Download - Led lighting outdoor design challenge dec2013

SPONSORED BY:

2 Five rules for designing roadway lighting

9 LED modules bring energy savings to high-mast outdoor lighting

15 Advanced thermal characterization improves LED street-light design

EDITORIAL DIGEST

Outdoor lighting challenges SSL component and system designersAlthough outdoor lighting can benefit from

the advantages of solid-state lighting — such

as lifetime, reliability, and energy efficiency —

the application presents particular challenges:

the high lumen output required by high-

mast lighting, environmental conditions,

and avoiding inefficient distribution of light,

to name a few. This digest will address

how outdoor lighting system designers can

apply thermal management techniques and

materials to improve resistance to ambient

conditions, and integrate LED sources that

meet the mechanical design and luminous

efficacy requirements of a changing outdoor

environment.

http://e-litestar.com/

LEDs Magazine :: EDITORIAL DIGEST

2

* This article was published in the April 2013 issue of LEDs Magazine.

Five rules for designing roadway lighting

Effective use of LED sources and emerging knowledge of human visual systems guide best practices for SSL roadway lighting.

OF THE MANY design challenges facing LED-based solid-state

lighting (SSL) applications, perhaps there is none greater than that

of expectations. There are expectations around the application.

There are expectations around the incumbent technology. There are

expectations around the way it has always been done, and, as a result, there are

expectations around the way it should be done going forward. What if we were

able, however, to design with a clean sheet of paper? Take roadway lighting as an

example. If we were to take that application, deconstruct it, and come at it from

a different angle, what might

we do differently, and how are

LEDs specifically suitable tools

in this redesign?

When we think about the

job of lighting a roadway,

we are conditioned to think

about what is happening

right in front of us. We think

about targets in the road and

response time in identification.

In fact, the entire series

of metrics for roadway

lighting is modeled around

these requirements. From this

Fig. 1. A driver’s view of a simulated roadway scene, illustrating conventional forward auto lighting combined with traditional roadway lighting.


3


standpoint, our examination of

roadway lighting is fundamentally

no different than our examination

of office lighting. The conditions

and demands of the tasks, however,

couldn’t be more different.

Rule #1: Zoom out and consider the bigger picture.

Once we step back, one of the

things we can appreciate regarding

roadway lighting is that we are

invariably talking about night-time

situations. While the human visual

system has an amazing ability to

tolerate a wide range of conditions,

the mechanisms that allow

for those ranges vary for different lighting levels — night-time environments

especially. To better appreciate how those mechanisms come into play, we need

to consider the retina and its component parts.

The retina is incredibly complex, but its basic role can be summarized by two

types of photoreceptors: cones and rods. Cones are located predominantly in

the center of the retina in the fovea. Rods, which greatly outnumber cones,

surround the fovea and encompass the periphery of the retina. The retina is

in simplest terms a camera. It produces images for the central nervous system

(CNS) to interpret.

The CNS-to-photoreceptor pathways best define the photoreceptor’s role in vision.

Each cone, in effect, has its own direct path to the CNS. A quanta of information

is personally escorted to the brain for processing. This one-to-one relationship

defines its role in higher order perception such as fine detail discrimination and

color analysis. The peripheral vision pathways to the CNS are shared by large

groups of neighboring rods. Light that grazes one edge of the group triggers a

response on the far edge. Through this mechanism, rods preform their basic role

of gross peripheral motion detection.

Fig. 2. A driver’s view of a simulated roadway scene, illustrating asymmetrical forward lighting for objects on the roadway combined with peripheral roadway lighting for detecting objects near the road.

http://e-litestar.com/


5


Using night-time driving as an example of the mechanism, our eyes are directed

for the majority of time at the roadway, where the cones are aiding in the analysis

of detail. When something appears in the periphery, say a deer approaching the

shoulder of the road, this sight registers across many groups of rods, signaling

movement to the CNS. At this point, the eyes move and perhaps the head pivots, so

that the cones can be engaged for better detail analysis and subsequent reaction.

Rule #2: Appreciate the importance of peripheral detection in night-time driving.

