Online Tools Optimise Solid-State Lighting Performance

Osama Mannan, Technical Marketing Engineer, Future Lighting Solutions (a division of Future Electronics)

READ THIS TO FIND OUT ABOUT: Obtaining real-world analysis of an LED-lighting system and determining the effect of design tradeoffs prior to building. Refining design parameters and achieving improved performance with tools such as the usable light tool, the LED-reliability tool and QLED.

 

 

Every engineer designing an LED luminaire will have a target for lumen output, longevity and efficiency. But as Figure 1 shows, the interactions of the key variables in solid-state lighting – heat and current – are complex. Osama Mannan, Technical Marketing Engineer, Future Lighting Solutions (a division of Future Electronics) shows how they also change between parts from different manufacturers.

Unfortunately, LED manufacturers’ datasheets do not necessarily make it easy for engineers to take these, and other performance variables, into account when making system design decisions. For instance, one factor which defines LED-system performance is the reliability of the LEDs. Unlike traditional incandescent light sources, which normally fail completely, LEDs show a decline in lumen output from a peak when the LED is new. So reliability in LEDs is expressed in terms of lumen maintenance, which is the rate of decline of lumen output against hours of use.

Generally, reliability data from LED manufacturers provides an average or typical representation of the lumen maintenance at a specific current and junction temperature. In reality, however, LEDs degrade differently over time, even with LEDs from the same reel, producing a distribution in both lumen depreciation and time across a complete population of LEDs. As a result, it is critical to include a probability factor when discussing lifetime and lumen maintenance.

 

For lighting designers to make informed design decisions, they will need access to such distribution information. LED manufacturer, Philips Lumileds, has addressed this requirement by expressing LED-lifetime data in terms of (Bxx, Lyy). For instance, (B10, L70) would mean that 10% of the LED population will fail to maintain 70% lumen maintenance at a specific current, junction temperature, and elapsed operating time. This lifetime data is presented graphically with reference to drive current and junction temperature (see Figure 2).

To incorporate all the above concepts, and to manipulate the design variables in order to achieve the best performance, the LED Reliability Tool (LRT) and the Usable Light Tool (ULT), available free at www.FutureLightingSolutions.com, can be used to make design trade-off decisions and to provide designers with a true real-world analysis of high-power LEDs.

 

 

The application of these tools is best illustrated through an example. Many lighting-equipment manufacturers now face the requirement to meet government-mandated performance specifications, such as the US government’s Energy Star Programme for solid-state lighting. The Energy Star standard for a commercial recessed downlight, for example, requires the luminaire to have a minimum light output of 575lm, minimum efficacy of 35lm/W, and 70% lumen maintenance at a minimum of 35,000 hours. The ULT can be used to calculate light output, junction temperature and efficacy for systems using LUXEON® LEDs from Philips Lumileds.

For instance, running the ULT using seven, 80-lumen LUXEON® Rebel LEDs, at 800mA current, with an ambient temperature of 30°C, using a heat-sink with thermal resistance of 5°C/W, will generate 810 lumens of usable light, a junction temperature of 127°C, and efficacy of 46.31lm/W, as shown in Figure 3.

 

 

The light output results in the ULT are adjusted for heat but not for optical losses due to secondary optics. Therefore, to account for these optical losses, an optical efficiency of 80% will be assumed for the secondary optics. Incorporating the optical losses will decrease the ULT results to 37lm/W efficacy and 648 lumen output, which still meet the Energy Star requirements.

Since the optical performance specification has been met, the LED Reliability Tool can now be used to determine whether the design will meet the lifetime requirement of 35,000 hours. Figure 4 illustrates an LRT lifetime output screenshot for a LUXEON Rebel LED with a probability distribution of (B10, L70), drive current of 800mA and junction temperature of 127°C. This scenario, where the data originates from the ULT output, generates an expected lifetime of 40,000 hours, thus surpassing the Energy Star lifetime target, as well as in addition to the lumen output and efficacy specifications for recessed downlight applications.

 

 

It is worth noting that all LRT results have a confidence interval of 90%. The above design can be further refined by adjusting the thermal management solution. The designer may wish to modify the LED drive current, heat sink, the luminaire fixture and the ambient conditions, in order to instantly verify the effects of these adjustments on the overall system. In order to accomplish this task, QLED thermal design and simulation software, available at www.FutureLightingSolutions.com, can be used. QLED guides users through step-by-step design wizards to select, place and simulate power LEDs mounted on FR-4 boards or Metal-Core Printed Circuit Boards (MCPCBs).

Additionally, users can seamlessly integrate thermal vias, heat sinks, fans and fixtures to generate the most accurate transient or steady-state thermal simulations. Figure 5 demonstrates a QLED project before and after the thermal simulation for a 7-LED recessed downlight application using seven LUXEON Rebel LEDs.

 

 

It is evident that obtaining a real-world analysis of an LED-lighting system and determining the effects of design trade-offs prior to building it can significantly save time, cost, and effort in the prototyping cycle. Likewise, using tools that perform calculations and help to instantly verify the effects of adjustments on the overall system will provide a more comprehensive end result.

Tools such as the Usable Light Tool, the LED-Reliability Tool, and QLED are all available at www.FutureLightingSolutions.com and can be used to ease the design and prototyping stage, and showcase immediate results.

These results enable designers to further refine design parameters and achieve improved performance for their design in order to develop efficient systems, both optically and thermally, to meet the specifications of emerging standards.

 

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