Design Tools for Optimizing the Performance of Solid-State Lighting Systems
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. They also differ from
one manufacturer’s part to another’s.
Figure 1. Relationship between variables affecting LED operation
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. LEDs do
not normally fail completely as a traditional incandescent
light source does. Instead, 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’, 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. Figure 2 illustrates the existence
of a distribution in both lumen depreciation
and time. As a result, it is critical to include a
probability factor when discussing lifetime and
lumen maintenance.
Figure 2. Lumen maintenance distribution curves.
Source: Philips Lumileds
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 3).
Figure 3. (Bxx,Lyy) Lifetime curves for LUXEON® LEDs
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
– one such is the US government’s Energy Star
Program 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 eight,
90-lumen Cool White LUXEON Rebel LEDs, at
750mA current with an ambient temperature
of 30°C, using a heatsink with thermal resistance
of 5°C/W, will generate 1021 lumens of usable
light, junction temperature of 127°C and efficacy
of 54.86lm/W, as shown in Figure 4.
Figure 4. ULT calculations for a recessed downlight example
The light output results in the ULT are adjusted for
heat but not for optical or electrical losses due to
secondary optics and driver efficiency. Therefore,
to account for the optical losses first, an optical
efficiency of 80% will be assumed for the secondary
optics. Incorporating the optical losses
will decrease the ULT results to to 43.9lm/W
efficacy and 816 lumen output, which meet the
Energy Star requirements. Furthermore, to account
for the electrical losses, which will only affect the
efficacy of the system, an electrical efficiency of
65% will be assumed. Incorporating those electrical
losses will finally decrease the ULT efficacy
result to 37.3lm/W, which still meets the Energy
Star requirements.
Figure 5. LRT lifetime calculations and curve
Since the optical and electrical performance specifications
have 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 5 illustrates an LRT lifetime output screenshot
for a LUXEON Rebel LED with a probability
distribution of (B10, L70), drive current of 750mA
and junction temperature of 127°C. This scenario,
where the data originates from the ULT output,
generates an expected lifetime of 42,000 hours,
thus surpassing the Energy Star lifetime target, 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, heatsink, the luminaire fixture, the
ambient conditions and 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 MCPCBs.
Additionally, users can seamlessly integrate
thermal vias, heatsinks, fans and fixtures to
generate the most accurate transient or steadystate
thermal simulations. Figure 6 demonstrates
a QLED project before and after the thermal
simulation for the 8-LED recessed downlight
application using LUXEON Rebel LEDs.
Figure 6. QLED Thermal simulation demonstration
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, all available at
www.FutureLightingSolutions.com, 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 specifications of emerging
standards.
About Future Lighting Solutions
Future Lighting Solutions is the leading provider
of LED lighting components and solution support
for lighting designers and OEMs interested
in taking advantage of solid-state lighting
technology. Future Lighting Solutions provides
LED lighting knowledge, resources, programs,
partners, solutions and logistics support to
promote the development of LED products and
installations. The company is a division of
Future Electronics, the third largest electronic
components distributor in the world. Both companies
operate in 167 locations in 42 countries in the
Americas, Europe and Asia. For more information,
visit www.FutureLightingSolutions.com.
Future Lighting Solutions; Making LED Lighting
Solutions Simple™.
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