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LEDS EXPLAINED A basic insight into this exciting technology Ever since Philips Lumileds created the high power LED over 10 years ago, the rapid advancement and development of solid state lighting components has facilitated exciting new applications and innovations. If we gaze around our environment, we notice just how wide. spread the use of LED technology has become. by Ross Spina (RMS Parts Pty Ltd) LEDs are everywhere, in just about every household appliance, in our automobiles, in our computers and also in our lighting fixtures. In fact it is the lighting industry which is gaining the most in the use of LED Technology. The push for greener, environmentally friendly lighting solutions means that LEDs have become the viable alternative solution to incandescent and fluorescent lamps. Where the reliability of CFL products is questionable, LEDs are being promoted as more reliable, consuming lower power and more adaptable in their applications. However the adoption rate of LED lighting in consumer homes is still relatively low. This is due to the high expectations which have been placed on the cheap imports and their failure to deliver. Within the industry there are many myths about the reliability of LED products. The most common relates to the lifetime of LEDs. There is a general belief that 14  Silicon Chip LED fittings will last 50,000 hours with 70% of the initial lumen output. This is simply an exaggeration. To put this statement into perspective, a LED fixture would need to be utilised for 12 hours a day over more than 11 years before we could expect partial or total failure. Any electronics engineer will know that the fitting is only as good as its weakest link. Given that LEDs often require complex circuitry and adequate cooling to operate, it is unrealistic that we should expect this level of reliability. It may be useful to summarise some of the The LED Dazzler, from our February 2011 issue used 3 high-brightness Seoul Semiconductor LEDs and came with a warning: don’t look into it! terminology used in High Brightness LED specifications. LED distributors are often asked to explain some of these terms to clients. Luminous Flux (or Lumens) This is probably the most prominent information provided by any highpower LED manufacturer – and also one of the least understood. It simply measures the total amount of light emitted by the light source and is measured in lumens (lm). On its own, this information is not very useful other than to provide a comparison of the total output of one light source when compared to another. Accordingly it is likely that this information will be marked on the packaging of most light bulbs or light fixtures. Traditionally the output of low power or indicator LEDs has been measured in candelas (cd) or milli-candelas (mcd). This is still a valid unit of measure for LEDs as it measures the luminous intensity of a light source in a given direction. If you focus a LED light source into a narrow beam, this will increase the siliconchip.com.au How does a LED produce light? All light is produced in the same way – from an electron giving out energy as it jumps from an excited state back into its standard orbit around its nucleus. That holds true whether we are talking about light from the sun, light from a chemical explosion, light from an incandescent bulb filament, light from a fluorescent tube, or light from a LED. Of course, the mechanism varies just a little between each. In a bulb, for example, the filament (resistance wire) is heated by current, forcing electrons to jump into a higher orbit around the nucleus. They’re not happy in that excited state and try to jump back into their “normal” orbit. Energy (photons of light) is given off as they do so. It’s not too dissimilar in a LED, except that the action does not rely on heating. An applied voltage forces electrons across the P-N junction. The electrons change state as they cross the P-N junction, losing energy (voltage) in the process, which is emitted in the form of a photon. With this happening countless millions of times, the photons all add up to produce light we can see. The colour of a LED is determined in two ways: (1) by the material used to make up the P-N junction, and (2) by phosphors which are coated on the LED surface and glow with particular colours. White LEDs, for example, use a yellow phosphor which mixes with the blue glow of the LED itself. light intensity, thus increasing the candela rating. While this is a practical measurement for indicator LEDs, it has no real purpose for lighting applications. LED lighting needs output in much more than one direction. However, it is still common to be asked for the relationship between candela and lumen output despite the fact they are measuring different lighting characteristics. There is no direct correlation, suffice to say that the narrower the