25742-Designing With LEDs eBook 09-29-2010

8/8/2019 25742-Designing With LEDs eBook 09-29-2010

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EDN 1

exclu v eDn -Book

CHICAGO, ILfrom theTechnical Seminar

September 29, 2010


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Designing with LeDs EDN 2

Welcome to the e-book version of EDNs 2010 Designing with LEDs seminarEDNs fourth Designing with LEDs event , held in Chicago on September 29, 2010 at the Rosemont/Stephens Convention

Center addressed high-brightness (HB) LED design challenges from the viewpoint of the hardware engineer. The event featuredkeynotes by lighting industry luminary Cary Eskow as well as a panel discussion among representatives from three leading LEDmanufacturers on The 50,000-hour lumen maintenance myth. Paper topics included power control, thermal management,and opticsall of which affect the cost, efficiency, and lifespan of LEDs. Technical papers also discussed the impact of lightingcontrol and communication on system cost and usability.

In order to broaden the reach of the event and make its content available to those unable to attend, we have published atranscript of the LED manufacturers panel discussion and six of the technical papers in this e-book. We hope you benefit fromthese and look forward to seeing many of you at our next LED Workshop!

Best Regards,Margery Conner

Technical Editor, Power and ComponentsEDN magazine [email protected]

4Panel Discussion: LEDs and the

50,000-hour lifetime myth Mark Hodapp of Philips Lumileds

7 Panel Discussion: What LM-80 Is & Is Not

Paul Scheidt, Cree

9White Paper: Design Challenges or

Solar-powered HBLED Lighting Heather Robertson, Avnet

12White Paper: Understanding E fcient Heat Removal

in HB LED Applications Barry Dagan, Cool Innovations

15White Paper: Practically Speaking:

LED Light Measurement Wolfgang Daehn & Bob Angelo, Gigahertz-Optik

20White Paper: What Mechanical Engineers

Should Know About LEDs Richard Zarr, National Semiconductor

21White Paper: Integrated Solar Powered

Lighting Solutions Luca Difalco, STMicroelectronics

25White Paper: On the Use ul Li etime

o LED Lighting Systems Geof Potter, Texas Instruments

Table o Contents


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Allied Electronics , is a small order, high service level distributor of electronic components andelectromechanical products with more than 50 sales branches across the United States and inCanada.

Avnet LightSpeed Whether you are considering a new application or are interested in re-visitingan existing design, Avnet Electronics Marketing has the LED technology and product experts youneed to get the job done. Our team of engineers can help with all areas of LED technology, thermalmanagement, power driver stage and secondary optics. From design to delivery Avnet ElectronicsMarketing brings together the worlds foremost LED, high-performance analog and optical/electromechanical manufacturers, along with best-in-class technical expertise and supply chainmanagement services.

Bergquist Thermal Clad is an insulated metal substrate circuit board providing complete thermalmanagement systems for surface mount and High Power LED applications. Available in standard andcustom configurations, Bergquist Thermal Clad solutions provide better thermal management withlower die temperatures, extended LED lifetimes, and increased light output. The Bergquist Company designs and manufactures high performance thermal management materials used to dissipateheat and keep electronic components cool. With some of the best-known brands in the business,including Sil-Pad, Gap Pad, Gap Fillers, Bond-Ply, and Hi-Flow phase change grease replacementmaterials, Bergquist is your total thermal management supplier.

Coilcra t:See how Coilcrafts LED Design Center makes it easy to pick the perfect inductor for yourLED driver circuit. Start with a specific IC, a driver topology, or inductor specs. In seconds, youll geta list of options with performance data, pricing, even detailed loss calculations.

International Rectifer (NYSE:IRF) is a world leader in power management technology. IRs analogand mixed-signal ICs, advanced circuit devices, integrated power systems and components enablehigh performance computing and reduce energy waste from motors, the worlds single largestconsumer of electricity. Leading manufacturers of computers, energy efficient appliances, lighting,automobiles, satellites, aircraft and defense systems rely on IRs power management benchmarks topower their next generation products.

Jameco has been supplying electronic components to design engineers, product developers, educatorsand hobbyists for over 35 years. Known for its personalized service, Jameco offers a wide range of name brand and house brand pricing options, a low-price guarantee and the highest quality catalogin the industry. Get your free catalog and start saving today.

National Semiconductor s energy-efficient LED drivers provide constant current to arrays of LEDs, enabling color and brightness matching over a wide temperature range. Through dimming,thermal management, and fault protection, Nationals drivers improve performance in a variety of applications. Use the new WEBENCH LED Architect design tool to build the optimal LED driversystem to meet your requirements. Check out Nationals LED drivers and design tools today.

Philips Lumileds delivers reliable, illumination-grade power LEDs and outstanding world-wideservice and the design support that customers need to rapidly develop LED lighting solutions. Asan industry leader, Philips Lumileds LUXEON LEDs are used widely in retail, entertainment,outdoor, automotive, and many other applications. Philips Lumileds is a technological pioneer inLED development, delivering the most lumens per watt, per package and per dollar with the widestoperating range, reliability and lumen maintenance. LUXEON LEDs are enabling lighting solutionsthat are more environmentally friendly, help reduce CO2 emissions and reduce the need for powerplant expansion..


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Panel Discussion Transcript: LEDs and the50,000-hour li etime mythPanel Discussion: LEDs and the 50,000-hour lifetime myth

Mark Hodapp of Philips Lumileds

Some things you may not know: 10 years ago the myth was theexception, not the rule. 10 years ago was about the date when [theindustry] started developing what we call high-brightness LEDS.Before that, LED technology was made with other technologies,gallium arsenide, gallium arsenide phosphide, the allen gaptechnology, and colors were red, yellow, and green And we usedepoxies for most of the LED construction. Then Nichia developedthe GaN LED that lead to the blue and white {LED color]technologies. [The industry] found out that those materials that

were used with building LEDs up until then really werent all thatcompatible with the blue and white [product] lines.

Source: Rensselaer Polytechnic Institute, Troy, NY; NationalLighting Product In ormation Program publication Lighting

Answers: Light Emitting Diode Lighting Systems, Volume 7,Issue 3, May 2003.

Here are some of the studies that were done roughly ten yearsago. This is work that was done by the Lighting and ResearchCenter [LRC] at Rensselaer. They looked at lumen maintenancefor different types of LEDs. These are the standard 5mm LED

which was the commonest led at the time. LRC found out thatthe shorter wavelength devices did not last all that long. The reddevices, gallium arsenide phosphide, really do last a long time:

You can see that at 10,000 hours of operating theyre still brandnew at 85% of their original light output. But they found that theshorter wavelength devices, particularly the blue and white, werentdoing that well. They found out that the epoxies werent all thatcompatible with this really short wavelength light. So, at least thefirst white LED technology really didnt do so well. Instead of lasting50k hours, we were lucky to get a couple thousand hours.

Source: Rensselaer Polytechnic Institute, Troy, NY; NationalLighting Product In ormation Program publication Lighting

Answers: Light Emitting Diode Lighting Systems, Volume 7,

Issue 3, May 2003, Figure 13

LRC also found out that these epoxies were really sensitive totemperature and drive current. When run at a higher temperature,the LEDs would degrade a lot faster. Drive them at higher drivecurrent, which also means higher flux levels, and these devices

would degrade a lot faster. So these LEDs really werent all they were cracked up to be. And realize that 10 years ago the market waspretty much all this type of LED technology [red, yellow, and green].There were just a few [LED manufacturers] starting to release high-power LEDs.

Source: Narendran, N., L. Deng, R.M. Pysar, Y. Gu, and H. Yu,2004, Per ormance characteristics o high-power light-emittingdiodes. Third International Con erence on Solid State Lighting,Proceedings o SPIE 5187: 267-275.

This is some work that LRC did on comparing two white LEDs one was a standard 5mm LED and one was a high-power LED. After18,000 hours of operation the high-power LED is still running at80% of its initial light output while the 5mm LED after 10,000hours was down to 40%. Their difference was in the packaging.Remember, an LED is not only the LED chip itself and it turns outthe packaging has a huge effect. [The industry]started using siliconmaterial instead of epoxies and this made all the difference.


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So basically this is what is meant by LM80 testing. At a minimum youre supposed to test for 6,000 hours, with data reported at1,000 hour increments. I believe all major LED manufacturers arecomplying with LM80 test protocols.

With this data I wanted to show a couple of things: This is actualdata for an LED, tested after 15,000 hours. It also shows the DOE(Department of Energy) 6,000 hour limits those are the little reddiamonds. Limits based on the equation shown on the slide they do a simple exponential extrapolation of lumen maintenance withsimple equation of flux as a measure of time using the exponentialequation. If you follow the limits in the spec, and you use thisequation what youll find is that if the device lumen maintenanceexceeds those limits at 6,000 hrs or actually met those limits at6,000 hrs then with this simple exponential model the lifetime

will be 25,000 or 35,000 hours.

