Project Lighting - April 2008 - (Page 21)

FeAtURe computational performance and even the lowest-cost 8-bit microcontroller can be used. This mechanism provides not only compensation for the aging of the LEDs, but also for the aging of the driver circuitry. LEDs arEn’t cooL incandescent lamps of a given power might be a poor fit for an LED lamp of the same power, as the heat conduction path might very well be too limited. powEr convErsion anD controL The other big challenge that general lighting applications pose is certainly the heat management. As we said before, high-power LEDs don’t waste energy producing electromagnetic radiation outside a narrow frequency range, but they still produce heat as a byproduct, and that heat has to be transferred by conduction instead of radiation like an incandescent lamp. This imposes some important limitations on the design of a general lighting system (for example, a complete fixture), when considering backward compatibility with incandescent systems. A fixture designed for 21 | Project lighting | aPr 08 With all the focus of the LED industry concentrated on obtaining the maximum luminous efficiency, the emphasis must be similarly strong on the efficiency of the driving/controlling circuits. LEDs are low-voltage devices (Vf of 3 to 4 V), a big mismatch with the power grids, and to operate at the best efficiency and maintain the luminous output constant, LEDs require precise current control. Only switch-mode power supplies can provide the high efficiency required for the conversion. Several constant-current driver topologies can be used. The requirement for isolation, power factor correction, and/or the need to retrofit existing solutions can sometimes require a two-stage approach. The input voltage is reduced to an intermediate level, with PFC and high-voltage isolation, while a second stage takes care of the LED current and thermal-control requirements. The diagram in Figure 2 employs a boost converter (the MCP1630) in a constantcurrent configuration. A small 8-bit microcontroller provides a flexible clock source, a programmable current set point to adapt the driver circuit to different LED modules, and dimming functionality, and can provide closed-loop temperature control using an external sensor. A microcontroller-based solution provides great flexibility. The power to the LED module can be gradually reduced as its temperature approaches a critical threshold—rather than providing an abrupt system shutdown or simply providing an alarm. This kind of feature can be critical, particularly when the LED lamp is designed and commercialized separately from the fixture and a correct thermal design cannot be guaranteed. thE aDvantagE of bEing smart Contents Viewpoint Selecting HBLED Drivers for Lighting Apps Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs Lighting News A smart driver design also means that adding more intelligence and features becomes just a matter of adding a few more lines of code. The microcontroller’s built-in serial communication peripherals can implement simple communications protocols, such as DMX-512 or DALI. Ethernet or ZigBee connectivity could be used for more advanced system integrations, and a whole new level of energy management can be designed to improve the efficiency of an entire home or office building system via a central processor. Home Page Product Training Module Online Ordering Suppliers Catalog http://digikey.com/ http://ad.doubleclick.net/clk;198717884;26055153;t http://ad.doubleclick.net/clk;198717989;26055167;e http://ad.doubleclick.net/clk;198717988;26055168;e http://ad.doubleclick.net/clk;198717986;26055170;v http://ad.doubleclick.net/clk;198717985;26055171;v

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Project Lighting - April 2008 Contents Viewpoint About Project Lighting Sponsor Simulation Technique for Optimizing White-LED Design An Analytical Approach to Cooling HBLEDs LED Lighting Needs Smarts to be Competitive Lighting News Enter to Win an iPhone Refernce Materials Lighting Quick Links

Project Lighting - April 2008

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