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by Julian Edgar U S manufacturer Lumileds Lighting has developed a whole new breed of LEDs – they use new technology, they look quite different and they produce more light than we have ever previously seen coming from a LED. The new LEDs are branded Luxeon. It is almost impossible to photograph either of the Luxeons in action – and do their amazing light output justice. Here the current to the 1W LED has been reduced, the camera has been placed well off-axis – and bad lens flare has still occurred. 64  Silicon Chip The manufacturer’s publicity suggests that the Luxeon LEDs are the start of a lighting revolution – and to an extent we agree. “Luxeon is today’s brightest solid-state light source, producing 10-20 times the output of a standard LED lamp,” suggests the company. “With a white source effi- cacy of 25 lumens/watt]. . . Luxeon is a realistic source for general and directional lighting.” Lumileds claims that the LEDs can be integrated into light fixtures – after all, with a rated life of 50,000 hours (other data suggests “up to 100,000 hours”), the ‘bulb’ probably never needs to be changed. Their publicity material shows the LEDs being used for internal commercial lighting and outside architectural lighting, in addition to emergency and portable lighting. However, Lumiled’s engineers are a little more conservative. “Today there is really no such thing as a commercial ‘solid state lamp’ for use in illumination,” they state in an IEEE paper. “However… differences are beginning to appear in the technologies used for low-power LED indicators and the high-power LED light sources that will evolve into lighting sources.” So what are the new technologies? And do the Luxeon LEDs live up to their publicity hype? We won’t be able to make a judgement on their life for nearly six years www.siliconchip.com.au The Star/O is a 1W LED which has a built-in collimator to provide a focused beam. The resulting package is extraordinarily bright and easy to use. (and only then if they are on continrelate mostly to dissipating heat – a lower than a conventional LED. A uously!) but having bought two difhigh thermal resistance and an epoxy high-temp soft gel inner encapsulant ferent versions of the Luxeon LEDs, limited to a maximum temperature optically couples the LED chip to a we can state categorically that these of about 120°C results in a maximum plastic lens. little beasties are simply awesome. input power of about 0.1W. (Consider In addition, Luxeon LEDs use InGaN Phenomenal. Bright enough to a 5mm white LED that might have a (Indium Gallium Nitride) materials blow away any thoughts you constructed in a so-called might still have that LEDs are ‘flip-chip’ structure that good only for panel indicators, The Luxeon gives the following benefits: LEDs are so bright not for illumination. The bulk of the light that they are officially cat egoThe LEDs can provide so much rised is extracted through the as a Class 2 Laser Prodlight that – with the right optics substrate rather than beuct. You should take great care – a beam with a reach, brightness ing attenuated through a not to look directly into the and whiteness can be created that partially absorbing layer as beam and in any applicatio n tha t is made of the LEDs, shi puts a battery-operated handheld occurs with a conventional elding should be provided spotlight to shame. Yes, that so that end users are similarly protected LED; . When working with the LED make sure bright! ♦ Metallisations within that the LED cannot be ina dvertently turned on when you the LED structure give low are looking closely at it. The technology resistance contact to the GaN Conventional 5mm LEDs are and act as excellent optical not very suited to high light outputs – reflectors; maximum current of 20mA at 3.5V – understandable since their packaging ♦ The thin current-spreading that’s 0.07W). was never designed for this purpose. layer in a conventional LED is reThe Luxeon LEDs use completely These LEDs use a small chip mountplaced with a thick, opaque metallic different packaging. A larger LED ed in a reflector cup, with the entire contact, allowing increased current chip is used and it is mounted on an device encased in an epoxy which is densities; aluminium or copper slug, resulting also shaped to act as a lens. ♦ Wire bonds over the top of the in a thermal resistance about 17 times The disadvantages of this approach device are not used, reducing the ab- Safety! www.siliconchip.com.au May 2003  65 A conventional 5mm LED package has a high thermal resistance and temperature-limited epoxy which help limit the maximum input power. [Lumileds] sorption of light within the package; ♦ The LED can be scaled-up in size without electrical