Equation: flux(t) = EXP (alpha t) where alpha = LN(EnergyStarlimit)/6,000hr

Thats how those limits were developed. The limits are from the USgovernment. These are the limits that we manufacturers would liketo have our products meet or exceed to meet those requirements.

Heres a slide of some independent studies with different types of LEDs. What you can see from these is that the LED wavelengthhas a huge effect particularly the shorter wavelengths . Materialstend to be more affected by the shorter wavelengths. When you seea product that advertises a 50,000 hour LED, you want to questionthat and see if there is actually lumen maintenance test data toback it up. Package construction and material had a huge affect onlumen maintenance. The drive conditions, particularly with thetemperature of material and the delta flux levels of the light have ahuge affect on lumen maintenance.

These studies were done several years ago. Since then the industry has developed a protocol for lumen maintenance testing calledthe LM80 test. My message here is that you shouldnt assume justbecause its an LED it will last for 20,000 hours: You really need tolook at the manufacturers test data to prove thats really the case.

And realize that when this myth was promulgated, 50,000 hours was the exception, not the rule. Today, I dont know that I would say that its the rule not the exception, but it really depends on how theLEDs were manufactured because theres still a huge industry outthere building epoxy-based LEDs that probably are not 50,000-hourLEDs.

So to go to the present, Id like to talk briefly about LM80 testing.Several years ago the LED manufacturers got together in conjunction

with the US government and wrote a spec on how to do lumenmaintenance testing. The spec says how to do testing in a way that

you can compare different manufacturers results. The key thingsin the spec are to do the test at different control case temperatures.

As a minimum there are three different temperatures: 55C, 85C,and a third temperature. It turns out that we werent able to getagreement on what that third temperature should be because we

were testing at different temperatures. So 55 and 85 were standardsbut not the third which we left it up to the manufacturer to pick.

When you build the product youre supposed to thermocouple theLEDs in the product and compare the thermocouple measurements

youre making with these temps. As long as youre tempmeasurement is within the range that that the LEDs were tested by

the manufacturer, you can use that test report to show how youreproduct will behave. So the key thing is if youre running at a hightemp then the third temperature needs to be similarly high.

The spec also sets limits for the effective air temperature in thechamber which needs to be approximately the same as the casetemperature because it sets tolerances for how the devices aremeasured.


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Paper available on reliabil ity: http://apps1.eere.energy.gov/buildings/publications/pdfs/ssl/led_luminaire-lifetime-guide.pdf

Evaluating the Lifetime Behavior of LED Systems (a methodology for LED testing) www.philipslumileds.com/pdfs/WP15.pdf

When the LM80 spec was finished it talked about how to test theparts for 6,000 hours minimum, but it didnt talk about how toextrapolate the data from, for example, 6,000 hrs to 25,000 hrs.

There is a fairly new committee called the TM21 committee which has been chartered with how to take the 6,000 hr data andextrapolate it out to 25,000 hr and onward. Our goal is to have thespec written this year.

Lumen maintenance is important, but its not the whole story onLED reliability. Lumen maintenance takes a really narrow focus,looking at the LED component only. It doesnt say that the light as a

whole will last 50,000 hours. The whole system is quite complicated you have drivers, you have optics. You have to look at the rest of the system as well.

You can imagine all the points of the previous slide as like the inksof this chain. We dont really know necessarily up front where the

weakest link is.


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Introduction You cant go to an LED or lighting conference without hearing

LM-80 come up so lets go to LM-80 School and address whatLM-80 is and is not.

AN LM-80 report is really cheap if you want to go buy one its only $25.

What LM-80 IsLM-80 covers the measurement of lumen maintenance forLED packages, arrays and modules and measures their lumenmaintenance over time. Where people get tripped up is that they think it has something to do with extrapolation methods, or hassomething to do with LED lifetime. What is it is an approvedmethod for doing the long term lumen maintenance testingof an LED but it does not provide any guidance or make any recommendation about prediction methods. In fact, we have to do

6,000 hrs of testing , and there is a note which many people dontknow about that recommends 10,000 hrs of testing in order to doextrapolation .

LM-80, covers the measurement of lumen maintenance of yinorganic LED-based packages, arrays and modules

This approved method does not provide guidance or makeyany recommendation regarding predictive estimations orextrapolation for lumen maintenance beyond the limits of thelumen maintenance determined from actual measurements.

..the unit shall be driven for at least 6,000 hours 10,000yhours are preferred for the purposes of improved predictivemodeling.

What LM-80 Reports Look LikeHeres what the report looks like its a bunch of numbers.

What LM-80 Is NOT An extrapolation of any kindy

A lifetime numbery- IES TM-21 (scheduled for Q1 2011) will provide anindustry-standard way to derive L70 lifetime fromLM-80 data

BUT only for each particular data set

A report cardy- There is no pass or fail, LM-80 is simply data

- Criteria must be applied to the dataSpecific to anyones real LED designy

-LM-80 requires that Ta=Tc, which is usually NOT thecase for real designs

Its not a report card. Its information overload and generally not what peope are looking for its just a manner of collecting data. Itsalso not a lifetime number. Like I said, LM-80 is not a report card,there is no passing or failing: LM-80 simply is data. There must becriteria applied to the LM-80 data in order to figure out whether youhave passed or failed anything.

Last point: Its not really specific to anyones real LED design.The LM-80 requires that the ambient temperature and the casetemperature of the LED are held within 5 degrees of each other,

and this is not actually the case for most real LED designs. So whenthis question comes, Do you have LM-80? it usually doesnt mean,please give me the complete LM-80 report, because I dont know how to interpret it.

Whats the Real Question?I want to get down to two more basic questions that are underneaththis Do you have LM-80? type of question.

First: Is your combination of thermal management & LEDchoice good enough to pass a certain criteria set?

Which criteria?

ENERGY STAR SSL: DOE pre-LM-80, DOE LM-80,yEPA (D&R)

ENERGY STAR ILL: no LM-80 optiony

Design Lights Consortium (DLC)y

Is your combination of thermal management in your LED systemand the LED that youve chosen to use in the system, are those twothings together good enough to pass a certain criteria set? And thisis a question asked of the system designers of the world -- not theLED manufacturer -- as to whether they made the right choice intheir LED system design.

And the question there is: Which criteria are you trying to pass?There have been several instances of the Energy Star SSL [solid-state lighting]. Theres now the Energy Star integrated LED lampprogram which doesnt have an LM-80 option you have to test thebulbs. And then theres the Design Lights Consortium, which wasput together to help validate designs that are not covered by Energy Star programs.

Second: Is your lifetime rating full of crap? Is your luminaire/lamp lifetime rating full of crap?

All LED manufacturers have their own modelsy(opinions)

TM-21 will provide a consensus opinion for certainydata sets(cont)

Panel Discussion Transcript: What LM-80 Is -and Is Notby Paul Scheidt, Product Marketing Manager at Cree


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Re erences

[1] ENERGY STAR SSL, Category A Applications

[2] ESILL Lumen Maintenance Requirements

[3] ESILL Lumen Maintenance Options

Cant prove anything unless you always want to use 6y year old LEDs

All the LED manufacturers have their own models for how toproject LED lifetime. They all boil down to the manufacturersopinions of how long the LEDs are going to last. TM 21 will providea consensus opinion for certain data sets, and again those datasets are provided under LM-80 conditions. But the note I want toleave you with is that you cant prove anything unless youre alwaysgoing to use 6 year old LEDs unless youre always willing to waitthe whole 50,000 hours --which is about 6 years --for that completetesting to be done. So again you have to ask yourself: How muchdata do you need in order to feel good about the lifetime ratingsthat are out there?.

All LED manufacturers have their own modelsy(opinions)

TM-21 will provide a consensus opinion for certainydata sets

Cant prove anything unless you always want to use 6y year old LEDs

y

Semiconductor Reliability Testing

Fortunately we have the model of semiconductor reliability to follow.First, a BIG caveat electron-based testing is different than photon-based testing. But we are learning. The more data we get, the better

we get at predicting lifetime with early data. But we do have decadesof knowledge in how accelerated lifetime testing works.

Reliability test methods and acceptance criteria forysemiconductor components have been standardized (JEDEC,EIAJ, others) and practiced for decades

Think: processors, regulators, microcontrollers, etc..