Our current metrics are concerned with foveal vision exclusively, yet the fovea

takes up a tiny percentage of the visual field. We essentially light the road

as depicted in Fig. 1. Mark Rea, director of the Lighting Research Center and

professor at Rensselaer Polytechnic Institute, has written extensively on the

subject. Rea has said that considering just the fovea in driving is akin to driving

while looking down a long, narrow tube. Given the choice, would we choose the

field of vision on the inside of the tube or the outside in order to drive? While

what is inside the tube is important, this example illustrates that the outside of

the tube — our peripheral vision, at the very least, deserves some consideration.

While rods work in groups, they are individually much more sensitive to light

than cones. Able to absorb and register even a single photon, one immediately

sees their advantage in night-time conditions. Indeed, as light levels drop, the

rod-to-cone activation ratio increases until rod sensitivities are at a peak level in

night-time conditions.

Rule #3: Consider the different sensitivities of the photoreceptors.

Where the spectrum of light is concerned, the rods and cones respond similarly

to higher wavelengths. Rods are, however, much more sensitive than cones

to lower wavelengths, especially after they have time to adapt to night-time

conditions. If one of our goals is to optimize the lighting to better aid in peripheral

target detection, we should be working with a spectrum that is optimized to that

task and optimized to the photoreceptors (rods) engaged in that task.

Rule #4: Eliminate double work.

Regardless of the importance of peripheral vision, we still need cones for sign

identification/reading and analysis of detail in the roadway. The metric that


6


matters, just as in office lighting, for example, is contrast. How do we present the

task in proper relief? Strong forward lighting (such as provided by car head lamps)

with narrow optics will optimally illuminate the vertical plane and present a

snappy, sharp shadow with an excellent dichotomy between light and dark. Current

roadway metrics, mostly concerned (again, like office lighting) with horizontal

illumination, don’t even consider the vertical plane. As written, the application

requirements only consider overhead lighting, which can have a deleterious effect

on contrast when combined with forward lighting on cars. Roadway lighting needs

to complement forward lighting on automobiles and aid in the creation of contrast

and clear, decipherable indicators to which our CNS can respond.

Rule #5: Light the edges.

More importantly, however, is the ability to identify hazards prior to them being

in the roadway. Rea has suggested, only partially in jest, that better viewing

conditions may be gained by simply pivoting roadway lighting 180o in order

to light the shoulder (Fig. 2). The job of lighting the roadway is then left to

headlights. The optimal solution is most likely a combination of that approach

and current practices, but the clues are there.

The issue with incumbent technology in roadway applications is the one-size-

fits-all limitations. We start with a high flux, high wattage, omnidirectional light

source, and we attempt to corral the beam to meet the application. The approach

is inherently inefficient from an optical perspective. There is no opportunity for

nuance or spectral shaping.

Fig. 3. Cree XSP street lights installed in Hollywood, CA, focus light on the roadway, limiting back light.


7


SSL in roadway lighting

With LED point sources, we build a fixture piece-wise until we have the perfect

distribution — no more; no less. As Fig. 3 shows, SSL fixtures can be designed to

produce almost no light behind the poles. Through proper binning, we are able to

spectrally shape the output in order to best match the visual needs. In the example

we have been using for roadway lighting, we can imagine many different designs or

a combination of attributes in one package.

We could have a component of the beam that lights the shoulder and surrounding

areas of the roadway for the optimal spectrum of the rods. We could concurrently

light the roadway with another spectrum ideal for foveal vision and contrast.

We could have peripheral lighting that stays on constantly in rural settings or in

areas of high deer traffic. Conversely, thanks to SSL instant start capabilities, we

could have peripheral lighting that comes on as a function of peripheral motion.

The fact is that a conversion of roadway lighting to SSL is happening at a rapid

pace, driven in many cases by energy efficiency and low maintenance. The city

of Los Angeles has retrofitted more than 115,000 street lights with LED fixtures

(see Fig. 4). However, SSL can go beyond saving energy by providing significant

enhancements to roadway safety.

