LED Technology
Looking for the best
Shane Callanan considers the key points that need to be taken into account when selecting an LED driver so you end up choosing the very best part for your application needs
W
ith so many companies offering a solution to drive High Brightness LEDs it is increasingly
important that you are satisfied with, as a design engineer, the many key performance points that have to be considered before making a commitment to selecting the part that best meets your needs. Highlights on a data sheet are all very well, but you need to understand how the part will function in your design to ensure you get the best performance. As LED designers are forced to produce systems with increased efficiency, and optimised performance, advancements in LED driver technology have to be taken into account. A key point when it comes to choosing the right LED driver is not only reviewing the top level specifications, but also understanding how the driver will perform once it has been integrated into the solution. Below we consider the salient points that need to be considered when choosing a suitable power supply to drive your high brightness LEDS.
Basic driver features Power supplies are a critical part of any system, and can vary significantly in price and performance. As power supply designers are forced to reduce cost a serious trade off, that has to be considered, is the degradation of performance. As a consumer of these parts you need to establish that this does not impact on the overall design requirements. Key points to consider will include: efficiency over line and load, life expectancy, protection features and size & weight. Efficiency over line & Load - It is easy to do a side-by-side comparison of LED driver performance if you are only looking at the highlights on the datasheet. However, this is not the best way to analyse the efficiency performance. If you happen to use the driver at say 80% of the load, then you need to know how things such as the THD, PFC and efficiency will
10 February 2012
change with this variation. This can be observed in Figure 1. The Figure shows the performance of two supplies. Here we are comparing the LDB100 with an alternative
Life expectancy - Any power losses will
increase the internal temperature of components, which in turn will have a direct impact on the reliability of these
lifetime calculation figures to address this issue. Protection features - Protection
features are also important, as these will ensure that damage does not occur to your LEDs in the event of something going wrong. If these are designed onto the driver it will make the design less complicated and ease the integration of the overall system. For example, the LDB product range recently released by Excelsys contains several modes of protection that will maintain the safety of a design in the event of any abnormal operating conditions. These include: Over Temperature Protection:
Figure 1: Efficiency over line and load performance
vendor. You will note that both of the designs can quote a market leading efficiency of 93%. If you choose the LDB200 plots you will get above 90% efficiencies down to 40% of the load, whereas the alternative profile can drop as much as 33 points in efficiency over the same loading range. If, for example, you had derated your
power requirements by say 20% (as is very typical with engineers), you would now be using the alternative design at an efficiency level of 90%, instead of 93%. This can equate to an additional 6 Watts being dissipated in a 200W design, which will lead to a rise in component temperatures. This might seem inert enough from the outset, but you must recall that for each 100 degrees Celsius rise in die temperature of a component there will be a 50% reduction in the reliability of that part, which will have a direct impact on the lifetime expectancy of the part itself.
Components in Electronics
components. Thermal management is a key item that has to be addressed by the design team. A significant number of LD drivers have potting material as part of their mechanical design. The role of this is not just to provide an IP67 rating, but also to disperse the heat away from heat generating components. Potting compound has a thermal conductivity much higher than air, and if used correctly can conduct heat to the surface of the enclosure.
A key feature in achieving high lifetime expectancy figures on any design is to reduce the amount of heat and this will be key to the lifetime of the driver and should be addressed at the outset. It will not be possible to 'add-in' this feature at the back end of the design process, so it would be imperative from the user point of view to have a full understanding as to how the supplier has dealt with this on their design. The vendor should be able to produce
be modelled and this can then be used to avoid harmful thermal conditions. A growing number of designs will have an onboard thermal sensor which will turn the power supply off in the event of an over temperature event occurring. Over Current Protection: Over current
protection is where the device itself can interpret an event where more than the maximum current is being taken by the
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Electronic components used in power supplies and other devices can be exposed to temperature extremes that can damage them or shorten their MTBF (Mean Time Between Failure). It is very important to avoid such potentially dangerous thermal conditions before they can damage the component. By measuring a base temperature (such as ambient or case temperature), component temperature can
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