If youve recently flown in an airplane, driven in a

car, or talked on a cell phone, youve trusted your life on this body of scientific work and testing

ENERGY STAR SSL, Category A Applications Residential

Kitchen under-cabinet 24 lm/W Portable desk lights 29 lm/W Recessed , pendant downlights 35 lm/W Ceiling-mounted luminaires 30 lm/W

Cove lighting 45 lm/W Surface-mounted directional lights 35 lm/W Outdoor porch lights 24 lm/W Outdoor step lights 20 lm/W Outdoor pathway lights 25 lm/W Outdoor decorative lights 35 lm/W

Non-residential Recessed downlights 35 lm/W Under-cabinet 29 lm/W Portable desk lights 29 lm/W Wall-wash luminaires 40 lm/W Bollards 35 l

http://www.energystar.gov/ia/partners/product_specs/program_reqs/SSL_prog_req_V1.1.pdf

CRI>75;2700K 3000K 3500K

CRI>75;2700K

-5000K

8

The Minimum Requirements Products may not be qualified on LED component testing alone (i.e. via LM-80) Full ENERGY STAR approval requires the following:

ESILL Lumen Maintenance Requirements

Testing Requirement Bulb Type Requirements

10 Lamp Test5 base-u 5 base-down

Decorative(15,000 hrs L 70 lifetime)

Ta ! 25C

.,LM-79 Report from

Accredited Testing Lab Temperature Stabilized

Non-Standard,Omnidirectional,Directional(25,000 hrs L 70 lifetime)

LED Lamp Power < 10W: Ta ! 25C LED Lamp Power ! 10W: Ta ! 45C

Average of 10 lamps > 91.8% LF at 6,000 hours (250 days)

Lumen Maintenance Options1. Claims of L 70 lifetime > 25,000 hours (up to 50,000 hours)

2. Early qualification with LM-80 data

ESILL Lumen Maintenance Options

10

Maximum L 70 Lifetime Initial Approval Final Approval

30,000 hours ! 93.1% LF @ 6000 hours (250 days) ! 91.2% LF @ 7,500 hours (312 days)



45,000 hours ! 95.4% LF @ 6000 hours (250 days) (not specified)



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Solar powered high brightness LED (HBLED) lighting systems are asmart energy choice, and one which will continue to see increasingadoption. Powered by a clean energy sourcethe sun, they also takeadvantage of an extremely efficient form of lighting in HBLEDs. Inthis whitepaper, we will discuss the major components of a solarpowered HBLED lighting system, as well as the design challenges inimplementing such a system.

Figure 1 Solar Powered HBLED Lighting System

In typical HBLED lighting applications, power conversion is animportant factorand even more so in solar power applications. Notonly must power be maximized from the solar panel, but it must alsobe conditioned for maximum energy storage in the battery array, andconverted for use powering the HBLEDs.

Fundamentals o Driving HBLEDs As a diode, LEDs exhibit a steep voltage-current (V-I) curve, whichmeans that even a small change in voltage will result in a largechange in current and brightness. It follows then, that the mostconvenient method to regulate their output is to control currentand not voltage. When multiple HBLEDs are used in a current-controlled configuration, they are often wired in series to ensureuniformity of brightness.

Based upon application, there are a large number of high efficiency,highly integrated HBLED driver solutions available. Common powerconversion topologies for HBLED constant current drivers are:boost, buck, sepic, and flyback.

HBLED drivers typically look like a well behaved load on thebatteryin most applications, there will be no need to support a

high current surge during start up due to the fact that LEDs are nota capacitive load, and most integrated HBLED driver circuits havesoft startup protection.

Estimating Current Load When designing a system, the size of batteries and solar panels mustbe established. To do this, an estimate of current consumption isrequired, usually measured in Amp-hours. Current consumptionin a design consists of standby power and active power . In this case,standby power would refer to times when the HBLEDs are off,such as in the daytime. In standby mode, current use would beminimized by design, with only a small amount of sense/controlcircuitry awake, or shut off completely with only leakage currents

remaining. Active power refers to power consumption when thesystem is operating, in this case when the HBLEDs are on. Designsmay have only one active/standby mode, or multiple active/standby modes. Active modes usually have a well understood duty cycle-- apercentage of time in a 24hr day in which the system is active.

IL=(IST*24)+(IAM*FDutyCycle*24) (1)IL =Current load (Amp Hours)IST=Standby current (Amps)I AM =Active mode current (Amps)FDutyCycle=Duty Cycle Factor (Percentage)

Sizing & Choosing Solar PanelsThere are a number of technologies available for solar panels, andeach has its own unique characteristics. Table 1 is an overview of

technologies and their notable features. For the majority of solarlighting applications, crystalline silicon panels are used because of their general availability and high efficiencies. Most HBLED lightingsystems require panels less than 100W. More so than any of theother technologies, crystalline silicon panels are available in theform factors and smaller sizes suitable for industrial applicationsranging from 5W to 100W. In an example of an interesting fit fora different technology, flexible amorphous silicon has been usedto wrap a light pole with solar materials, forming the solar panelaround the light pole itself.See Margery Connors article on off-grid lighting on the EDN

website for more information.

Table 1 Solar Panel Technology Summary

To size the solar panels, a number of factors need to be considered.Optimum performance for solar panels is achieved (assuming noactive tracking) when the panels are pointed south, with a tilt angleapproximately equal to the latitude of the instal lation + 20 degrees.Further optimization can be achieved by adjusting the tilt angle of the panels 2 or 4 times per year to track seasonal changes in the pathof the sun. If the solar panels cannot be placed with optimum tiltangle, or facing south, they wil l not be able to generate maximum

power. As an example, some systems require panels to be placedparallel to the ground, with no tilt angle at all, or, they may be placedsubject to significant shading. An orientation/shading multiplicationfactor for these situations must be figured into the panel sizingequation. In addition, it is prudent to factor degradation into the sizeof the panels as well. Over a 20 year period, solar panels may lose asmuch as 20% of their rated output.

It is important to understand the amount and power of sunlight thatis received in a given geographic location in order to plan a solarpowered lighting system. Insolation is the term used to describethe amount of solar irradiation received on a given surface area in agiven time, usually measured in KW-hrs/m2 per day.

Design Challenges orSolar-Powered HBLED Lightingby Heather Robertson, Avnet


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it is important to include a design factor to account for the impactof ambient temperature, as lead acid battery capacity is reducedin cold temperatures. Table 3 shows design factors for lead acidbatteries.

Table 3 Lead Acid Battery Temperature Design Factor

A simple equation for battery capacity is shown below:

(3)C battery =Battery Capacity IL =Current LoadTsolar=Number of Solar Autonomy DaysFTemperature =Temperature Design FactorF

DoD=Max Battery Discharge %

This equation is a good estimate for lead acid batteries where therated discharge rate is similar to the expected discharge rate. If thedischarge rate is higher or lower than the specified discharge rate of the battery, this estimate must be adjusted accordingly. Peukerts Law may be used to calculate effects due to discharge rate. Lithium ionbatteries generally do not have a significant sensitivity to dischargerate.

Maximum Power Point TrackingSolar panels have a characteristic I-V curve which varies dependingupon irradiance and temperature. As can be seen in Figure 3 below,there is a point on the IV curve where the panel will be generating

maximum power. In many solar applications, the system is designedto operate the panel at this point, generating maximum power. Thisis called MPPT (maximum power point tracking). Many MPPTalgorithms exist, but the goal of all of them is the sameto operatethe panel as close as possible to the characteristic peak power pointof the power curve.

Figure 3 Solar Panel Voltage/Current and Power/VoltageCharacteristics

The National Renewable Energy Lab (NREL) has a variety of solarinsolation data available. When sizing solar panels for a system,another fact that must be considered is the number of days that thesystem must be able to run without any sun at all; this is known assolar autonomy days. Estimates for general and conservative systemsare shown in Figure 2 below, based on the worst case winter solarinsolation for the regions where the system will be installed.

Figure 2 Solar Autonomy Days

Wpanel (2) W panel=Panel SizeILoad=Current LoadForientation =Orientation/Shading Factor/Degradation

V battery =Nominal Battery Voltage Winter Peak Insolation=Number of Winter Sun Hours (NREL data typically used)

Sizing & Choosing Batteries Another aspect of a system is energy storage. While sealed lead acid(SLA) batteries are the most common battery type used in off-gridsolar powered HBLED lighting applications, other options may befeasible, as shown in Table 2 below. SLA batteries can be chargedbelow freezing, which is desirable in solar powered applications.

While other technologies may be able to operate below freezing,charging may be an issue at low temperature.

Table 2 Bat tery Technology Summary

Lead acid battery performance is impacted significantly by temperature, depth of discharge, and rate of discharge. Thesebatteries are often labeled by not only capacity (AH), but also by rate of discharge. The rate of discharge impacts the capacity of a

batterythe faster the rate of discharge, the lower the capacity. Theconverse is also truethe slower the rate of discharge, the higher thecapacity of the battery.

Depth of discharge (DoD) also needs to be carefully considered.DoD is the amount of energy (expressed as a percent of totalcapacity) that will be discharged from the battery. The greater theDoD, the fewer cycles that the battery will be able to supportthereis a direct relationship between DoD and battery life. 50% of capacity is the generally recommended limit for deep-cycle lead acidbatteries. Lower DoDs will extend battery life.

