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Perfecting Performance


Alastair Froggett, Director of Operations at Low Energy Designs, discusses the thermal management of high power LED lighting.


The effects of aging on traditional incandescent and fluorescent tube technologies manifests itself in the gradual darkening of the enclosure or tube walls, as deposits from the filaments or gasses contaminate the inner surface of the lamp. LEDs, by contrast, tend to dim or change colour (darken) slightly over time. LED manufacturers quote Lumen Maintenance figures, which is a way of describing how bright an LED will be over time compared to when it was manufactured. Most quote the time to the 70% level of output (L70), which is typically in the order of tens of thousands of hours. What the casual viewer may miss, however, are the conditions that are needed in order to achieve anything like the stated lifetime – chief among these being the control of the LED junction temperature.


The LED junction temperature is


directly influenced by the size of the forward current used to drive the LED and the surrounding ambient air temperature. For a given specific LED, the harder the LED is driven, the brighter it will shine and, without additional heat sinking, the hotter it will get. For example, an LED driven with a


forward current of 350mA and a junction 48


temperature maintained at 80 degrees Celsius could have an L70 life expectation of 105,000 hours. The same LED driven at 350mA but with a 20 degree Celsius increase in junction temperature is likely to see a decrease to around 77,000 hours in the L70 figure. Even with effective heat management, higher current use tends to reduce the life expectancy of the LED. If we consider the example above, but drive the LED with a 700mA forward current, the L70 life expectancy at 80 degrees Celsius is still likely to be as low as 85,000 hours. The use of high power LEDs necessitates the use of metal core PCBs, thermal transfer mediums and heat sinks. Metal core PCB (MCPCB) typically uses an aluminium alloy substrate base separated from the copper top sheet that carries the printed circuit by a dielectric polymer layer. The dielectric layer provides electrical insulation but high thermal conductivity between the copper and aluminium surfaces. The alloy backing of the MPCPB may then require mounting on a heat sink. The interface between the alloy and the heat sink may incorporate a heat transfer paste, conductive adhesive or graphite sheet to improve conductivity between the board and heat sink.


The heat sink may be an independent element or form part of the luminaire housing. In any event, it must have the necessary size, form and finish to effectively transfer heat from the LED to the surrounding air.


Contact


Low Energy Designs T: +44 (0)1258 858 171 www.lowenergydesigns.com


www.a1lightingmagazine.com


A1


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