用全新的陶瓷方法来简化LED散热设计(三)

高工LED技术中心 发布时间：2009-08-19 15:36:06
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在本文第二部分中,我们讨论了如何计算LED和散热器的热阻,以及如何将陶瓷进行一物两用。在接下来的本文第三部分,我们将讨论该思路的灵活性、定制解决方案的仿真模型、现有灯具改造和隔离、以及改善现有LED系统设计的分装方法。 这一思路是灵活的,可用于不同的目标。你可以选择将LED运行在一个最佳温度,以确保长寿命和每瓦高流明数,你也可选择将LED运行在一个更高的温度,但接受更短的LED使用寿命和效率。从50℃至110℃的温度分布是常见的。如果需要更多的流明数,5或6W LED设计可以配备4W散热器。将如此高功率分解成数个1W功率LED,有助于改善热量扩散： 5W时65℃和6W时70℃(见图4)。 随著芯片永久和可靠地邦定到电气绝缘CeramCool上,该陶瓷散热器就承载了更多的热量,变得更热。它可消除LED的散热负担,有效地冷却这个关键元件。降低的裸片温度也允许采用更小的表面,因此陶瓷散热器可做得更小。 LED散热金属块离冷却水距离仅2mm
在非常高的功率密度下,空气冷却性能已达到其极限。此时,液体冷却就是一个可选项。一个这样的例子是CeramCool水冷却,它得益于陶瓷的惰性。其思路与风冷散热器的目标是一样的,即热源与散热通道之间的距离最短。在陶瓷散热器上,冷却水与LED散热金属块的距离可做到只有2毫米。没有任何其他的方法可以做到这一点,同时又具有陶瓷的坚固性。多层电路可以直接印刷在陶瓷上,同时又不会造成任何热障碍。
定制解决方案的仿真模型 由于大多数使用CeramCool的应用是客户定制解决方案,因此在第一个昂贵的原型出来之前,有必要对其性能进行验证。为此进行了大量的研究来建立仿真模型。这些仿真模型已通过各种测试验证,并证明了与测试结果的可靠相关性。基于这一认识,新的思路或变化就很容易进行评估。 灯具改装和隔离 灯具改装问题主要牵涉隔离。任何改装灯必须是Class II结构,因为你不能保证提供电气地。这意味着任何暴露的金属块必须通过双重或增强型绝缘与交流电源走线隔离。 金属散热器的改装常常不能遵守这一点,因为它们需要更大的距离(如空中6毫米)或双层绝缘,而这会影响散热器正常工作。GU10 LED中集成的电子驱动器在空间上是如此受限制,以致于该产品变得非常复杂。借助陶瓷散热器,即使驱动器完全不工作,散热器也不会导通交流电,因此产品还是安全的。 CeramCool GU10 LED聚光灯可与任何LED一起工作。插座和反射器采用同一高性能陶瓷材料制造。因此,它具有安全绝缘的简单的Class II结构。一个高压4W LED的最高温度只能达到60℃,既提高了LED使用寿命也增强了光输出。 在所有CeramCool散热器中,基板就变成了散热器。在这里,它充当灯甚至灯具。这种简化的设计提供了极高的可靠性。此外,GU10 LED聚光灯的安装和反射器通常用不同材料制成。这一解决方案采用少得多的材料,陶瓷开发用于电气绝缘、良好的EMC和高的机械/化学稳定性。 SUBMOUNTS FOR IMPROVING EXISTING LED SYSTEMS Ceramics can greatly improve both new and existing LED systems. Using a ceramic CeramCool Submount, the PCB between LEDs and the metallic heatsink can be replaced with ease, considerably reducing the total thermal resistance of the system. This offers important advantages, such as very good thermal conductivity while delivering perfect electrical insulation and high temperature stability. Whether it’s a submount or complete circuit board, ceramic is absolutely corrosion-resistant, which eliminates galvanic corrosion, especially outdoors. High-power applications, especially those targeted for outdoor usage, are well-suited for CeramCool. In fact, a family of round heatsinks that will meet the demands of different power levels is under development. The concept combines cost-efficient production with high flexibility of usage. What will ultimately result is a “semi-customized” product family. METALLIZATION AND COMPONENT PART CARRIER To fully exploit the optimization potential, we have to look at metallization possibilities as well. Ceramic can be coated directly with proven thick-layer technologies and their high adhesion force (WNi(Au), Ag, AgPd, Au, DCB, AMB...) or thin-film processes with their smooth surfaces (allowing precise light angles). A finish for better soldering can be obtained using electroless nickel or gold (immersion or cathodic deposition). The possibility of metallization makes the heatsink’s entire surface usable as a circuit carrier, which can be firmly packed with LEDs and drivers on customized circuit layouts while providing reliable electrical insulation. The process can be simplified by bonding the chip directly onto the specially designed metallic surface. ASSEMBLING AND QUALITY CHECK One potential assembler is BMK, a well-known German center for electronic services. The company performs prototyping as well as series production. The heatsinks are mechanically set in a production framework, and a solder cream that’s automatically applied is pressed through a template. In the next step of the process, LEDs and other components are mounted onto the heatsinks, followed by subsequent processing in reflow ovens. A durable bond is established after cooling. The soldering points and position of the components are visually inspected, followed by a 100% functional test. At this point, the product is complete and ready to be sent to the customer. 作者：Armin Veitl 设计总监
欧洲Altair Engineering GmbH