In choosing the capacity of a battery, especially lead acid batteries,


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Figure 4 Cypress PowerPSOC Re erence Design

The Cypress architecture uses a current controlled buck regulatorfor MPPT and battery charging. The MPPT and battery chargingalgorithm embedded in the PowerPSoC uses voltage and currentfeedback from the panel and operates the panel at its peak powerby controlling the switches in the buck regulator. The switches inthe synchronous buck regulator circuit are also operated in a way toensure that the current delivered to the battery is per requirement of the current charge state of the battery.

Figure 5 MPPT/Charge Control Implementation Block Diagram

The solution features two LED drivers: one floating load buck driverfor LED loads up to 8V and 2A where the forward voltage is lessthan the battery voltage and the other is a boost driver for LEDloads up to 40V and 2A where the forward voltage is more than thebattery voltage.

ST Microelectronics is developing a highly integrated solar MPPTcharger/HBLED driver. The fully integrated solution features alead acid battery charger with MPPT optimization, and integratedHBLED drivers. This high level of integration reduces cost, improvesreliability, and simplifies design. Releasing in late 2010, this productis ideal for HBLED street lighting applications.

Additional Resources

For additional information on solutions in this whitepaper, as wellas industry solar and HBLED related articles, reference designs andsolutions, please visit Avnets solar and HBLED web pages:

http://www.em.avnet.com/solarhttp://www.em.avnet.com/Lightspeed

Heather RobertsonTechnology Director Solar

Avnet Electronics Marketing

PowerPSoC is a registered trademark of Cypress SemiconductorCorporation

Implementations of MPPT can be fully analog, or mixed signal,and often include a microcontroller or state machine. In designinga system, a cost benefit analysis should be performed to determineif adding MPPT functionality increases energy capture enough tooffset the cost of implementation.

Charge ControlCharge controllers are used to charge batteries in a safe, efficientmanner. Depending on the application, charge controllers can bebought off the shelf, or designed for a specific application; often withthe MPPT and charge controller circuits combined. As mentionedin a previous section one of the most common battery types forHBLED solar powered lighting applications is lead acid batteries.Efficient charging of lead acid batteries requires a variety of chargingmodes, including bulk charge, absorption, float and equalize.Each state requires charging with different current and voltagecharacteristics, making sensing/feedback and control an importantelement in the controller.

A common architecture for an off-grid MPPT charge controllerimplementation is the use of a boost, buck, or buck/boost switchingregulator, and a microcontroller with analog inputs for sensingcurrent and voltages from both the solar panel and the batteries, andPWM outputs to control switches in the regulator.

Control and CommunicationHBLED lighting can be networked, or stand alone. Networkedlighting enables energy saving control and dimming, as well ascommunication of environmental activity such as movement,traffic, etc., as well as battery and fault status. Both wired and

wireless networks are common. Standards based wireless protocols(such as Zigbee, etc.) and proprietary wireless protocols runningover the ISM bands of 902-928MHz and 2.4GHz are often used.For wired networking, power line modems (PLM) are often used,communicating over the grid.

While it may seem contradictory to have a grid connected solarpowered HBLED luminaire, the grid would primarily be used fornetworked communication, as well as for an optional power source

for battery charging. A potential application using both wired and wireless communication would be a wireless link connecting asubnet of lights, with each subnet controlled by a node connected tothe main control center through PLM.

System ExamplesElectronics for solar lighting applications lend themselves wellto integration. On the market today are integrated solutionsspecifically designed for solar lighting, as well as products currently in development.

Cypress Semiconductor has a complete solar charger HBLEDreference board designed around their PowerPSOC processor.Developed to be powered by a 12V solar panel, and to charge 12V lead acid batteries, the reference design includes MPPT optimization,a battery charger and both buck and boost HBLED driver circuits.


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You dont have to be in the lighting business to know that greentechnologies are in very high demand and that at the forefront of sustainable lighting are LED lights. LEDs have very high luminousto watt efficiencies, very long lifetime, and are not made of toxicmaterials. These factors result in reduced down time, reducedmaintenance and reduced costs. From architectural illumination tolit street lights/signboards to ordinary home/office lighting, LEDsare rapidly becoming ubiquitous in our reduced carbon consumption

world.

However, there is one significant limiting factor with LEDlighting. That is heat, or more specifically, temperature. Like allsemiconductors, LEDs are very sensitive to high temperatures. Toget more brightness, LEDs have to run at higher power, meaningincreased heat, and therefore higher temperatures. Running an LEDhot is unfavorable for two reasons. The first is that as temperatureincreases, LEDs actually become dimmer and lose lighting efficiency.Second and perhaps more serious, operating too hot will cut downthe lifetime of an LED, wiping out one of the most beneficialfeatures of the technology.

Through the wise selection of appropriate heat sink technology, life-sapping heat can be conducted away from the LED where it is leastdesirable to the surrounding air. To understand how to effectively heat sink an LED application, it is important to know some of thephysics behind heat transfer.

Understanding Natural ConvectionMost High Brightness (HB) LED lighting will be designed withnatural convection cooling, that is no forced air circulation by fanor other inducement, for a variety of reasons. Fans generate noise,

would typically need to be replaced over the lifetime of the LED andadd to the complexity and cost of the lighting appliance. As a result,natural convection is preferred in most scenarios.

The motivating force behind natural convection is buoyancy. As theair in direct contact with the heat sink surface heats up, like any gas,it expands and becomes less dense. The less dense pocket of hot air

will float upwards like a hot air balloon, drawing cooler denser airinto the heat sink. Assuming the rising hot air can be exhausted anda source of cooler air is present, this process will cycle as long as theheat sink remains hot.

These natural convection air currents unfortunately are very weak,typically one to two orders of magnitude less than any forcedconvection. As a result, natural convection can easily be hinderedby friction along the surfaces of the heat sink as well as frictionbetween the individual air molecules themselves.

Maximizing Natural Convection Per ormance What heat sinks do fundamentally is multiply the surface area of aheat producing device, such that the device can come into contact

with more air simultaneously. The more surface area a heat sink possesses, the more heat it can transfer to the air. The flip sidethough is that the more surface area a heat sink has, the morefriction there will be between the moving air and the heat sink. If there is too much friction, the air will not move at all, rendering theheat sink equivalent to a block of solid metal.

For good heat sink design, it is especially important in naturalconvection to find the right balance of trying to maximize surfacearea but not chocking off the air flow.

Heat sink Geometry ConsiderationsThere are a lot of geometric considerations when selecting a heatsink. These include choosing between fins or pins, the density of thefins or pins, the size of the heat sink, the foot print of the heat sink,the material of the heat sink, etc.

Fin density refers to the amount of fins or pins per square inchof the heat sink footprint. The optimum fin density shouldbe determined by the available air flow, in the case of naturalconvection, low density is required due to low airflow. Having a low fin density results in less surface area, but natural convection aircurrents will be less encumbered resulting in better performance. If the fin density is not sufficient for the required cooling, then the useof a fan or other non-passive forms of cooling may be required.

Another important consideration is whether to choose a continuousfinned heat sink like most extrusions or a pin finned heat sink. In afinned heat sink, the air can travel along the length of the fins andto a limited extent up and down the fins perpendicular to the base.

When used in natural convection, a finned heat sink works best when the fins are aligned parallel to gravity so that the rising air cantravel along the fins. When the base is horizontal, air can only bedrawn in from the two open sides of the heat sink, some air can fallinto the heat sink from above but it will be fighting the rising hot aircurrents. This tends to lead to poorer performance for finned heatsinks when oriented horizontally; they perform worst when the baseis vertical but the fins have been oriented perpendicular to gravity.

With respect to natural convection cooling, forged pin fin heat sinksare a more advanced heat sink technology. Since the heat sink isopen on all sides, air can be drawn in and exhausted from all sidesof the heat sink except the base. This means that the heat sink willperform well in any given orientation. Another not very obviousbenefit of pin fins is that since each pin is an individual element, itis very easy to modify pin finned heat sinks into special shapes by trimming or removing only specific pins without damaging the restof the heat sink. This makes it very easy to add installation featuresor fit oddly shaped cavities. Pin finned heat sinks can also be forgedinto round or other shape bases.

Traditional pin finned heat sinks are surpassed in performanceonly by advanced flared pin finned design heat sinks. A flared pinfin heat sink differs from a traditional vertical pin fin heat sink inthat all the pins expand outwards in a radial fashion from the base,resembling a hedgehog.

Flared pin fin designs can significantly improve natural convectionperformance by increasing the spacing between each pin row whilemaintaining the same amount of total surface area. The additionalspace serves to reduce air flow restriction, making it easier for hotair to escape and fresh cold air to enter the pin matrix.

