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Illumination & Displays

Solid-state lighting requires specialized cooling technologies

Commercial-grade LED light fixtures can deliver more lumens than comparable compact fluorescent lighting but demand improved design for optimal output and long life.
20 June 2008, SPIE Newsroom. DOI: 10.1117/2.1200806.0952

Commercial lighting technologies evolved slowly over the past hundred years, involving primarily incandescent, fluorescent, and high-intensity-discharge (HID) lamps. But just recently, since 2006, revolutionary advances in white, high-power light-emitting diodes (LEDs) have made it possible to design ‘solid-state’ fixtures that produce more lumen output than comparable compact fluorescent (CF) fixtures. However, these LEDs pose design challenges, including the need for adequate thermal management.

Although designers of traditional fixtures have had to address heat-related issues, such as radiation of infrared energy from incandescent lamps, removing heat from LED semiconductor materials presents issues never before encountered. As a result, some recently-introduced fixtures lack proper thermal management and are likely to deliver lower levels of lumen output than promised and to have shorter lifetimes.

At Gallium Lighting, we have designed commercial-grade LED lighting fixtures that, by incorporating a number of heat management technologies, offer substantial advantages in terms of quality. First, metal-core printed circuit boards (MCPCBs), for example, provide excellent thermal transfer from the heat sinks embedded in the bases of LEDs. More common circuit board materials, such as flame resistant 4 (FR-4), generally lack the thermal transfer characteristics required for high-power LEDs. Second, the extruded aluminum from which our sinks are manufactured dissipate heat into the surrounding environment. Third, the heat sinks also use thermal interface materials that can properly bond the MCPCB to the heat sink to provide the best possible thermal transfer. Finally, the lighting fixtures are vented to provide effective air flow and heat dissipation through convection. Figure 1 shows a cross-section of the Gallium GS6 series.

Figure 1. Cross-section of LED fixture. Heat is dissipated using five strategies to maintain proper operating temperature.

Optimal thermal management combines excellent heat dissipation with high performance. Our fixtures can deliver more than 1000lumen, which represent greater output than a comparable 26W CF fixture. Further, the associated cooling technologies ensure the full rated life of LEDs, typically stated by manufacturers to be from 40,000—50,000h, or about four times the expected life of a CF lamp.

The critical measurement for LED thermal performance takes place in the die containing the positive-negative junction (or PN junction) at which the diode creates photons from electricity. The major suppliers publish maximum die temperatures that range from 120°C to 150°C, but operating LEDs at or near these levels is not recommended. Rather, junction temperatures should be maintained well below 100°C for typical applications. In a 25°C ambient environment, our fixtures operate at below 75°C, ensuring both maximum lumen output and long life.

A side benefit of effective thermal management is the ability to use LED fixtures in insulated ceilings, which are commonly found in residences. Underwriters' Laboratories (UL), the organization that ensures compliance to product safety guidelines, provides maximum housing temperatures for lighting fixtures intended for use in insulation. Installations that easily meet these requirements can be used in either commercial or residential environments.

Specialized semiconductor cooling technologies, which are not commonly used by lighting fixture designers, are critical components for use with the latest high-power LEDs. Further progress may be anticipated in active cooling technologies, including cooling fans, heat pipes, thermoelectric coolers, piezoelectric fans, synthetic jets, and microchannels.1,2 However, these advanced technologies are only expected to be required when LEDs are driven at operating currents higher than 350mA and in the most demanding thermal environments.

Keith Bahde
Gallium Lighting
Fayetteville, GA