Fig. 4. The City of Los Angeles has replaced more than 115,000 street lights with energy-efficient LED fixtures.


8


The options are open-ended. What is clear is that new technology allows

designers the opportunity to not only work with new tools but also return to the

applications themselves and rethink the way things are done. When we do that,

the value of lighting is optimized in its abilities to help people. We escape the

morass of expectations, and we evolve as an industry.

DON PEIFER is a senior product portfolio manager at Cree.

9

* This article was published in the June 2012 issue of LEDs Magazine.


LED modules bring energy savings to high-mast outdoor lighting

While LEDs have pervaded a variety of street- and roadway-lighting applications, most owners of high-mast lights have stayed with HID lamps. A Maine case study indicates significant potential for SSL in the higher-power lights used in places such as freeway interchanges.

WE ROUTINELY COVER case

studies of LEDs used in outdoor,

street- and area-lighting

applications where solid-state

lighting (SSL) is delivering significant savings

in both energy and maintenance costs. But

repeatedly at conferences the prevailing wisdom

among speakers has been that the high lumen

output required in high-mast applications would

require SSL fixtures that cost far more than

metal-halide (MH) or high-pressure sodium

(HPS) sources – an even greater cost differential

than is the case with normal street lights.

Presumably the high cost can stretch the payback

beyond what municipalities or transportation

departments are comfortable with. The Maine

Department of Transportation (MaineDOT),

however, is testing LED-based lights in a high-mast retrofit and the results

are promising.

Fig. 1. Global Tech LED’s high-mast retrofit module.


10


High-mast lights are quite different

in nature from more typical street or

roadway lights. High-mast fixtures are

regularly mounted at 60 ft to more than

100 ft above ground level and occasionally

as high as 250 ft. Normal street lights are

typically mounted at heights lower than

60 ft, and many are in the 30-ft range.

The applications for high-mast lights

include installations at transportation

terminals, other large, outdoor

maintenance or storage yards, and

specialty roadway applications. The

aforementioned freeway interchange

installations are probably the most

common roadway application, although

you will find some high-mast lights within

municipalities in busy areas.

In street-light installations, the lighting

designer normally specifies a rectangular

beam distribution or pattern that directs the lumens precisely and eliminates

light spill. The pattern is designed to evenly illuminate the roadway with

maximum spacing between poles. High-mast applications rely on more of a

circular or square pattern and are designed to distribute light evenly over a

maximum-sized radius or area.

If you look at legacy lights installed in North America, you can generalize about

the two disparate applications in terms of energy usage. Municipalities typically

install 250-400W HPS lights individually on a pole in street-light applications.

High-mast installations regularly gang 2, 4, 6 or 8 1000W HPS fixtures spaced

evenly around a single pole.

Potential savings

Clearly there is potential for savings in such high-mast applications. Including

Fig. 2. Workers retrofit a lowered high-mast fixture.


11


the ballast, a 1000W HPS

light actually consumes as

much as 1200W. LEDs could

certainly cut that energy usage.

Plus consider the potential

maintenance savings. About

high-mast light owners, Jeffrey

Newman, president of Global

Tech LED, said “They have been

replacing lamps once per year.”

Global Tech manufactures LED

modules designed for use in

high-mast retrofit applications.

The modules include six

clusters of seven LEDs for a

total of 42 Philips Lumileds LEDs per module (Fig. 1). Global Tech has developed

customized lenses that cover each LED cluster to control the beam pattern.

Depending on the application, as many as four of the Global Tech modules might

be used to replace a single high-output HID lamp.

Newman is quick to attack the question of affordability of LEDs in the high-

mast application. He laid out a theoretical comparison where the LED alternative

dissipates 600W while the incumbent lamp is the 1200W HPS lamp and ballast.

According to Newman the 600W LED reference case is a very conservative

example, because most likely you would use a lower-power LED configuration.