Another important consideration is the effect of pinned and finnedsurfaces with regards to the aerodynamic flow of air. Air travellingalong a straight fin will develop what is called a boundary layeralong the fin wall. A boundary layer is a region of disrupted airflow

Understanding E fcient Heat Removalin HB LED Applicationsby Barry Dagan, Cool Innovations


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Natural Convection Extrusion Heat sink

Flared Pin Fin Heat sink

Table 1Physical Characteristics o the Tested Heat sinks

Extrusion Vertical Pin Flared Pin

Length (in) 5.0 5.0 5.0

Width (in) 5.0 5.0 5.0

Height (in) 2.5 2.5 2.5

Number o fns/ pins

13 360 360

Pitch (in) 0.38 0.25 0.25

Sur ace area (in2) 332 358 358

Weight (lbs) 2.11 1.84 1.84

Power (Watts) 60 60 60

Table 2Thermal Per ormance Results o the Tested Heat sinks

Extrusion Vertical Pin Fin Flared Pin Fin

Horizontal

position0.83(C/W) 0.77(C/W) 0.62(C/W)

H-V45 Deg.position 0.85(C/W) 1.07(C/W) 0.78(C/W)

Verticalposition 0.84(C/W) 1.26(C/W) 0.88(C/W)

be cooled without the use of forced air convection. The flared pinfin heat sink is also suitable for lights that can be installed in any orientation as it exhibits very strong performance characteristics inall orientations.

The natural convection finned heat sink performed best when thelight source was oriented vertically. The finned heat sink is sensitiveto orientation as the fins must be parallel to gravity. Whethermounted in the horizontal or vertical orientation, the air must beable to travel up along the fin walls in order to develop natural

convection.The straight pin finned heat sink is omni-directional and canhandle high power levels. It performs better than the extrusion inthe typical horizontal position. The straight pin finned heat sink issuitable for LED fixtures with tight space constraints.

Looking Forward When looking at the expectations of light output from LEDs andtheir associated power levels, it is obvious that heat is going to be asignificant factor in LED designs. Good High Brightness LED design

will require the incorporation of more efficient and more compactthermal solutions.

Natural convection is going to be the dominant method of coolingLEDs as it requires no power, no maintenance and has no obviousfailure mechanisms. In order to maximize natural convectioncooling, it is important to design a system with air flow in mind:this means having an unlimited supply of fresh cool air, having anexhaust for hot air and choosing a heat sink with low aerodynamicresistance to natural convection airflow yet a high cooling capacity.The heat sink must also be able to operate in the given orientation(s)of the application and require minimal space.

Traditional round based light fixtures will have to be replaced withmore efficient square and rectangular fixtures in order to maximizeheat sink surface area. Rectangular fixtures can also be more costeffective when designing for LED applications that dont haveproduction volumes to justify a dedicated heat sink die.

Further down the road, very high powered LEDs may necessitatethe development of no-noise, extremely long life air movers. Otherpossibilities include developing conduction cooling mechanismsthat use pre-existing structural elements. These may be uncommonsolutions, but successful high brightness LED lighting will only bebrought about by experimentation, persistence and innovation.

Images and Tables

Straight Pin Finned Heat sink


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Luminance Lv describes the measurable photometric brightnessyof a certain location on a reflecting or emitting surface when

viewed from a certain direction. It describes the luminous flux emitted or reflected from a certain location on an emitting orreflecting surface in a particular direction (the CIE definition of luminance is more general. In detail, the (differential) luminousflux d v emitted by a (differential) surface element dA in thedirection of the (differential) solid angle element d is given by d v = Lv cos( ) dA d with denoting the angle betweenthe direction of the solid angle element d and the normalof the emitting or reflecting surface element dA. The unit of luminance is 1 lm m-2 sr-1 = 1 cd m-2

Illuminance Ev describes the luminous flux per area impingingyupon a certain location of an irradiated surface. In detail, the(differential) luminous flux d v upon the (differential) surfaceelement dA is given by d v = Ev dA. Generally, the surfaceelement can be oriented at any angle towards the directionof the beam. Similar to the respective relation for irradiance,illuminance Ev upon a surface with arbitrary orientation isrelated to illuminance Ev,normal upon a surface perpendicularto the beam by Ev = Ev,normal cos(J) with J denoting the anglebetween the beam and the surfaces normal.

The unit of illuminance is lux (lx) and also foot-candle.

1lx = 0.0929 fc (lm/ft)

Beside brightness, sensitivity to color sensations are human sensory perceptions and light measurement technology must express themin descriptive and comprehensible quantities. In light measurementapplications luminous color of incident light and light sources isof main interest. According to the tristimulus theory, every color

which can be perceived by the normal sighted human eye can bedescribed by three numbers which quantify the stimulation of red,green and blue cones. If two color stimuli result in the same valuesfor these three numbers, they produce the same color perceptioneven when their spectral distributions are different. Around 1930,

Wright and Guild performed experiments during which observershad to combine light at 435.8 nm, 546.1 nm and 700 nm in such a

match to that specified by CIE and DIN is one of the key parametersfor photometer specifications. Spectral mismatch error is the key source for measurement uncertainty with light sources other thantungsten lamps.

The most common photopic measurements quantities are:

Luminous f lux y v is the basic photometric quantity anddescribes the total amount of electromagnetic radiation emittedby a source, spectrally weighted with the human eyes spectralluminous efficiency function V( ). Luminous flux is thephotometric counterpart to radiant power. The unit of luminousflux is lumen (lm), and at 555 nm, where the human eye has itsmaximum sensitivity, a radiant power of 1 W corresponds to aluminous flux of 683 lm.

Luminous intensity Iv quantifies the luminous flux emitted by aysource in a certain direction. In detail, the sources (differential)luminous flux d v emitted in the direction of the (differential)solid angle element dW is given by

d v = Iv d and thus

The unit of luminous intensity is lumen per steradian (lm / sr), which is abbreviated with the expression candela (cd).

1 cd = 1 lm / sr and also foot-Lambert (1 cd/m = 0.2919 fL)


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used if the chromaticity differs more than uv=5x10-2 from thePlanckian radiator.

Color rendering is the effect of a light on the color appearance of objects. Sources that include light of all spectral colors, e.g. sunlight, effect natural color sensations from illuminated objects. Herethe color rendering is good. Light sources with irregular spectralcolor distribution effect unnatural color sensations. Here the colorrendering is poor. If for example the color of the object is notincluded in the source spectrum the color rendering is gray.

The Color Rendering Index CRI specifies the quality of the colorrendering of illuminants. The CRI is calculated by comparing thecolor rendering of a sample source to that of a reference source. Forexample a black body radiator with CCT below 5000K as comparedto day light source like D65 with CCT higher that 5000K. A selection of reflective test color samples (TCS), specified by the CIEare used to calculate the CRI of a test lamp. The first eight samples

with relative low saturation are used to calculate the general CRI Raof a light source. The other seven samples provide supplementary information. Four are with high saturation the others represent wellknown objects.

Light Emitting Diodes (LED) are semiconductor device incoherentlight sources with high electr ical power to light power conversionefficacy. As with any semiconductor device, operating temperatureeffects changes in light output and color performance. Thisis referred to as a devices temperature coefficient. Thermalmanagement is therefore of primary importance in the successfulimplementation of LEDs. Due to thermal drift LEDs areoften operated in pulsed mode. High peak intensities can begenerated in this mode with reduced average electrical powerand therefore reduced junction temperature. Sorting or gradingof individual LEDs by color differences caused by tolerances inthe semiconductor process is a common practice offered by most

way that the resulting color perception matched the color perceptionproduced by monochromatic light at a certain wavelength of the

visible spectrum. Evaluation of these experiments resulted in thedefinition of the standardize RGB color matching functions whichhave been transformed into the CIE 1931 XYZ color matchingfunctions.

These color matching functions define the CIE 1931 standardcolorimetric observer and are valid for an observers field of view of 2. Practically, this observer can be used for any field of view smaller than 4. Although the XYZ tristimulus values definea three-dimensional color space representing all possible colorperceptions, for most applications the representation of color ina two-dimensional plane is sufficient. One possibility for a two-dimensional representation is the CIE 1931 (x, y) chromaticity diagram with its coordinates x and y calculated from a projection of the X, Y and Z values.

Color Temperature (CT) is a specification for visible light andused to specify lighting conditions in lighting, photography, filmrecording, publishing, and other applications. The color temperatureof a light source is determined by comparing its chromaticity

with that of an ideal black body source. The color temperaturedescribes the emission spectrum of a black body sources or sources

which match the color temperature of a black body source. Mostartificial light sources such as fluorescent or discharge lamps andLEDs are only nearly-Planckian black body sources. They can bejudged by their correlated color temperature (CCT). The CCTcan be calculated for any chromaticity coordinate but the resultis meaningful only if the light sources are nearly white. The CIErecommends that the correlated color temperature should not be


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One large source of measurement uncertainty inherent withintegrating sphere use is the absorption effect. During calibrationof the sphere photometer some of the light irradiated into thesphere will exit the sphere through the measurement port and beabsorbed in the dark room. But during actual use, the measurementport of the integrating sphere will be fully or partially covered by the device under test DUT. So light leaving the sphere through themeasurement port will be reflected back into the sphere addingerroneously to the DUT light signal. Depending on the spectralreflectivity and color of the DUT the re-reflected light will vary in intensity and color and affect an unknown measurement error.

