PLW7070* Series I2LED High Power LED Thermal Management Report

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Introduction State of the art high power LED light sources, such as Plessey’s PLW7070* series, now achieve record levels of efficacy that promise significant benefits for both the end user and the environment. The level of heat produced by LEDs as a by-product of light generation is comparatively small compared with historical light sources, but it is not insignificant and requires management at the system level if the LED is to provide a long life with high performance.

Figure 1:

PLW7070GA, 12V, high power LED.

This technical note covers the management of heat in LED systems.

Thermal Resistance At the heart of each member of Plessey’s PLW7070* family of high power LEDs is a single multi-junction LED chip. The chip layout for a Vf 12V and Vf 24V is shown below.

Figure 2: (left) PLW7070GA,Vf=12V, and (right) PLW7070OA, Vf=24V, high power LED chips These are fabricated using Plessey’s unique GaN on Silicon technology and this allows the construction of these integrated/monolithic LEDs with forward voltages that are approximately multiples of 3V. This allows the LED light source to be built with a single LED chip rather than the traditional hybrid construction of multiple, individually bonded, LED die.

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Another benefit of the technology is the ability to produce very large area LEDs. In common with all LED devices, the heat produced by the device in operation has to be accommodated in the design of the system in which it will be used. When a device is forward biased the current passes through the junctions and they collectively emit blue light with a wavelength in the range 450 to 460nm. This blue light excites a phosphor whose emission, when combined with some of the blue, produces a wide spectrum light output engineered to replicate the spectrum of a black body at that target temperature. The colour rendition (CRI) of the light source can be close to 100%. There are primarily two heat sources in the LED. The first is the emission region within the LED chip. This is located within a few nanometres of the chip surface. The second is the phosphor that has