beam angle of the light source, the greater the cd/lm ratio. Frequently you will see that LED manufacturers will market a product with a very high luminous flux output. This is not a determinant for a very high quality LED. It is simply a comparative measure that designers may take into account if they require a light source with this much output. LED manufacturers (that is, the actual LED) use this measure as the primary means of sorting or “binning” their products after the manufacturing process. A white LED data sheet usually provides a typical luminous flux output for a particular part. However a separate, more detailed listing will usually advise the range of available “bins”. The highest output bins will be sold at a premium price where the manufacturing yield is not so high. Using the basic fact that a typicalwhite LED is basically a blue LED associated with a yellow phosphor coating, the cooler the shade of white, the higher the luminous flux. In the February 2011 edition of SILICON CHIP, the LED Dazzler article siliconchip.com.au demonstrated the luminous flux of the (white) P7 LED made by Seoul Semiconductor. This device can deliver around 900lm when fully driven at 2.8A. It sells for approximately $18ea in small volume. Another high power LED is the XM-L made by Cree. This device can deliver up to 1000lm at 3A. It can be purchased in Australia for around $10ea. These products both represent exceptional performance in terms of Luminous Flux from a single LED. However, just to confuse the discussion, Luminaire (that is, the fittings incorporating LEDs) manufacturers do not specify their fixtures just in terms of luminous flux. This is because this is a measurement taken from the light source. It is usually more practical to explain the performance of a light fitting in terms of its effective light output at a measured distance from the source. This measurement, also known as illumanance, is used to quantify the incident light radiated from the light source and is described as the lumens per square metre (lm/m2) or Lux. Luminous Efficacy (Lumens/Watt) Luminous efficacy is a more useful measure of determining LED efficiency. It is the ratio of luminous flux to power consumed and is measured as lumens per watt (lm/W). Accordingly, the higher the lumen output per watt, the less power required to generate the same amount of total luminous flux. In terms of LED performance, this September 2011  15 Est.1978 5th Generation MR16 & GU10 5 Watt LED Replacements Ultra bright 400 lumens =45W Wide beam 60° Long life 35,000 hours Cool operation Cool, natural & warm white 2 year conditional warranty MR16 Dimmmable MR16 (1+) $22.00 (10+) $24.00 GU10 (1+) $23.00 (10+) $25.00 Incandescent & CFL Led Replacements Long life 30,000 hours Cool operation Cool & warm white 6 Watt 620 lumens (cw) = 60 Watt 7 Watt 740 lumens (cw) = 70 Watt 9 Watt 915 lumens (cw) = 90 Watt 5 year conditional warranty 6W E27/B22 $19.00 7W E27/B22 $24.00 9W E27/B22 $28.00 Queensland Bowen Hills Ph: (07) 3252 7466 Southport Ph: (07) 5531 2599 New South Wales Homebush Ph: (02) 9704 9000 www.prime-electronics.com.au 16  Silicon Chip figure is constantly improving for manufacturers and some top performing white LEDs are currently able to offer in excess of 120lm/W. When compared to Halogen bulbs or CFL tubes which offer between 10-50lm/W, it is possible to see why it is quite practical to save power by utilising LED technology fixtures. Fluorescent tubes range from about 16 to over 100lm/W, depending on their size/type and the ballast type. Lighting manufacturers use this information as the basis for developing LED lighting fixtures with the following consideration. Inherently, high power LEDs get very hot as large amounts of current are passed through their small profile. This heat must be dissipated efficiently to keep LEDs working within their design constraints. As the temperature of the LED rises, luminous efficacy reduces proportionately. Most LED dies have a relatively linear luminous efficacy up to their maximum operating temperature. This is typically around 85°, although some manufacturers will provide operating temperatures up to 125° before their efficacy is compromised. Efficacy directly affects the size of heatsink required to dissipate heat. The greater the efficacy, the less power is needed to generate the same amount of light and the need for more elaborate heat-sinking is reduced. The Luxeon Rebel, for example (manufactured by Philips Lumileds) has one of the highest maximum operating temperatures within the industry. The Rebel will operate at 125° while still delivering a high luminous efficacy. This is an important consideration where a light fixture needs to operate at a very high ambient temperature or there are issues in dissipating heat effectively. Correlated Colour Temperature (CCT) In basic terms, the correlated colour temperature (CCT) describes the colour