The LED approach saves 600W. Based on a burn time of 12 hours per night, the

savings amount to 2628 kWh per year. At a rate of $0.12 per kWh, that electricity

saving equates to around $315 per year. The price a municipality would pay

for the retrofit would depend on distributor pricing, but Newman said that

MaineDOT is paying in the range of $1200 to $1300 per kit including credits

supplied by the state. So the payback is in the four-year range before you consider

maintenance costs, and perhaps a lower-power LED implementation.

Fig. 3. An LED-based high-mast pole in Waterville, Maine.


12


According to Newman, the LED

project in Maine is about more than

savings and payback and is focused

on keeping the lights on. He said,

“They were shutting the lights off

at 11 pm at night because of the

expense.” The LED retrofit will

allow the lights to burn all night,

although the long-term plan may

also entail dimming the lights late

at night.

Maine Interstate 295

Ron Cote with MaineDOT said

that he was doubtful that an LED-

based product could serve in the

high-mast application when Global

Tech first approached the state. But

after seeing the modular approach,

MaineDOT retrofitted one high-

mast pole with the Global Tech

modules eight months ago. Cote reports that the retrofitted fixtures on the pole

have been problem free.

The LED kits replaced 1000W HPS lamps. The project used four of the Global Tech

modules in place of the 1000W lamps. Each module dissipates 98W for a total of

392W per fixture. The retrofit relies on a metal mounting plate with four holes for

the modules, and the plate is attached to the reflector of the existing fixture.

Once installed Cote said that the LEDs provide 1 fc at ground level out to a

distance of 200-300 ft. He said, “Up until now, there hasn’t been an LED fixture

that could touch the light distribution of HPS.” But Cote said that the installed

LEDs are providing comparable performance.

After testing the one pole, MaineDOT is retrofitting eight additional poles at two

freeway interchanges. Cote reports that the retrofit process is relatively simple.

Fig. 4. LED high-mast lighting (top) compared with HPS high-mast lighting (bottom).


13


Typically high-mast lights are mounted in such a way that cables can be used

to lower the fixtures to ground level as opposed to requiring a bucket truck

for service (Fig. 2). Cote said it typically takes about 20 minutes to lower a set

of lights and another 20 minutes to raise the fixtures back up the pole once

service is finished. He said it also takes workers about 20 minutes per fixture to

install the retrofit.

MaineDOT is able to afford to burn the LED lights all night. Cote said that the SSL

retrofit is delivering about 66% in energy savings. The energy cost per freeway

interchange has dropped from $800 to $266 per month.

The lights are also superior in terms of quality. Fig. 3. shows one of the Maine

LED high-mast lights. Cote said the broad-spectrum light and 5000K CCT

provide better visibility. Before and after photos weren’t available for the Maine

installation. But Fig. 4 shows LED and HPS high-mast lights from a Global Tech

project at a Florida shipping container terminal.

The energy-conservation-oriented Efficiency Maine organization also commented

on the quality of the SSL retrofit. “I went by the Waterville exits this morning

on the way in,” said Michael Watson, project engineer at Efficiency Maine. “The

Kennedy Memorial Drive exit is done, all four towers have the LED fixtures and it

looks great. They also had one done at the Main Street exit and what a difference

it makes compared to the HPS fixtures. The LEDs really light it up nice.”

Controls and dimming

Looking forward, Cote said that MaineDOT is contemplating a retrofit of 108

additional poles – the entire high-mast inventory along I-295. Moreover the

department may consider dimming the lights for five to six hours each night to

further reduce energy consumption.

Newman estimates that with dimming the energy savings could stretch to 80%.

Global Tech uses a combination of a custom microcontroller (MCU)-based control

circuit on each module along with a modular Philips Lighting driver. The MCU

can dim the lights to any level required. The MCU bases the dimming operation

on the photocell that is already used on each pole to turn the lights on. Newman

said a typical scenario is what he calls 561. Five hours after the lights come on,


14


the MCU dims the lights. The lights remain dimmed for six hours and are brought

back to full brightness for one hour.

Newman said Global Tech has also developed a wireless control network that

can optionally be installed in the retrofit modules. For now, MaineDOT is not

installing modules with wireless support.

Cote said that MaineDOT will likely test dimming at a single interchange in the

next phase of the project. The department will then seek input from the public

and other interested parties on the light levels.

