• led light 23.11.2011 No Comments

    In LED lamps, as most power LEDs are surface-mounted, some sort interface is needed before the heatsink.

    This can be a metal-cored PCB (MCPCB), a ceramic substrate, or even an FR4 PCB riddled with thermal vias - the latter of which can hit a creditable 3-4K/W if done well, according to Lumileds’ Kruger.

    All have different thermal conductivity and different coefficients of expansion, and picking the wrong expansion coefficient can ruin reliability in products that undergo a lot of thermal cycling.

    For limited runs, custom heatsinks may not be necessary. “There are off-the-shelf circular LED heatsinks and heatsinks for strips of LEDs,” says Paul Ward, opto-products manager at distributor Farnell.

    And for large runs, there is an alternative to aluminium heatsinks.

    “There’s a new thermally-conductive plastic that can be moulded. You can form the body and heatsink from the same material,” says Ward. “Philips has done this in one of its MR16s.”

    He holds out some hope for people who do all the design optimisation and still find there is not enough natural convection to cool their designs: “Nuventix has a very clever way of generating turbulent flow using magnets and a diaphragm.”

    Called SynJet, they are small DC-powered capsules that puff air out in a way that pulls in ambient air. Nuventix claims they will not clog with dust and can run silently. A range of circular finned heatsinks are available from the same company that are matched to specific puffers and specific LEDs.

    Fans are used inside LED car headlights. Are these any use in luminaries for buildings?

    “I have not found anybody using fans,” says Ward.

    One last piece of advice:

    “People always focus on the thermals of LEDs,” said Lumileds’ Kruger. “Quite often you will have the driver inside heatsink, and sometimes it will have electrolytics - whose reliability goes down very rapidly above 100°C. A system is only as strong as its weakest link.”

    On the other hand, you could make a batch of different mechanical prototypes, which is increasingly difficult with the smooth curving geometry and complex shape of modern designs, but made easier by rapid prototyping techniques.

    “Batches of runs are generally the argument for computation,” says Parry, pointing out that time optimising heat spreading and conduction in the ’stack’ is seldom wasted: “If you can take a few cents out of an LED light bulb, its really does warrant putting that effort into design.”

    In designing a stack, “the more you can spread heat before it gets to the heatsink the better, but you need a stack-up that meets your cost objective. You have to think about this in 3D”, he adds.

    As an example of ‘Rolls-Royce’ heat spreading, Parry cites some telecoms laser diodes where the die sits on a synthetic diamond heat-spreader, on a copper second-level spreader, on the aluminium package base.

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