Systems design approach optimizes high-brightness LED performance

Using a system-level approach, the performance of LED packages can be optimized to maximize the amount of light captured into the required aperture. The result is high brightness, high wall-plug efficiency LED systems for surgical illumination and industrial and dental sealant curing.

Aug 7th, 2004
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Although the efficiency of commercially available LED die has improved in recent years, many applications are limited by the performance of commercial packages. According to Innovations in Optics Inc (IOI), a company based in Woburn, Massachusetts, the package design should be driven by product specifications and by examining the system-level requirements.

In a paper presented at Photonics West in 2004, IOI's Thomas Brukilacchio and Charles DeMilo explained that the optical, thermal, electrical and mechanical aspects of the package should be refined in an iterative process with consideration for manufacturability, reliability, performance and cost to arrive at the optimum LED package.

In focusing on high performance packages, IOI has developed highly efficient primary optics that utilize non-imaging concentrators. Such optical systems overcome one of the most overlooked aspects of system design, which is the efficient coupling of the LED output to the target illumination area.

Illumination system
By considering the concept of conservation of radiance, or etendue, IOI has designed optics such as the compound parabolic concentrator (CPC). This comes within 4% of the performance of an ideal concentrator, and is ideally suited for use with LED die or arrays.

Improved efficiency

As well as optical performance, the CPC-coupled package has other advantages. One is wall-plug efficiency, a property that decreases with increasing current density. For a given output aperture and half angle, the CPC approach maximizes the number of die that can be used, which in turn minimizes the current density for a given value of optical output power. Hence, the wall plug efficiency is maximized.

Also, the non-imaging nature of the CPC tends to homogenize the output from the package. This can counteract the variation of LED output when the devices are driven in parallel, which can be caused by different die having different forward voltages.

Industrial curing
Thermal issues are also important, since operating temperature affects LED die lifetime and maintaining a sufficiently low junction temperature is critical to achieving long life. The optimized package developed by IOI contains a thermally efficient header mounted to a high performance heat sink, with a low thermal impedance material at the interface. The die are mounted about a die width apart to prevent light being reabsorbed by adjacent devices. LEDs that emit all their light from the top of the die will allow devices to be more densely packed inside the CPC.

IOI has developed a number of illumination systems based around its CPC package. One is a dental and industrial curing tool in which the output from the CPC is coupled into a light pipe and delivered to the work area. The output of 1200 mW/cm2 at 470 nm from an 8-mm diameter pipe rivals that of much larger tungsten halogen systems used in dentistry. At 405 nm, the higher intensity of the LEDs results in an output of 3000 mW/cm2 for handheld industrial curing.

IOI has also designed a surgical lighting system with 1000 die per header that delivers 1500 lumens with high efficiency and uniformity, as well as an LED scanning system to replace 300 W xenon lamp at a fraction of the cost.

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