The savings potential of LEDs on the Maine interstate system is significant. Cote

said that the state spends $750,000 annually on interstate highway lighting. Not

all of the lighting is high-mast. But Cote thinks the state could definitely save a

third of the total just through a move to LEDs on high-mast poles.

MaineDOT also expects to realize significant maintenance savings, although they

haven’t projected a figure. Cote said, however, that they were expecting 50,000

hours of life from the LEDs. That would certainly curtail the maintenance cycles

for replacing HPS lamps.

MAURY WRIGHT is the Editor of LEDs Magazine.


15

Advanced thermal characterization improves LED street-light design

A street light is hot-lumens tested in compliance with JEDEC standards

SOLID-STATE LIGHTING (SSL) designers who consider thermal properties in

their LED based design are more likely to produce luminaires with long-

term consistent light output and longer lifetime. In addition, in the case of

LED street lights, illumination often needs to be consistent over a range

of ambient conditions, which can be assured using the appropriate simulation and

thermal testing techniques.

This article demonstrates

how thermal simulation using

computational flow dynamics (CFD),

and thermal testing to the latest

Joint Electron Devices Engineering

Council (JEDEC) standards, can

provide the luminous flux of a

street-light luminaire under various

conditions. The test methods shown

can be used in prototype development,

product testing or failure analysis of

luminaires.

What constitutes good thermal design?

LEDs, as one of the most efficient

light sources available today, are

* This article was published in the July/August 2012 issue of LEDs Magazine.

Fig. 1. Mentor Graphics’ LED characterization flow.


16


becoming more widely used in indoor lighting, outdoor lighting and automotive

lighting. Good thermal design based on the application is essential to ensuring

the longevity of the LED luminaire because both LED lifetime and light output are

closely related to the LED’s junction temperature.

When an LED’s pn-junction

temperature is hotter, the

performance of the LED is

impacted in terms of shorter

lifetime and decreased light

output. In applications such

as headlights of cars or street

lighting where lives might be

at stake, lighting standards are

very strict. In addition to the

prescribed spatial distribution

patterns that are required,

illumination levels also need

to be provided consistently; for

example, even on hot summer

nights, luminous flux of LED-

based luminaires must meet

the lighting standards. This

necessitates having the appropriate knowledge about the thermal and light-

output properties of LEDs.

As of today, diligent lighting design with LEDs cannot be based solely on

a manufacturer’s data-sheet values. Information needs to be gathered

experimentally by physical testing of LEDs, and the gathered LED characteristics

need to be provided for thermal simulation using, for instance, CFD.

Thermal characterization of LEDs

From a semiconductor standpoint, LEDs are simple pn-junctions, thus it seems that

they should be easier to measure, when in actuality they are not. LEDs present a

number of thermal characterization challenges. They are often very small, and

measuring them un-mounted is difficult. Fortunately, parts can be mounted on

Fig. 2. Close-up view of the top cover of the housing (heat sink) of the HungaroLux LED based street-lighting luminaire.


17


Fig. 3. CFD thermal simulation results of LED-based street-lighting luminaire where the applied LEDs were all represented by their compact thermal models obtained from T3Ster TeraLED results.

two different substrates and the dual-

interface measurement principle can

be applied for obtaining their junction-

to-case thermal resistance. A greater

challenge comes from the fact that

LEDs, unlike other semiconductors,

emit light.

Light emission must be considered

when measuring the LED’s thermal

resistance. For the majority of

semiconductor devices, thermal

resistance can be calculated by simply

dividing the temperature rise by

the electrical power applied to the

package. This is because all of the

supplied electrical power is converted

to heat. However, this is not the

case for LEDs because a significant

proportion of the supplied energy is converted into and emitted as light, making

it an efficient light source. Depending on the LED, energy conversion efficiency

can be as high as 30-40%.

