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The Flexible Definition of High Brightness LEDs

Shortly after the first transistor was invented some talented folks had the idea of crafting several on the same piece of silicon and the integrated circuit was born.  A few more transistors were added and the integrated circuit was renamed the Large Scale Integrated circuit, then the Very Large Scale Integrated Circuit, then the industry ran out of adjectives and concentrated instead on putting 4bn transistors on a chip.  So it is with the LED industry, where the definition of ‘high brightness’ is somewhat flexible, depending on whether the conversation is about LED die, light engines or luminaires.

Most LEDs emit visible light, with much of the current focus on the development of white LEDs to replace tungsten and fluorescent lights in general illumination.  A white LED that is 100 percent efficient would run slightly north of 400 Lm/W.  Modern high brightness LEDs are rapidly approaching 40 percent efficiency, meaning the remaining 60 percent comes out as heat. Unless this heat is removed, the LEDs will fail faster than you can say “tungsten filament” and a major part of developing LED lighting products is in devising effective thermal management solutions.

Of particular importance to the thermal management of LEDs is the substrate or circuit board to which the semiconductor components are attached.  High brightness LEDs are physically small, usually measuring less than 0.5 mm on a side.  This means that not only does a significant quantity of heat need to be removed but, more importantly, the thermal flux, which is the quantity of heat per unit area, is intense.  Consequently only a limited selection of materials are suitable for this application since candidates must possess the combination of low thermal resistance and good dielectric properties.  Tiles made from ceramics like aluminium nitride and beryllium oxide are technically superb but very expensive, while metal-in-board PCBs such as those based on Nanoceramic coated aluminium can provide similar thermal performance at a fraction of the cost with the added benefits of physical robustness, availability in large panels and 3D profiles.

It should be noted that the definition of “high brightness” is also influenced by the wavelength of the LEDs.  Aside from visible LEDs there is a rapidly growing industry, currently worth around $100m/annum that manufactures LEDs to emit UV radiation.  These are used in all manner of industrial processes from disinfection and sterilization through plant growth, printing to scientific instruments.  A key difference with white LEDs is that UV LEDs are incredibly inefficient.  A “high brightness” UVC LED might only be 5 percent efficient.  In other words, these LEDs are actually high power electrical heaters and produce the odd photon every second Tuesday.  Not only do UVC LEDs produce significantly greater heat than white LEDs, to make matters worse the small light output means the LED die have to be packed extremely densely in order to achieve acceptable optical output.  In this industry, it is not uncommon to find UVC LEDs that are operating at power levels of 150 W/cm2.  To put that figure in perspective, a COB domestic white LED ceiling down lighter is running at about 5 W/cm2, some 30 times lower.

For these high brightness UVC LEDs, the approach to thermal management is completely different. Rather than a heat sink to dissipate the heat to air, the LEDs are mounted on a water-cooled metal block, held at constant temperature by an industrially-sized chiller.  Between the LEDs and the cold plate there still needs to be a circuit board to provide electrical connection to the LEDs.  All organic materials are excluded since they would be rapidly destroyed by the UVC radiation.  Thus the choice reduces to metallized ceramic plates with their attendant economic and physical limitations or Nanoceramic coated aluminium, which, with a bit of ingenuity, can incorporate water cooling channels.

Efficiency gains probably mean that the next definition of “high brightness” in the context of white LEDs will not have to resort to water cooling.  Nevertheless, it is comforting to know that thanks to the UVC LED industry the thermal solution has already been proven and the required materials and components are available off-the-shelf, today.

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