetime at Tc (max), if a luminaire is expected to run at the top end of the specified operating temperature range, driver lifetime may be compromised. Some, but by no means all, datasheets will show a graph depicting how lifetime decreases as temperature rises above ambient. Some might state the reduced lifetime for operation at Tc (max). But it is the responsibility of the design engineer to winkle out the detail buried in the datasheet. Another popular ‘get out’ is to specify a ‘maximum failure rate’, only beyond which will a manufacturer consider warranty claims. This figure might be 3, 5 or 10% depending on the complexity and the performance of the driver. In such an instance, an engineer may strictly follow all the required design parameters, but unless more than 10% of devices are failing, the driver may not be covered by the warranty. And in most commercial/professional applications, a failure rate of higher than 1 in 10 is probably not acceptable. One driver supplier states that it only covers product failures when the ‘nominal failure rate’ is exceeded, but that rate is not at all evident in any literature or on the website. Some drivers will boast a temperature protection mechanism. However, it is important to know at precisely what temperature it is triggered. There are cases where the lifetime of a driver at 70°C is 50,000h, dropping to just 18,000h at Tc (max) stated at 85°C. Yet the temperature protection mechanism is only triggered at 110°C. At this point, not only is any guarantee already invalidated, but also lifetime falls dramatically further. Even if the temperature protection mechanism is triggered earlier, it may mean that non-critical damage may have already occurred to some components, which could impact lifetime in the longer term. No recompense Research has shown that most luminaire failures can be attributed to the driver. The problem is that despite warranties and guarantees, driver manufacturers have become expert at avoiding any liability. There is a wide range of ‘get out’ clauses even allowing for the technical design and operational parameters already mentioned. Some vendors will blame failures on poor installation, surge conditions, or even outdated software (in the case of one DALI driver). A major contributory factor to the problem of LED driver selection is that there is no industry standard yet published on LED driver testing, or on how to present information in datasheets so that end users can make easy comparisons on features and performance. LED driver manufacturers need to be challenged more often and more rigorously to present all the relevant information clearly, especially on how their products truly perform and those parameters that cause drivers to fail. Show by example European LED driver manufacturer, Lumotech, is aiming to pioneer better documentation in the LED driver market. It has produced an informative White Paper highlighting the issues, and which includes a checklist for designers to test LED drivers to ensure the right choice. Lumotech also takes a no-nonsense approach regarding its own product datasheets, clearly indicating the technical performance and reliability parameters and operating conditions. Graphical data shows efficiency with respect to power, load and temperature conditions, power factor and power output, and harmonic distortion performance across current ranges. In addition, the company offers a totally transparent five-year warranty on all its products, and lifetimes are clearly stated at Tc (max). In summary, LED drivers cannot be selected on cost alone. It is essential to ask the right questions of your potential supplier, and to test driver performance across expected conditions. Above all, read the specifications in the data sheets very carefully. 36 Electronic Engineering Times Europe November 2015 www.electronics-eetimes.com


EETE NOV 2015
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