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Half-Watt LEDs Fill the Gap Between 0.25 W and 1 W LEDs

Fri, 05/21/2010 - 12:32pm
Keat-Chuan Ng and Margaret Tan, Avago Technologies

Margaret TanNG KEAT CHUANHalf-Watt (0.5 W) LEDs are gaining more attention from product designers because they fulfill the need for a device that falls between 0.25 W and 1 W LEDs. As more consumer products become smaller and more energy efficient, more designers are selecting 0.5 W LEDs for use in their designs. Prior to the availability of 0.5 W LEDs, lighting module designers were forced to use 1 W LEDs that fell short of their design requirements and caused their own environmental and cost issues.

Taking into consideration the growing number of applications that could benefit from using a 0.5 W LED, a new series of 0.5W LEDs has been developed. Careful consideration was given to the cost, manufacturing methods, component size, Moisture Sensitivity Level (MSL), and thermal performance over the lifetime of the product, with the goal of encouraging product designs to easily adopt 0.5 W LEDs to meet their specific design requirements.

The Evolution of the 0.5 W LED
In the 1990s, mainstream LEDs were less than 0.25 W, but when solid state lighting started to get attention, LEDs at 1 W and above were extensively developed to fulfill the needs in the market. Manufacturers jumped onto bandwagon to develop high-power LEDs with the goal of eventually replacing the incandescent bulb. In the meantime, applications that use lower than 0.25 W LEDs also evolved.

Today, higher power LEDs are needed according to different application needs, such as using fewer LEDs to fit into smaller spaces due to shrinking product size, higher brightness required by each LED, different lighting effects/outlook needs (e.g., spotlight illumination), and other requirements.

These LED lighting requirements translate to the need for LEDs of over 0.25 W. Although, some application needs may be fulfilled by LEDs at 1 W and above, most need only a one-fold increase to 0.5W. Not all designs require a drastic change from 0.25 W to 1 W of power, since designs greater than 1W normally means using larger LEDs. However, product designers are sometimes forced to drive a 1W LED to 0.5W to fulfill their design requirement; but if the designer under drives the LED that was originally designed for 1 W, it will not always operate as efficiently (lm/W). Further drawbacks of under driving 1W LEDs to achieve the performance of a 0.5W LED result in a solution that wastes resources, is not environmental friendly, and increase the cost of final product. Therefore, 0.5W LEDs that fill the gap between 0.25 W and 1 W LEDs are gaining more attention from product designers.

LED Design Considerations
A growing number of applications can benefit from using 0.5 W LEDs. For example, a 0.5 W series of LEDs developed by Avago Technologies uses a Plastic Leaded Chip Carrier (PLCC) platform, which is a mainstream SMT LED package in use today. The LED die used is also at its highest efficiency (lm/W) when operating at a 0.5 W condition (around 150 mA). In designing this compact LED, careful consideration was given to cost effectiveness, manufacturing methods, component size, Moisture Sensitivity Level (MSL), and thermal performance over the lifetime of the product.

To ease the product designer into adopting an 0.5W LED, the device was developed to provide the highest design flexibility and to reduce design cycle time in its class of LED modules. The 0.5 W series of LED, therefore, was made to be exactly the same size as the mainstream 0.25 W LED (2.8 mm x 3.0 mm x 1.9 mm) and has a similar foot print. This way, the product designer can redesign the electrical circuitry routing on PCB with less effort and with minimum consideration for the space necessary to switch from a 0.25 W to 0.5 W LED.

Another design consideration is to ensure a sufficient heat dissipation path is installed on PCB to drain out the heat generated by the higher power LED (which may be already available to another electronic component on the PCB). Therefore, the component packaging for the 0.5 LED series achieves 60 K/W by adding additional heat paths closer to the LED heat generator, the LED die. In Figure 1, the schematic shows the additional heat paths added to the 0.5 W LED. An additional lead not only provides a shorter heat-path length but also increases the area of normal to heat flow. These two important factors contribute to increasing thermal conductivity according to the equation for steady-state heat flow which can be expressed by:  

Equation-Avago

Where:
Q = heat flow per second (W)
A = area normal to heat flow (m²)
k = thermal conductivity (W/mK)
L = length of heat path
?T = temperature gradient


Figure 1: The heat path of a conventional <=0.25 W LED versus a 0.5 W SMT PLCC LED with multiple heat paths.

This new 0.5 W platform is also highly scalable to cater to both single and multiple dies, depending on the application needs, without compromise on thermal performance.

To stay competitive, the 0.5 W LED was innovatively designed with the 0.5 W heat sink fabricated on the same metal sheet as the electrical contact terminal and die pad. This way, a minimum of process steps were added to the existing process steps already in place to produce the 0.25 W PLCC LED. In comparison, 1 W LEDs require a large heat slug, which need to be produced with more process steps and are generally bigger. This cost-effective approach further encourages designers to use 0.5 W instead of 1W LEDs.

Additionally, the product designer should also consider other important performance parameters of a 0.5 W LED: moisture sensitivity level and the optical performance of the product over life time. The higher the MSL level, the longer the devices will be able to stay in a manufacturing environment before it is soldered on board, which translates to easy handling in a mass production environment. The 0.5 W series LED can meet the JEDEC MSL2 standards, resulting in a one-year floor life without re-bake of the unit. Other contributing factors are the careful design of the LED to have better anchorage in a lead-frame-plastic housing, as compared to conventional Plastic Leaded Chip Carrier (PLCC) LEDs.

Moreover, the light output performance over the LED’s lifetime should be considered by the product designer. In general, when an LED’s power goes higher, more heat is generated and more light is emitted. This, however, will speed the deterioration of the housing material and lead to faster Light Output Power (LOP) degradation. The 0.5 W LEDs were developed with a high-reflective housing material coupled with an enhanced performance silicone as window layer to ensure minimum optical degradation.

Other specific considerations needed by the product designer depend on the field of usage. For example, they must consider if the LEDs will used for automotive designs or if they must be suitable for outdoor signage purposes, in which case they must fulfill the ingress protection rating according to NEMA IEC 60529.

All in all, there are many advantages of using 0.5 W LEDs, but the designer must consider a few key factors like adoption time, ease of use and performance of the product, and whether they must meet specific requirements according to the field of use. Therefore, the designer should spend time carefully selecting the correct high-performance 0.5 W LED suitable for their particular application.

Keat Chuan Ng and Margaret Tan are R&D team leaders at Avago Technologies’ Optoelectronic Products Division. For more information, contact Avago Technologies, 350 West Trimble Road, Building 90, San Jose, CA 95131; 877-673-9442; www.avagotech.com

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