emitted by an LED and is measured in Kelvin (K). For white LEDs this is normally used to describe light output as either a warm, natural or cool white. Warm white tends to be towards the red end of the spectrum while cool white is towards the blue. Theoretically, natural white contains an even mix of colours and roughly equates to the colour produced by the sun. Generally LEDs with a CCT of between 2700-3500K are considered as warm white LEDs and typically replicate the colour temperature of a standard incandescent lamp or metal halogen bulb. LEDs with a colour temperature of between 3500 – 5000K are considered natural white. LEDs above 5000K are considered cool white and will output light with a bluish tint. Reproducing the same colour temperature on a LED is a difficult process and not an exact science. Because a typical white LED is merely a blue LED with a phosphor coating, it is often the thickness of this phosphor coating that determines the colour characteristic of the LED. Unlike coloured LEDs which have a dominant visible light wavelength, white LEDs filtered from a monochromatic light source (such as blue or UV) will produce a broad spectrum of wavelengths. It is easier to “bin” them according to their Chromaticity (x and y) coordinates on the CIE chromaticity chart. Colour binning is an important issue which affects luminaire manufacturers. Surprisingly, minor differences in colour temperature on LED arrays can often be easily spotted and impacts the overall quality of the fixture. Therefore it is important to specify exact requirements in relation to CCT. Generally the LED distributor or reseller will offer “open” bins at the lowest price which means you don’t necessarily get the exact same product on subsequent orders – buyer beware. It’s a bit like colour variations in ceramic tiles, bricks and wallpaper. Colour Rendering Index (CRI) Another important criteria of LED Lighting is determining how faithfully the light is able to reproduce the colours of various objects in comparison to natural light. This is referred to the Colour Rendering Index. If we assume that sunlight is able to faithfully reproduce all colours perfectly, then it would have a CRI of 100. The incandescent bulb and most halogen bulbs have a CRI greater than 85. By comparison, some of the low pressure sodium lighting which illuminates our roads has a CRI of around 20 or less. LEDs in the warm-to-natural colour temperature range typically have the highest CRI of around 85-95. Because siliconchip.com.au siliconchip.com.au September 2011  17 LED Lighting - Saving Energy & the Environment ecoLED Tube The friendlier alternative to fluorescent lamps No mercury, no lead, environmentally friendly Less power, Longer life, Less maintenance Can retrofit T8 Fluorescent Lamps No strobing, no flicker, no buzzing, no irritation Half the power, energy cost saving Longer life, very low maintenance Flexible LED Lights RGB Multi-colour, White, Warm White. 24VDC. Cut to length. Remote controls for colour & dimming. With waterproof seal and adhesive taping (non-seal version also available) Cove lighting Bar lighting Console Kickboard lighting Colour changing & effects via remote control. Sets the mood & atmosphere for your venue. Website: www.tenrod.com.au E-mail: sales<at>tenrod.com.au 18  Silicon Chip Sydney: Melbourne: Brisbane: Auckland: Tel. 02 9748 0655 Tel. 03 9886 7800 Tel. 07 3879 2133 Tel 09 298 4346 Fax. 02 9748 0258 Fax. 03 9886 7799 Fax. 07 3879 2188 Fax. 09 353 1317 Soanar’s Ecolume DL Series of down lights is the perfect replacement for power-hungry halogen down lights. The DL series operates on only 15W, with an operational life of over 60,000 hours. of the nature of the broad spectrum white light radiated from a white LED, the subjective quality of the colour reproduction differs, depending on the dominant wavelength which may be present within this visible spectrum of light. However the general consensus is that the greater the CRI, the better the quality of the light for illumination applications. Forward Voltage (VF) LEDs require a certain voltage across them in order to make them emit light. Because LEDs are obviously diodes (with an anode and cathode) current can only run in one direction – from anode to cathode. This is called the forward direction and the electromotive force, or voltage, required to push the current in this direction is called the forward voltage. Most low power LEDs, such as those used for indicators, would normally have a forward voltage of around 2 – 3.5V (DC). Different colour LEDs have different voltage requirements – reds and yellows tend to be lowest, blues and whites highest. However, high power White, Blue and Green LEDs are made from Indium Gallium Nitride (InGaN). This material is employed in the epitaxial LED layer because it is able to produce the shorter wavelengths needed for these colours. The disadvantage of this material