Auxiliary lamps are used to compensate this absorption errorby measuring the signal of the auxiliary lamp with and withoutthe DUT at the measurement port of the integrating sphere. Thedifference in intensity is used as a correction factor for subsequentmeasurements of the same kind of DUT.

Along with light intensity and color data, spectral intensity distribution is another important test property in LED analysis.Spectral based light meters are used for this type of measurement.Filter type light meters employing photometric or tristimulus (RGB)

detectors are restricted to comparative or relative measurements,e.g. LED sorting and binning against gold-standards. Howeverspectrometers offer different levels of quality levels, especially diode array type spectrometers which are often limited by intensity linearity and stray light characteristics.

An alternative method is to mate a photodiode with a diodearray like Gigahertz-Optiks BTS256P Bi-Technology Sensor. Itsphotodiode with a precise photometric response provides a highly linear ratio between light input and signal output over a very widedynamic range for very accurate luminous flux measurements. Thephotodiode offers a fast response time mostly independent from thelight intensity so that the measurement signal of the photodiode can

semiconductor manufacturers. But due to differences in LEDmanufacturers sorting processes and environmental conditions,the LED lighting industry is forced to do in-house qualificationmeasurements.

The most common light measurement quantity used in LED testingis luminous flux measured in lumen. This quantity corresponds toLED efficacy by correlation of the total light output to the electricalpower. Measurement of the total light output in lm instead of

luminous intensity in cd produces much better reproducibility because it is independent of spatial light distribution which may beinfluenced by temperature, humidity, distance, different viewingangles, misalignment and other experimental error. In research andindustry the most commonly used measurement devices forluminous flux are light meters with an integrating sphere . Theintegrating sphere acts as a light integrator for spatially emitted light.The light source may be mounted inside or outside the sphere. Theintegration effect is the result of multiple diffuse reflections on thediffuse reflecting surface of the hollow sphere which results in auniform light distribution at the sphere surface. The illuminancemeasured at any position on the integrating sphere surface istherefore an indicator of the total flux generated by a light sourceinside or outside of the sphere.

The size of the integrating sphere should be much larger than thesize of the test sample to keep measurement uncertainty low andindependent of the spatial light emission characteristics of thetest sample. If a smaller integrating sphere is used this must beaccounted for in the calibration procedure of the measurementdevice.


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ResourcesCIE 127 Measurement of LEDsy

Colorimetry and Color Quality of Solid-State Light Sources,y Wendy Davis, Optical Technology Division, NIST

IES LM-79-08 IES Approved Method for the Electrical andyPhotometric Measurements of Solid-State Lighting Products

IES LM-80-08 Approved Method for Measuring LumenyMaintenance of LED Light Sources

ANSI/IESNA RP-16-05 Nomenclature and Definitions foryIlluminating Engineering

Works in Progress by IESNA, CIE, IEC, ANSI, DOEy

be used for fast data logging and pulse synchronized measurementapplications.

Spectral distribution data is provided by a separate diode array sensor. The spectral data enables the measurement device tocalculate color data e.g. xy and uv color coordinates, colortemperature CT, correlated color temperature CCT, colorrendering index CRI, peak wavelength peak and dominant

wavelength. The sensitivity of photodiode array is controlled

by integration time so the lower the light level the longer themeasurement time. This makes diode arrays unsuitable for fastmeasurements. Longer integration times effect increases in both realsignal and dark signal. To improve the signal to noise ratio offsetcompensation becomes an important rule for diode array sensors.Best offset compensation is done with a dark signal measurementusing the same integration time as the signal measurement. A remote controlled shutter is built into the BTS256-LED tester tosupport synchronized integration time on-line offset compensation.Low light detection with CMOS photodiode array technology can therefore be achieved by employing offset compensation incombination with very long integration times.

As shown, accurate light and color measurement requires a goodbasic understanding of radiometric, photometric and colorimetricprinciples and quantities.

With this knowledge you will be better prepared to make decisionsregarding objectives, instrumentation and compromise solutions.

A general plan of action would be to:

1. Determine the goals and purposes for the measurements

2. Estimate the acceptable levels of accuracy required

3. Decide which instrumentation best suits 1 &2 and if it meetsbudgetary requirements

4. Juggle all decisions for best fit trade-offs may be necessary


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Abstract With more lighting companies embracing LEDs as a move to improve

efficiency, mechanical engineers are being asked to redesign fixturesto accommodate this technology. However, LEDs have differentrequirements from incandescent bulbs and florescent tubes whichmechanical engineers need to take into account. This paper addressesseveral of these issues in making the move to LEDs.

Introduction As lighting fixture manufacturers begin to move away fromincandescent and fluorescent light sources in favor of Light EmittingDiodes (LEDs), engineers need to adapt their designs to accommodatethe changes solid-state lighting technology requires. Most of the focuson these transitions has been on the electrical changes to provide thecorrect currents to high-brightness LEDs. What is often overlookedare the mechanical considerations and how they affect the life of theemitters as well as the physical implementation.

There are several key differences between LEDs and other lightsources. Solid-state lighting is more closely related to fluorescent tubessince both require more complex electrical drive, are efficient andemit little IR. However, there are other considerations plus new foundadvantages that can be exploited to provide new and exciting lightingfixture designs with LEDs.

LED BasicsLight Emitting Diodes, commonly referred to as LEDs, create lightthrough a process called electroluminescence. During this processelectrons give up energy in the form of photons when traversing theLED diode junction. The materials selected to make the junction(i.e. gallium, arsenic, phosphorous, indium, etc.) provide very specificproperties. On one side of the junction the materials will have anabundance of charge (the n-type with electrons) and the other willhave a deficit of charge (the p-type with electron holes). Betweenthem is a zone where the electrons cannot exist (much like the shellsof an atom). This zone is called the energy band gap or forbiddenzone and the materials used in LEDs form band-gaps that have specificenergies that provide photon emission. These are called direct band-gaps.

As positive charge enters the p side and negative charge enters then side, the electrons combine with the holes and fall to a lowerenergy level emitting a photon. This is analogous to water flowingover a dam driving hydroelectric generators. The water is losing energy as it falls which turns the turbines and creates electricity. As theelectron crosses the junction, the wavelength of the photon createdis determined by the band-gap energy. By varying the band-gap,engineers can create LEDs that emit anywhere from the infrared,through the visible spectrum and all the way to the ultraviolet.

Modern high-brightness LEDs go further and use quantumcontainment to increase the efficiency of photon generation withinthe LED and some even provide optical wave-guides formed inthe device to allow trapped photons to reach the surface. Someincorporate secondary phosphors which absorb some or all of theLEDs emission and through a phenomenon called Stokes Shift areable to create white light. Anyway you look at it these little devices

would have impressed even Edison in their efficiency in convertingelectricity to light. But they have characteristics that fi xture designersmust consider when including LEDs in a product.

LED Advantages and Disadvantages

Light Emitting Diodes are current devicesthat is, they require aconstant current (not voltage) to operate correctly. Also, because of the

construction of many white LEDs, you cannot simply vary the currentand get smooth dimming. LEDs incorporating phosphors will shiftblue if the current gets too low. There are special drive circuits such asthe LM3445 made by National Semiconductor that will allow fixturesto be used in retrofit applications with existing TRIAC dimmersthisis quite difficult to accomplish due to the nature of LEDs. TheLM3445 driver simplifies this by reading the TRIAC signal fromstandard dimmers and correctly controlling pulses of constant currentto dim the LEDs providing a smooth, linear range familiar toeveryone. Figure 1 shows a typical drive circuit.

LEDs for general lighting are very energy efficient since the emissionis tailored to be visible to humans. Unlike incandescent bulbs, generallighting LEDs emit very little infrared energy. One big advantage of LEDs, especially in task lighting for kitchens, is they do not warmfood or other items sitting below thema common problem withincandescent task lighting. This can prematurely cause food productsto spoil or melt. A fun little test is to place an unwrapped chocolatebar 18 inches from a 60 watt light bulb it will melt quickly. This isnot due to the visible light, but the large amount of infrared energy that is emitted and invisible to humans, but absorbed easily by thechocolate. An equivalent LED fixture would not cause this to happen.

LEDs do have a dark side. The biggest advantagethe lack of IR emissionis also a problem. Incandescent light bulbs shed most of their waste energy through the emission of infrared. LEDs mustconduct their waste heat away through thermal management. TheLEDs may not melt the food below them in a kitchen, but they mightbake the cabinets above them (and anything in them). Also, if theLEDs are allowed to get hot, their lifespan will decrease. LEDs rarely die a sudden death (as incandescent filament bulbs do). They willlose intensity over time. An LED is said to need replacing where theemission has fallen to either the 70% or 50% point from the originalintensity. These are called the L70 or L50 points. Figure 2 shows atypical L50 point based on the temperature of the LED.