resistance or light extraction losses disproportionately increasing. Lumileds state that the ‘flip-chip’ approach is 1.6 times as efficient as a standard power LED, both over the whole range of visible wavelengths and with forward currents of 251000mA. The Luxeon LEDs use a phosphor-converted approach to generating white light – a blue LED is used to pump visible light-emitting phosphors integrated into the package. However, in the Luxeon white LEDs the phosphor granules are applied in a thin conformal layer – rather than being heaped into place – which gives better colour accuracy. Electrostatic discharge (ESD) protec- The Luxeon high power LEDs use packaging which is much better at shedding heat. A more efficient internal architecture is used and ESD protection is incorporated. [Lumileds] tion is built into the LED – shunt diodes are located within the submount that provide electrostatic protection from up to 16kV (human body model) or 2kV (machine model). Two different groups of LEDs are available: a 1W device that uses a 1mm x 1mm chip and an incredible 5W LED that uses a 2mm x 2mm chip. The 5W LEDs have internal Lumileds’ performance records of a drive current density of 50 amps per square centimetre, an efficiency of 44.3 lumens/watt and a flux output of 187 lumens. The 1W Star/O One of the most useful packages is the Star/O. This is a 1W Luxeon LED mounted on a 25mm square piece of aluminium-core PC board. A collimator (focusing lens) constructed from optical grade acrylic results in a beam narrower than would otherwise be obtained. Wiring connections are made to solder pads placed on the upper surface of the PC board, with the positive connection represented by a small dot or ‘+’ sign. The LED assembly can be mounted by small machine screws placed through the U-shaped cut-outs provided on opposite corners of the PC board. The beam produced by the optics has a viewing angle of 10° (defined as the off-axis angle from the centreline where the luminous intensity is half of the peak value). On axis, the 1W Star/O has a typical brightness of 180 Candela. No, that’s not a misprint: 180 Candela (or 180,000mCd….)! The LEDs use an aluminium heat-conducting PC board. The The collimator is a Total Internal Reflection design made wiring connections are on the other side, allowing the LED to from injection-moulded acrylic. It has an optical efficiency of up to 90 per cent and can be used with both be easily mounted on a heatsink. 1-watt and 5-watt Luxeon LEDs. 66  Silicon Chip www.siliconchip.com.au This heatsink is too small for continuous use of the 5W Luxeon LED – although it’s fine for the short, infrequent use that is being made of this LED. Thermal management of the heat dissipated from the back of the LED is critical to longevity. The 5-watt Luxeon Star V Portable produces a stunning amount of light. It is mounted on a hexagonal-shaped aluminium PC board with multiple mounting holes and connection solder pads available. It must not be operated without being mounted on a heatsink. The absolute maximum ratings for touch – and that’s in an ambient of frightening. Frightening not only bethe 1W units are 350mA forward curonly 24°C. cause of the way that you need to be rent (typically at 3.4V) and a PC board careful to avoid blinding yourself, but At the time of writing the Star/O temperature of 105°C. If that temperaalso because its thermal demands are costs $32 from the on-line shop of the ture raises some eyebrows, stay tuned, very high indeed. Australian Alternative Technology because we’ll come back to thermal Association (www.ata.org.au). Importantly, the 5W white Luxeon management in a moment. also has a much more limited life than Considering the number of convenOut of the box, the Star/O LEDs the 1W design – the 5-watter it is rated tional 5mm white LEDs required to are ideal for directional lighting at only 500 hours. such as torches, reading lights This is in contrast and so on. with the other colours As with conventional LEDs Although the assembly is far available in 5W versions , the forward current throu gh the LED must be regulated bigger than a conventional LED, (green, blue and cyan), . The resistance and powe r of the resistors used to limit the fact that it’s a ‘complete packwhich are all rated at up the current flow are select ed using conventional LED des age’ incorporating the light source, to 100,000 hours. ign procedures, although of optics, solder pads, heatsink and The LED is sold in course the resistor powe r dissipation will often be much a mounting plate make its applia hexagonal aluminihigher than is normally enc ountered! Variations in typ cations easy to implement. um PC board package ical values from LED to LE D must be catered for in the When powered