is a higher forward voltage – typically the forward voltage is in the range of 3 - 4Vdc. This specification becomes more critical when determining luminous efficacy. Using Ohm’s Law, the lower the forward voltage of the LED device, the lower the power dissipation at any given current. And as the applied current through the LED is increased, so does the forward voltage. Hence, the luminous efficacy tends to drop as the current (and thus power) is increased An example of the heatsinking required on Ultrabright LEDs – this CREE LMH6 has a light output of 2900 lumens – but it also needs to get rid of a significant amount of heat. siliconchip.com.au So peak efficacy is not usually at full power and is often quoted at below maximum power. Ultrabright LEDs need drivers Thermal resistance This characteristic of a LED is an important issue in determining how hot it is likely to get during its operation. The higher the thermal resistance of the LED, the more difficult it is to get the heat away. Unfortunately the performance of a LED is compromised as the temperature of the device rises. At very high temperatures, the lifetime of the LED is significantly reduced; even catastrophic failure can occur. It’s much better to have a lower thermal resistance because it is easier it is to dissipate the heat away from the LED chip. Thermal resistance is measured in degrees per Watt (°C/W). The thermal resistance of a LED is related to its structure/composition. The original high brightness Luxeon 1 LEDs manufactured by Lumileds were constructed in a plastic case with a plastic primary optic. These devices had a 1W rating and had a junction thermal resistance of approximately 15°/W. The current Luxeon Rebel ES series, with a ceramic substrate and a bonded metal interconnection layer, as well as a silicon lens, has a rating of approximately 6°/W. Manufacturers use various materials in order to reduce thermal resistance and increase thermal efficiencies. However ALL high-brightness LEDs must utilise some form of heatsinking to ensure that the junction temperature is kept to a minimum during operation. Earlier, we mentioned the fallacy of LED fixtures operating for 50,000 hours. The lumen maintenance data provided by many LED manufacturers is extrapolated from a simulation which tests the LED for a shorter peri- Give your lighting projects a SEOUL LED Dazzler Kit (As seen in SC Feb’11) Drive up to 3 powerful P7 LEDs Or most other high power LEDs Includes all parts and PCB LEDs not included KIT-LED_DAZZLER $39.95 + GST Getting light from a low-power LED is easy: just hook up DC power in series with a suitable resistor (which you can work out easily from Ohm’s law). Ultrabright LEDs can be driven the same way but it’s very inefficient. They really need a purpose-built constant-current driver, such as this MORNSUN KC24W. It’s a high-power LED driver designed as a step-down constant current source. With its high efficiency, wide input voltage range, and PWM dimming and analog dimming function selectable, also with remote shutdown capability. The KC24W series can be widely used in 12V, 24V, 36V and 48V landscape lighting, special lighting controls, commercial lighting, automotive lighting and many other commercial and domestic lighting system applications. The leaded package allows for more convenient use where no PCB is required or desired. The KC24W series is a high efficiency device (up to 96%) with an ultra-wide range voltage input and output (5.5-48 VDC), output current accuracy (±2%) with high output current stability(±1%) and low ripple & noise (<100mV) suitable for use with large capacitive loads (1000F). The KC24W series is also suitable for either PWM or analog dimming and is even waterproof to the IP67 standard. LEDs in automotive use Not long ago the festoon (dome) light in my car failed – and instead of replacing it I bought these LED replacements (about the same price as the globe!). They are dramatically brighter, especially the 12-LED version at left (the 6-LED is exactly the same size as the festoon globe). Just as importantly, the current drain has reduced from ~250mA to <100mA and the heat generated is also way down. Of course, these are just one example of LED use in vehicles these days: tail lights, siliconchip.com.au Give your lighting projects a P7 Power LED 10W Pure White Emitter Approx. 