Many new high-brightness LEDs include thermistors or RTDs to helpprotect the device from damage in extreme conditions. The thermaldevice can be used to fold back the current or energy being suppliedto the LED which in turn will reduce the temperature of the device.The LM3424 is an example of a special LED driver designed toincorporate thermal fold-back to protect the LED. This type of controlcan greatly increase the lifespan of LEDs where temperatures my riseand cause damage such as in emergency lighting, out door lighting, orhigh intensity applications.

Conclusions

LEDs have great advantages over other light sources. They are very efficient in producing light, are tiny point emitters which providedesigners with an array of options for fixture designs and come inevery imaginable color (or colors that can be mixed dynamically). They also have very long life-spans when driven correctlyon the order of 20,000 to 50,000 hours or longer. The major downside is managingthe waste heat of the current devices. Heat sinks and other passivemethods are best suited for low cost fixtures, and active cooling such asthat produced by Nuventix can be applied to commercial high-intensity applications. With a bit of thermal management and mechanical tricks,LED fixtures can be engineered for a long life and novel appearance as

well as saving significant energy.

What Mechanical Engineers ShouldKnow About LEDsby Richard Zarr, National Semiconductor


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1. IntroductionThe need for light has become one of the highest priority problemsto deal with for the electronic world. And when this need links very stringent geographical and environmental conditions with life savingsituations, the priority immediately becomes an urgencyeveryone inthe electronics world is called to duty to find a solution.

In the engineering world, the best way to solve a very complex problem is to reduce it to a combination of smaller, simplerproblems so as to address each of the resulting ones with a quicker,state-of-the-art solution.

2. The Problem IdentifcationIn order to comply with this approach, we can split the mainproblems into two categories:

Need for lighty

Lack of a power network y

The need for light issue reflects several potential situations, whichcan be grouped into four main areas:

Entertainment: Outdoor night time activities such as beachygames, tournaments, and camping, often require bright lightingsources for safety and accuracy.

Research and Monitoring: Special weather monitoring andygeological monitoring systems reside in stations very farfrom civilization and definitely as far as possible from any electromagnetically noisy environments. Roads leading to theseremote locations are typically off the grid and dangerous, andovernight expeditions require adequate lighting for safety andobservation.

Standard Life/Educational: in non-developed areas, there can beya complete lack of light during night disruptions and night timeemergencies; lighting is also a must for schools with both day and evening classes in remote places where the daylight hoursare used for work and family survival.

Life-threatening situations: emergency camps or temporary ymovable hospitals are normally situated in non-serviced areas

where bright, stationary light is crucial for medical support andfor even the simplest life saving operations.

The need of a power network in non-developed areas seems obvious:remote villages need lighting for schools, hospitals, and jobs. Andalthough the very lack of power networks in natural oases like

parks and paradise landscapes is part of their appeal, there areunfortunately cases where the lack of power suddenly becomes atop priority problem for l ife improvement situations and for morecritical life saving situations as well.

Many of the most unfortunate situations happen in cases wherethere is a pre-existence of a power network but due either to naturaldisasters (tornados, volcano eruptions, etc.) or to war conditions,the network is inaccessible or very dangerous due to uncappedconnections and open wires with high voltage floating around. Inthese horrible situations, the above-mentioned need for light foremergency camps and any other life saving activities comes intoplay.

3. Solar Powered LED solutionsThe needs described above require a set of solutions that combinedtogether will result in a single system that can address all of theissues triggered by difficult environments.

Requirements for our solutions:

Use the Suny

Make it Lasty

Make it Brighty

Lighting in rural areas and in developing countries is generally provided by wax candles or combustion lamps (e.g. kerosene), whilebattery-powered flashlights are typically only used as occasional,portable lights for intermittent use.

Combustible sources are cheaper than any form of electricity. On

the other hand the low efficiency, the poor quality of the light, andthe intrinsic fire risk advocate the use of electronic lighting in off-grid locations. The unavailability of the power grid implies that theelectrical energy must be produced locally. Among the methods by

which energy can be produced, photovoltaic systems (solar cells) areby far the most universally applicable.

The general principle is to convert the sunlight, in particular energy carried by photons, into electrical energy.

The use of photovoltaic systems brings some advantages:

Solar energy can be produced locally; hence the solar panels canybe installed everywhere (also in areas of difficult access) withoutthe need for infrastructures. This also minimizes transmission/distribution losses.

Renewable energy source; it does not impact the environmenty with pollution.

Facilities can operate with little maintenance or interventionyafter initial setup; this contributes to reduce the energy cost.

Low voltage generation; this simplifies the downstreamyconversion of the power.

However, energy production by photovoltaic systems must take intoaccount the intrinsic periodicity of the solar source (e.g. during thenight the energy source is absent). Electrical storage into batteriesassures a continuous availability of energy.

Once the electrical energy has been produced and stored, the nextstep is generating light in a wise way, in terms of energy saving andenvironmental respect.

Towards this end, the use of white LEDs is moving to the forefrontfor several reasons:

High luminous efficiency (more than 100 lumen/watt), whichyimplies less wasted power in comparison with other lightsources (e.g. incandescent bulbs).

Hazardous materials free (mercury or toxic gases), which makesyLEDs the cleanest light in ecological terms.

Low driving voltages, making LEDs particularly suitable foryphotovoltaic systems.

Integrated Solar PoweredLighting Solutionsby Luca Difalco, STMicroelectronics


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Taking into account less than ideal illumination conditions, theactual power delivered by the panel can be halved; thus consideringon average ten hours of sunlight, the energy produced by the panelduring the day can be estimated as around 200Wh.

This implies that the energy produced by the panel would be enoughto supply the lamp for about 16 hours, fully covering nightly lightingneeds in any season.

When the application requires a boost conversion, a solution couldbe provided, for example, by the LED7707. This is an appropriatesolution for all the typical input voltages (6V, 12V and 24V batteries), whenever the output voltage (fixed by the number of LEDsin series) is higher than the input voltage.

Each one of the integrated six current generators can provide from20mA up to 85mA, and the current can be simply adjusted by aresistor (see Figure 7).

In solar LED lanterns, a typical battery voltage is 6V (ranging from5.5V up to 7V).

ROW1

ROW2

ROW3

ROW4

ROW5

ROW6

LDO5

BILIM

RILIM

SS

SGND

S L O P E

V I N L X

O V S E L

S W F

A V C C

F A U L T

E N

S Y N C

M O D E

P G N D

D I M

VIN = 6V VOUT

COMP

+5V

Enable

Dimming

Fault

LED7707

Sync Output

Faults Management Selection

OVP selection

Switching Frequencyselection

Slope Compensation

Rows current selection

Internal MOS OCP

3W LED string

ROW1

ROW2

ROW3

ROW4

ROW5

ROW6

LDO5

BILIM

RILIM

SS

SGND

S L O P E

V I N L X

O V S E L

S W F

A V C C

F A U L T

E N

S Y N C

M O D E

P G N D

D I M

VIN = 6V VOUT

COMP

+5V

Enable

Dimming

Fault

LED7707

Sync Output

Faults Management Selection

OVP selection

Switching Frequencyselection

Slope Compensation

Rows current selection

Internal MOS OCP

3W LED string

Figure 7. 3W LED string driven by LED7707

Typically, a 15W solar panel is appropriate for solar LED lanternsolutions. Under the same hypothesis of sunlight duration andillumination conditions mentioned in the previous section, theenergy produced by a 15W panel is 75Wh.

Therefore, the energy produced by the 15W panel is by farsufficient, assuring more than 20 hours of autonomy.

6. ST Solutions and urther integrationThe below block schematic (see Figure 8) shows how the ST systemsolution approaches the all above requirements in one single systemfrom the MPPT algorithm to the battery charger related profile, theLED driving, and every kind of system protection towards the Panel,the battery and even the lamp, making the entire architecture safe.

Figure 8 Block Schematic o the ST Solar Battery Charger LEDStreet Light Solution

Figure 5 Evolution o Lighting and LED positioning

However, the method of driving LEDs should also be optimized toimprove the overall efficiency.

Because they are current controlled devices, the main requirementto drive LEDs is to control the current, which, in turn, determinesthe brightness of the light emitted.

And any LED driving solution must consider the applicationconditions:

Input voltage rangeaccording to the use, different battery typesycan be chosen: from 6V batteries in the case of solar lanterns, to

12V batteries for home lighting applications, up to 24V batteriesmainly dedicated to streetlight solutions.

Number of LEDs and how they are connectedLEDs can beyconnected in series in a single string or arranged in multipleparallel strings. The number of LEDs in series defines the output

voltage of the conversion (typical voltage drop across a whiteLED is around 3.5V), whereas the number of parallel stringsindicates the total current to provide.