Based on these efficiency figures, if the supplied electrical power rather than the

correct (heating) power is used to calculate the package’s thermal resistance, the

thermal resistance value would be significantly lower, suggesting that the package

(of a less efficient LED) would be far better at dissipating the heat generated in the

LED than it actually is. The emitted optical power can be precisely measured to

account for the calculation of the real thermal resistance if thermal testing of the

LED in question is performed in a CIE 127-2007-compliant total-flux measurement

environment, such as a TeraLED system from Mentor Graphics.

In this system, the temperature of the LED under test can be precisely set to a

desired value by a temperature-controlled cold plate. Such a measurement setup is

also suggested by one of the most recent LED thermal testing standards, JESD51-52,

which provide guidelines on methods to measure LED light output in connection


18


Fig. 4. Luminaire surface temperature map as shown by infrared imaging at an ambient temperature of 20°C.

with LED thermal measurements. This standard is one of a group of four new

international thermal test standards for LEDs that were published in May 2012.

As for the thermal characteristics of LED components, the junction-to-

case resistance is the most appropriate metric for packaged LEDs because it

characterizes the heat flow path from the point of heat generation at the pn-

junction down to the bottom of the case – exactly how LED packages are designed

to be cooled. A relatively new standard, JEDEC JESD51-14, for junction-to-case

thermal resistance measurement, is based on the latest thermal-transient

measurement techniques.

This method uses a

dual-interface approach

in which the thermal

resistance of the part

is measured against

a cold plate with and

without thermal grease.

The junction-to-case

resistance is determined

by examining where the

two measurements differ.

Very high measurement

repeatability is required

because the thermal

impedance curves for

the two measurements must be identical up to the point where the heat starts

to leave the package and enter the thermal interface between the package and

the cold plate. This ensures that the point where the curves deviate is clear. It

compares to LEDs mounted on a cold plate attached to an integrating sphere (as

the JESD51-52 standard recommends). This method provides the real junction-

to-case thermal resistance metric for LED packages if during the two subsequent

measurements the cold plate with LED under test is attached to an integrating

sphere (as the new JESD51-52 standard recommends).


19


Solutions for LED thermal characterization

The Mentor Graphics T3Ster thermal transient tester uses a smart

implementation of the static test version of the JEDEC JESD51-1 electrical test

method that allows for continuous measurement during a heating or cooling

transient, which also forms the basis of the JESD51-14 test method for the

junction-to-case thermal resistance measurements. This is also the preferred test

method in the LED-specific thermal measurement guidelines that are provided in

the JESD51-51 standard. The combination of Mentor Graphics’ T3Ster and TeraLED

products provide a comprehensive solution for LED testing which meets the

requirements of all the mentioned standards (Fig. 1).

In high-throughput bulk-testing applications (e.g., in large scale reliability

analysis), a multi-channel T3Ster system can characterize many thousands

of LEDs in an hour. T3Ster’s accurate measurements capture transient

responses of LEDs just 1 microsecond after switching the power off with a

temperature resolution of 0.01°C. This means that the earliest possible part of

the LED’s thermal response is captured; thus, you can see the influence of key

constructional features close to the heat source within the LED package, such as

the thermal resistance of the die attach, after a short time.

The T3Ster Master post-processing software fully supports the JESD51-14 standard

for junction-to-case thermal resistance measurement, allowing the temperature

versus time curve obtained directly from the measurement to be re-cast as

“structure functions” (described in JESD51-14 Annex A), and then automatically

determine the junction-to-case thermal resistance value. Structure functions are

also widely used in failure analysis as part of reliability studies mentioned earlier.

This combined with LM-80-compliant lifetime tests of LEDs helps establish

correlation between LED lifetime and degradation of different thermal interfaces

in the junction-to-ambient heat-flow path of LED components (see mycite.omikk.

bme.hu/doc/102602.pdf).

Because the JESD51-14 methodology yields the junction-to-case thermal resistance

as a side product, the step-wise approximation of the structure function up to

this thermal resistance value provides the dynamic compact thermal model of

the LED package automatically. The identified junction-to-case thermal resistance

values may be published on the product datasheet, and the automatically


20


generated dynamic compact thermal model of the LED package can be applied

directly in CFD analysis software such as Mentor Graphics FloTHERM.