900lm <at> 2.8A Ideal for torch applications PCB available to suit W724C0-D1 $16.00+GST P4 Star 4W LEDs Power LEDs mounted on 20mm Star PCB. Various Colours available. Pure White W42182 $3.90+GST Nat. White S42182 $3.90+GST Warm White N42182 $3.90+GST P3-II Star 2W LEDs Power LEDs mounted on 20mm Star PCB. Various Colours available. Pure White WS2182 $2.95+GST Warm White NS2182 $2.95+GST P5-II RGB Power LED High power RGB LED mounted On 20mm Star PCB Drive each colour <at> 350mA Ideal for wall wash applications F50360-STAR $14.95+GST SMD RGB LED General purpose RGB LED in PLCC-6 package Drive each colour <at> 20mA SFT722N-S $0.95ea+GST Channel Lighting Modules 12v Operation, Cool White Ideal for Sign illumination 3 LED – 41lm min. 21H0007 $2.70ea+GST 4 LED – 55lm min. 21H0008 $3.60ea+GST VOLUME DISCOUNTS APPLY AUSTRALIAN DISTRIBUTOR stop lights, truck clearance lights, indicator and dashboard lights . . . and there are even some vehicles now with ultrabright LED headlights! Ph. 07 3390 3302 Fx. 07 3390 3329 Email: sales<at>rmsparts.com.au www.rmsparts.com.au September 2011  19 MXA026 Pre-built module Stop-Watch and Clock timing to 1/100th of a second enough heat from the LED junction. Also required is an efficient heat conduction mechanism such as thermally conductive tape or grease compound which interfaces the LED and heatsink. AC LEDs vs DC LEDs $66.73 Only inc GST plus Pack and Post Shop on-line at: www.kitstop.com.au electronics-the fun starts here 56mm Super Brite Display Battery Back-Up Facility Also in 4 Digit Version The top spot for 100s of innovative & affordable projects for hobby, school & industry October 2011 od under multiple adverse conditions. The accepted standard for lumen maintenance is known as the IES LM-80 test standard. The test requires manufacturers to test LEDs for a minimum of 6000 hours at 1000 hour intervals and at three case temperatures (55°C, 85°C and one other temperature as selected by the manufacturer). The test requires an ambient temperature of 25°C. Unfortunately most fixtures do not operate in an ambient temperature of 25°C. Instead it is likely that a downlight fixture could be operating in a harsh environment of 60°C or higher. Accordingly it is important to use appropriate heatsinking materials to ensure that heat is dissipated away from LEDs. This forms the most crucial aspect to LED Lighting and the most difficult to design. The problem which arises is the large cooling area (including complex fin design) required to dissipate While AC LEDs have been available for several years, the adoption rate in Australia has been slow. This technology incorporates complex dies made up of multiple LED junctions which are able to withstand a forward voltage equivalent to 230VAC (for Australian requirements). There are several immediate benefits in using AC LEDs for lighting applications. Firstly, the design is much simpler, allowing a fast time to market. This is because there are no requirements for costly or complex power supplies to drive the LEDs. Secondly, there is a reduction in size of the light fixture since there are no other components required. Most importantly, AC LEDs are not polarity conscious, whereas DC LEDs can be damaged or destroyed if connected in reverse. However there are some implications which need to be considered. Like any other 230V device, there are safety/legal issues which need to be taken into account within the design. This is the main reason why AC LED technology has been slow to develop but there are other reasons – the complex dies do not have the same lifespan as DC LEDs and they generally have a lower efficacy meaning that their overall performance is lower than their DC counterparts. To overcome the some of the safety implications, both Seoul Semiconductor and Philips Lumileds have developed a range AC LEDs with an operating voltage of between 50 – 55V which can be configured for 230V operation and have an RMS operating current of 20-30mA. These devices require minimal additional components to operate and they are an excellent alternative to the low voltage DC LEDs in some applications. Summary It becomes obvious that there is more to it than just connecting a power supply to a LED to make a lamp fitting. Without an understanding of the realtionship between all the technical factors mentioned, it’s not an easy task to design LED lighting for the home or office. It’s best to understand the various pitfalls so as to avoid them. Fortunately there is a truckload of information available from the internet which is useful for both buyers and designers alike. Most of the major LED manufacturers also provide ample information about their products including some excellent design briefs. For pricing on any products from the manufacturers listed in this article, please contact the relevant distributors. In Australia, Cree is distributed by Cutter Electronics, based in Melbourne. Seoul Semiconductor is distributed by RMS Parts based in Brisbane. Lumileds are distributed globally by Future Electronics. SC Why Verbattim m LED D? High power LED light Wide range including: A Type, MR16, R63, PAR16, PAR30, PAR38, AR111 Long Lifetime 80% energy saving Unique Temperature Control System (TCS) for security and long life performance  Comfortable and soft warm white to neutral white  Very good colour rendering with CRI of >=80      to view our range and find a distributor visit us at www.verbatimlighting.com.au 20  Silicon Chip siliconchip.com.au