The highly reflective OneLED white package provides high light extraction while the black colored surface makes it possible to achieve a high contrast ratio from an array of LEDs in outdoor applications. The LED achieves a typical brightness of 745mcd for red, 1600mcd for green and 380mcd for blue. The achievable high brightness of the devices enables customers to achieve higher luminance (nits) with the same pitch size.
The colors of the individual OneLED chips are quite saturated (typ. 622nm for red, 530nm for green and 470nm for blue), which helps to create pure mixed colors. The OneLED color bins are “floating bins” that overlap quite a bit to support easy calibration and uniform visual appearance of the overall video wall. As an example, to achieve a target white point of D65, based on the LED RGB mixing ratio calculation, the estimation ratio between red, green and blue is approximately 3.2: 7.5:1 for wavelengths of 622nm, 530nm and 470nm. Thereby, with 210mcd blue light, you will need 672mcd red and 1575mcd green light to generate a neutral white color from an RGB multicolor LED. If the wavelengths are different, the RGB ratio will vary accordingly. Avago and other LED manufacturers can support the display maker by selecting the appropriate wavelength and intensity of the product to be used in the LEDs' assembly.
For indoor use, the completely black package option of the OneLED series (ASMT-YTC2) helps to boost the contrast ratio of the entire screen even more. Though the brightness of the chips is reduced by 40-60 percent compared to the white package, they can be used to design large video walls that look like huge LCD screens to people passing them. The completely black LED package enables high resolution indoor video walls with a very high contrast ratio and vivid mixed colors.
With the low thermal resistance of the black package, the ASMT-YTC2 LEDs can be driven at the maximum current at up to 55°C ambient temperature, as shown in Figure 3. With this feature, it is possible to get the maximum light output from the video wall even at elevated ambient levels, such as the conditions found in a football stadium during a hot summer afternoon.
Figure 3: Maximum forward current vs. ambient temperature for the Avago ASMT-YTC2 OneLED device
For outdoor video walls, protection of the screen from water ingress is a key consideration. Most surface mount LEDs generally require protection from outside moisture, as water could possibly enter the package causing short circuits or corrosion within the LED. Usually, this protection is done by using a transparent plastic cover above the LEDs. In contrast, Avago OneLED ASMT-YTB2 devices are tested according to IEC 60529:2001 with an IP rating of X6, which means it is ingress protected against strong water jets. If cleaning of the display is required, it is recommended that the pressure of the water jet stays below 30kPa in order to avoid excessive mechanical stress on the inside of the package.
Though the opening of the package is protected against water ingress, the electrical terminals of the LED need to be protected against water to avoid short circuits or corrosion from water exposure over time. As LEDs are usually not the only electrical components on the PCB, it is advisable to do a potting in any case. This potting can be done using electronic grade potting materials which are based on silicone, polyurethane or epoxy, as is shown in Figure 4
. However, apart from the leads of the OneLED devices, there is no need to protect the LEDs themselves from water. The opening of the PLCC-6 package can be directly exposed to outdoor environments in which rainfall, mist or fog directly contact the LED surface. Hence, it is possible to save a lot of cost and weight by eliminating the need for a complex covering system.
Figure 4: Potting provides additional water protection for the Avago OneLED device
In summary, ingress-protected surface mount LEDs can help to reduce the overall system cost of outdoor full-video displays. PLCC-6 LEDs offer the advantages of being small and robust, making them suitable for a wide range of applications. With a wide operating temperature range at high brightness output, they are the ideal choice for outdoor video displays.
Avago Application Note 5487 “IPx6 Rated Surface Mount LED”, AV02-2678EN, November 4, 2010
About the author:
Andreas Pohl is European Field Application Engineer for Avago Technologies' Optoelectronic Products Division with a focus on Solid-State Lighting.
Andreas joined Avago Technologies (formerly Agilent) in 2003 and is located in Boeblingen, Germany. He is a graduate of the University of Cooperative Education in Stuttgart, Germany, where he earned his degree in Electrical Engineering in 2006.
This article was first published on EE Times Europe.
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