The challenge of correct LED thermal characterization is compounded because an

LED’s efficiency is adversely affected by the junction temperature. This presents

a challenge for both LED vendors and SSL designers. The LED’s light output,

junction temperature, and power draw need to stabilize before measurements

can be taken. Consequently, the static measurement method used to capture the

cooling curve is the only correct approach to characterize LEDs.

The combination of the light output measurement (performed with equipment

such as TeraLED), and thermal transient testing allows measurement of the

light-output characteristics as a function of the temperature. Providing these

data as a function of the reference temperature of the cold plate is useful

information for SSL designers. But the same data is also available as a function

of the LEDs’ junction temperature, which is required for the correct physical

modeling of the light output of LEDs, in other words, the input data for hot lumen

calculations. Such a combined thermal and radiometric/photometric test setup

is recommended by the most recently published JESD51-5x series of LED thermal

testing standards.

Street-light luminaires

Hungary, was to develop street-lighting luminaires with the minimal number

of LEDs per luminaire such that all requirements of the rather strict European

street-lighting standards could be met for a wide range of road categories. The

two principal goals to reach were to obtain the required spatial light distribution

pattern (batwing pattern) and to reach the required level of luminance on the

road surface under all possible environmental conditions.

The first goal required careful optical design for which LED vendors typically

publish their LEDs’ so-called trace files.

Careful thermal design is required to achieve the second goal because the

required level of light output must also be ensured on a hot summer evening. For

this, reliable thermal simulations are needed that properly predict the junction

temperatures of LEDs assembled into the luminaire. Unfortunately, luminaire


21


vendors have not yet published LED thermal models. Thermal data on their data

sheet are sometimes questionable because, so far, no testing standard has been

explicit about the combined thermal and radiometric/photometric testing of LEDs

to be able to yield the real thermal metrics of LEDs. The solution to the thermal

design problem of HungaroLux was provided by the combination of Mentor

Graphics thermal testing and CFD analysis tools.

As described in the previous section, thermal testing of LEDs can yield compact

thermal models of their packages that are directly applicable in CFD simulation

tools. The CAD file of the HungaroLux street-lighting luminaire (Fig. 2) was also

directly used to build the final, detailed system-level thermal model. All 48 LEDs

were replaced by their compact models along with a compact thermal model of

the LED driver circuitry.

From the dissipation of the individual LEDs driven by the nominal forward current

(350 mA, 700 mA, 1500 mA), the driver’s dissipation is also calculated. In this way,

the luminaire-level CFD analysis is performed with real data that represent the

LEDs’ junction temperatures (Fig. 3). The CFD thermal simulation results have been

verified by measuring the surface temperature of the luminaire (Fig. 4).

Because the temperature dependence of the light output characteristics of the

LEDs was known from the same measurements that formed the basis of the

LEDs’ compact thermal models, the total luminous flux output of the luminaire

also could be calculated. Using this method, the luminaire could be properly sized

in terms of the number of LEDs needed to provide the required road luminance

level and for the LEDs’ junction temperature.

Conclusions

Recently published LED thermal testing standards and their commercial

implementations provide tools for comprehensive physical testing of power LED

components. Measurement results can be easily turned into LED compact models

that are directly applicable in CFD-based thermal analysis on the luminaire level.

The system-level CFD simulation results also allow the calculation of the hot

lumens of the entire luminaire because the combined thermal and radiometric/

photometric test setup used in the physical characterization of LEDs yields data

regarding the temperature dependence of the total luminous flux of LEDs. With


22


such a diligent and comprehensive characterization method, SSL designers can

be assured that their final LED-based products will meet the applicable lighting

standards and will provide the expected long lifetime.

ANDRÁS POPPE is a marketing manager at Mentor Graphics and an associate

professor at the Budapest University of Technology. ANDRÁS SZALAI is the chief

financial officer of HungaroLux Light. JOHN PARRY is a research manager at

Mentor Graphics Mechanical Analysis Division.

Download - Led lighting outdoor design challenge dec2013

Top Related