up, the LED design, (shaped in this way to so it’s best to measure act ual cur casts a superbright beam of light allow easy side-by-side ren t val ues wit h the individual Luxeon LE Ds that are being used. Lu – in the room in which I am now stacking) with multiple xeo n technical literature sugges ts that DC (rather than pu working, with the Star/O sitting screw cut-outs and also lsed operation) is the most sim on my desk pointing upwards, a multiple solder pads. ple and efficient way of dri ving LEDs, with pulsed opera pool of light about one metre in Centre-to-centre across tion used only when the LEDs diameter is thrown onto the ceil- need to be dimmable. the mounting cut-outs is ing. The light is clearly visible, 19mm and the assembly even with the room brightly lit stands 7.5mm high. by two windows… The LEDs have a forgain the same output (let alone the When viewed on-axis The LED is ward current of 700ma at a forward difficulty of focusing the resulting uncomfortably bright, even from 10 voltage of typically 6.8V. The maxlight) we think that the Star/O is good metres away – again, that’s inside imum rated temperature of the PC value – despite the sudden intake of with daylight streaming in thorough board is 70°C, however a heatsink breath that occurs whenever someone windows. temperature of less than 35°C is refirst sees that price! quired to retain a 90% lumen output However, despite the presence of after 500 hours. the inbuilt heatsink and the LED’s high 5W Luxeon Star V Portable efficiency, it takes only a few minutes And keeping that PC board temIf the 1W Luxeon is impressive, of operation at its peak 350mA before perature down is damn hard, let us then the 5-watter is perhaps a little the aluminium PC board is warm to tell you! Company literature says, Driving the LEDs www.siliconchip.com.au May 2003  67 “We do not recommend lighting a is kept low. any optics the LED will dimly light a 5W Luxeon Power Light Source for whole room at night; suspended from The Luxeon Star Portable comes more than a few seconds at its rated a high ceiling it would be completely without focusing optics, however the current without first mounting to appropriate as a room illumination if collimator used in the 1W Star/O is an appropriate heatsink” – they’re matched with individual spot lighting available separately. certainly right! for reading, etc. With the use of appropriate glue, it While the 1W LED can be used in With the collimating optics in place, is easily mounted on the more powcool ambient conditions (especially the effective brightness is of course erful LED. on a non-continuous basis) without much higher; we then went a step The acrylic optics can be used further heatsinking, that is certainly further and matched this combination without problems on the 5W not true for the 5W LED. with a coated glass lens to give an extraordinary bright spotAn appropriate recommendlight beam. ed heatsink is a finned extruded did aluminium design with a flat The 5W Luxeon Star V Portere wh gy, olo hn king tec So with such ground-brea face dimension 44mm x 44mm able costs $75 from the ATA y pan com e Th m? e fro the Lumileds company com and with six fins each 38mm while the collimating optics to the Hewlett-Packard long. are $6.50 each. can trace its antecedents of (OED). OED became part The heatsink that we used was Optoelectronics Division divided from Hewlett-Pack Conclusion about half this size and proved Agilent when Agilent was hting was formed Lig s led mi Lu 99 suitable for use when the LED The Luxeon LEDs redefine 19 in n ard, and the ture. ven t join s was used with a low duty cycle the current state of the art in ilip Ph and t as an Agilen (ie, was switched on only for short LED technology. bursts). While the relatively short With the LED continuously life of the 5W Luxeon is disLED; the heat is conducted through running at maximum rated current for appointing (but it is still much longer the rear aluminium PC board, with three minutes, the heatsink temperathan most incandescent bulbs used the optical surface of the LED (on ture became uncomfortably warm – a in torches and the like) and careful which the collimator sits) staying near bigger heatsink is essential! thermal design needs to be carried ambient temperature. In continuous use it’s worth again out when using the LEDs, the sheer And so what is the performance remembering the point that life will brilliance of these devices simply has like of the monster LED? Without be prolonged if the LED temperature to be seen to be believed. SC Lumileds? MicroZed.com.au PHONE (02) 6772 2777 9-5 FAX (02) 6772 8987 24 Hours There is more to PICAXE than just the chips. MicroZed have the whole lot on their shelves 68  Silicon Chip www.siliconchip.com.au