LED currenthigh brightness LEDs are supplied by currents of yhundreds of milliamperes (up to more than 1A).

Depending on the battery voltage and the number of LEDsconnected in series in one string, a buck or a boost conversion canbe the most suitable solution.

STMicroelectronics offers, among a wide range of products for LEDdriving, both a buck converter (L6902D, [2]) and a boost converter(LED7707, [3]) dedicated to LED driving. Both devices featuretechniques to control the current and a high efficiency conversion.

The L6902D is a step-down switching regulator mainly used forhome and street lighting.

6W LED string

L6902

FB

CS+

CS-

IN VOUT

senseresistor24V

350mA

100mV

6W LED string

L6902

FB

CS+

CS-

IN VOUT

senseresistor24V

350mA

100mV

6W LED string

L6902

FB

CS+

CS-

IN VOUT

senseresistor24V

350mA

100mV

Figure 6. 6W LED string driven by L6902D

Figure 6 shows a 6W LED string driven by L6902D.

An example of the L6902D used as an LED driver is in streetlighting. A 40W solar panel is suitable for this application, and a12W lamp, considering the high efficiency of LEDs, could be morethan enough for illumination of local roads in rural areas.

In a typical application, a 12W lamp can be realized by two stringsof 6 LEDs (connecting in series two 3W LED modules), each onedriven by one L6902D.


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to further improve the digital function integration, delivering anoptimum battery charger profile on top of the already achievedfunctions with the previous integration levels.

The resulting device main features and characteristics are listed inTable 1.

Table 1 Integrated Solar Powered Battery Charger

Solar lanterns Streetlights

PV panel

Peak power 24W 50-100WOpen circuit 17V 34V

@ MPPT 12V 24V

Lead acid battery

Charge level 6V 13.8V

Energy storage 4.5Ah 20Ah

LED

Power 5-10W 20-40W

Below is a list of the resulting IC features:

Power switch and synchronous rectifier integrated inside ICy

Operating voltage range 3V to 38V y

Perturbation and observation method for MPPTy

Constant current for bulk battery charge and constant voltageyfor floating charge

Battery status indication and automatic day/night detection viaysolar panel

Input pin for battery temperature feedback y

Protection for solar cell short circuit, battery over voltage andyover temperature

7. ConclusionsThe challenge of providing light in rural areas can be achieved by

producing energy locally. Photovoltaic systems make it possibleto exploit a readily available energy source while at the same timerespecting the environment.

The storage of the energy into batteries overcomes the intrinsicdiscontinuity of the solar energy.

Light emitting diodes, ever more present in lighting solutions, seemthe most appropriate choice for energy saving thanks to their highluminous efficiency.

Moreover, the availability of different LED drivers, with buck orboost configuration, provides flexibility in lighting systems designand high efficient power conversion solutions.

STs technologies enable a ready-to-go discrete implementation of all the above features as well as an integration path towards a oneor two chip implementation, achieving miniaturization and the bestefficiency targets.

Bibliography:[1] Imaging India. Ideas for the new century , pages 258, 454, 458Nandan Nilekani, Penguin Books India.

[2] Low Voltage LED Driver Using L6920D, L4971 and L6902D Application Note AN1941, STMicroelectronics.

[3] 6 rows85 mA LEDs driver with boost converter for LCD panelsbacklightApplication Note AN2810, STMicroelectronics.

A real implementation of the above block schematic has also beenrealized to prove the technology and system performance (see below Figure 9). This reference design uses state of the art microcontrollertechnology as well as best in class analog drivers and discretepower componentsall from the ST portfolio of technologies andproducts.

Figure 9 Real Implementation o the ST Solar Battery ChargerLed Street Light Solution

For this demanding industrial application, the challenges of innovation and miniaturization that a leading semiconductorcompany has to achieve are even more stringent than usualrequirements for the power conversion segment. To remain at theleading edge of technology and solutions, ST is engaged in anintegration path whose roadmap is seen in Figure 10.

Figure 10. Solar Roadmap Integration Path

Since most of the challenges described above were related to theMPPT and Battery charging profile setting, the first and mostimportant activity in the integration path has engaged research anddevelopment resources in first integrating the panel needs and thenthe battery charging needs, leaving the LED driving as a separatefunction. This will give the final system designer the flexibility todecide on configurations and other system parameters that could notbe made as flexible in an integrated design.

The first and most simple step taken on the integration pathhas been towards efficiency improvement right out of the panel,making the widely used Bypass schottky diode, an intelligent oneintegrating best-in-class analog IC technology with ultra low RdsonMOSFETs, to reduce by over one third the losses at that portion of the conversion.

The next and most important integration level was implemented when achieving the MPPT function together with a high powerDCDC boost converter into a single silicon delivering over 80W

with 92% efficiency. The last portion of the integration has been


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On the Use ul Li etime oLED Lighting Systemsby Geof Potter, Power Technologist, Texas Instruments, November 2009

Abstract Although they are relatively new players in industrial and residentiallighting, high-brightness light-emitting diodes (HBLED) are actively being characterized for field reliability prediction by many of themajor manufacturers of lighting products. That process requirescollection of meaningful long- termexperience data upon whichcredible estimates of expected operating life under actual fieldconditions can be published. Without credible estimates, fieldacceptance of LED luminaires will suffer.

This paper reviews what is known about the lifetime of LEDs in thefield and discusses how to estimate the operating life of LED-basedlighting systems by including reliability factors associated with LEDdrivers (sometimes called ballasts) that provide controlled voltage orcurrent to power the devices. Coordination of LED and driver lifeexpectancy is necessary for commercial lighting products if they areto fulfill the long-life promise of solid-state lighting.

Users of LED lighting products are legitimately concerned aboutuseful life, because the price of HBLED based luminaires has not

yet seen the cost benefits of sustained mass production on the scaleof incandescent or florescent lighting devices. Reasonably accurateprediction of time-to-failure is only practical where substantialinformation about the design, construction and environment foractual applications of LED-based products is available. Norms havenot yet been established in the minds of the public by high volumemanufacturers based on consistent field performance. Consequently,this paper approaches LED lighting longevity by discussing severalkey factors that influence useful life in order to identify ranges of probable lifetimes based on estimation of application conditions andthe general nature of the product design.

It is important to distinguish lifetime from reliability as applied toluminaires. Lifetime is the amount of useful operating time availablefrom the vast majority of lighting product units, under prescribedconditions, exclusive of random failures. Reliability is a term usedto describe how often such products fail randomly (i.e. exclusive of infant mortality or wear-out conditions). This document aims topresent factors influencing lifetime. Poor reliability resulting fromdesign or construction of luminaires can adversely affect the productlifetime by distorting the impact of premature failures and theapparent onset of wear-out.

A hypothetical LED LuminaireFor reference, in Fig. 1, below, is a simple diagram of an assembly consisting of a light diffuser, a box (chassis), an LED mounting

/ cooling plate and a circuit board containing components of aconstant current driver that provides regulated power to the LEDarray. Of course this is a representation of some basic elements of a luminaire, not a real device, and is intended to emphasize thepoint that such a product has more parts to consider from a lifetime

viewpoint than just the LED devices themselves.

CHASSIS

( SIDE VIEW )

DBCsubstrate

COVER

LEDMOUNTED

&CONNECTED

DRIVER CIRCUITBOARD

FRONT VIEW(14 LEDs )

DRIVER OUTPUTABOUT 20 WATTS

ALUMINUMELECTROLYTIC

CAPACITORS (e-caps)

DIAGRAM OF HYPOTHETICAL LUMINARIE PRODUCT SHOWING THE ENTIRE ASSEMBY INCLUDINGLEDs, DRIVER BOARD, CHASSIS AND DIFFUSER

Diffuser

Diffuser

Fig. 1

HBLED Li etimeOne advantage that LED devices have over incandescent lamps istheir lack of hard failures. LEDs display a wear-out mechanismrather that a propensity for catastrophic failure. In fact, the lightingindustry has standardized on a definition of failure that declaresend-of-life to be the point at which LED output (Lumens) hasdiminished by 30%. The so-called L70 point, is the point at which

most people can perceive a loss in intensity and is often used as themeasurement standard for failure.

LED output degradation occurs, in part, as a result of reducedtransmission of light through the LED package, that is, a lens andthe encapsulant that reduce internal reflections within the assembly.There is also a reduction in actual light generation within the diedue to an accumulation of defects in the lattice occurring with use.In devices that have external phosphor coatings to adjust color, thereis also a loss in phosphorescence that occurs with use.

Fig. 2 Side-view (cut-away) o mounted HBLED with associatedstructure [source: CREE ]

The net effect of both degradation modes is presented as a set of


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Most often, LED devices for lighting applications are mounted ontosome sort of

25742-Designing With LEDs eBook 09-29-2010

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