Philips Lumileds' New LED Lifetime Graphs Make Designing with LUXEON® LEDs Even Easier

By Ken Fee, Technical Marketing Manager, Future Electronics

Philips Lumileds is an industry leader in manufacturing reliable products, offering the world the longest lasting power LEDs. Since Philips Lumileds introduced the high power LED to the world in 1999, testing continues to be a focus in order to formulate accurate reliability information. Up until now, the reliability claims were covered under the blanket lumen maintenance statement “70% lumen maintenance at 50K hours,” a statement still used today by many LED manufacturers. To make this claim more accurate, a maintained LED die temperature is necessary at the typical rated forward current. The challenge exists when an application falls outside of those parameters; it is impossible for general lighting customers to accurately determine how long the LEDs would last. In order to understand how the new Lumileds reliability claims bridge the gap between the semiconductor and general lighting world, an analysis of how conventional bulbs are rated is required.

A conventional bulbs’ lifetime is usually specified on the package as seen below in Figure 1. Conventional bulbs’ reliability is characterized by a mortality curve as seen in Figure 2. This mortality curve refers to a percentage of bulbs that do catastrophically fail. For instance, the most common mortality rating is B50, which represents the time at which 50 per cent of bulbs will have failed catastrophically. In Figure 1, it can be seen that this Philips bulb is rated for 1500 hours. If it is a B50 bulb, then 50% of the bulbs would fail to operate at that specified time.

 

Figure 1

Figure 2

 

LEDs experience a gradual reduction in light output during operation and generally do not catastrophically fail. This phenomenon is called light output degradation and may stem either from a reduction in the light-emitting efficiency of the LED chip or a reduction in the light transmission of the optical path within the LED package. In fact, Philips Lumileds has a defect rate of < 1 DPPM (defective parts per million) on the LUXEON® emitter. As a result, high power LED lifetimes are not characterized in terms of catastrophic failure, but rather in terms of the percentage of original light remaining over time; this is called lumen maintenance (L). As stated previously, the lumen maintenance of Philips Lumileds’ LUXEON® LEDs has traditionally been 70% after 50K hours, at nominal drive currents and recommended junction temperatures.

This fundamental difference between the method of characterizing conventional bulb and power LED lifetimes has made it difficult for specifiers to specify LED based luminaries and OEMs to market their products. Realizing the gap between both industries, Philips Lumileds has worked closely with the Alliance for Solid-State Illumination Systems and Technologies (ASSIST) to identify a new way to characterize LED reliability.

The first step in developing new methods of characterizing LEDs is to perform real life elevated temperature testing of various LEDs from different production yields to ensure a representative sample set of full distribution products. The longer “real life” testing you can obtain, the more accurate the extrapolated lumen maintenance curve. A reasonable engineering estimate for power LEDs is about 6:1. This means that for every 1000 hours of real life data, you can extrapolate to about 6000 hours of lifetime. It is possible to extrapolate from lower amounts of test data; however, the model loses accuracy. To date, Philips Lumileds has published about 8000 and 6000 hours of testing on the LUXEON® K2 and LUXEON® Rebel respectively, but are gathering data daily. Please refer to Figures 3 and 4.

 

Figure 3

Figure 4

 

As mentioned, lumen maintenance is a function of drive current and junction temperature. As more forward current is provided to the LED, the LED’s lifetime is shortened. In addition, as the LED junction temperature is increased, the lifetime of the LED is decreased. This is represented graphically in Figures 5 and 6.

 

Figure 5

Figure 6

 

To enable a simplified graphical approach, more information than that expressed by the L70 (70% lumen maintenance) figures is required. An equivalent to the traditional B-lifetime (failure or mortality) figure is also necessary for designers to understand the percentage of LEDs in which the lumen output has fallen below the agreed threshold. Remember, an LED exhibiting “lumen maintenance failure” still provides illumination, but slightly below the established threshold. When combining mortality (B) and lumen maintenance (L) curves, the associated graphs are referred to as power LED lifetime predictions (B) and (L) curves. Using this method, Philips Lumileds has specified the expected lifetimes of the LUXEON® Rebel and LUXEON® K2 product families as a function of drive current and junction temperature. For example, the lifetime at (B10, L70) is the expected stress time at which 10% of the population is expected to have either failed catastrophically (i.e. opens or shorts) or has degraded by more than 30% from the initial light output.

The availability of (B50, L70) and (B10, L70) curves allows lighting designers to manage their system parameters (i.e. drive current and junction temperature) to target the desired lifetime of their fixtures. It enables the confident design of a solidstate lighting fixture. Below are the various lumen maintenance curves for the LUXEON® Rebel and LUXEON® K2.

Example: A customer’s application is driving 5 LUXEON® K2 LEDs at 1000mA and the customer has reached a junction temperature of 140°C. By using Figures 7 through 10, it is possible to accurately determine how long the LEDs will last. Using the B50, L70 curve in Figure 7, it is accurate to state that the 50% of the LED population will meet a 70% lumen maintenance at 37K hour lumen maintenance and in Figure 8, 90% of the LED population will retain its 70% lumen maintenance at 20K hours.

 

Figure 7

Figure 8

 

Figure 9

Figure 10

 

This is a new and revolutionary way to measure LED lifetime and it offers customers the advantage of knowing how long their LUXEON® LEDs will last. As these graphs were generated by real life test data, it is impossible to correlate this data to other high power LEDs on the market.

For more technical information about reliability of the LUXEON® K2 and LUXEON® Rebel and how these curves were generated, please refer to reliability datasheets RD06 and RD07 and the LED lifetime analysis white paper WP12. These documents can be viewed at www.FutureLightingSolutions.com/media_center/docs.asp.

 

About Future Lighting Solutions

A division of Future Electronics, the third largest electronic components distributor in the world, Future Lighting Solutions is dedicated to the LED lighting industry. With more than 7 years of power LED experience, Future Lighting Solutions is the pioneer in delivering the most comprehensive worldwide LED lighting solutions support structure in the industry. Future Lighting Solutions offers customers LED lighting knowledge, resources, programs, partners, solutions and logistics support, focused on enabling the adoption of LUXEON®-based solid-state lighting technology. Future Lighting Solutions’ innovative approach is built around its team of world class LED lighting experts, its highly specialized and experienced partners, and its exclusive worldwide relationship with Philips Lumileds Lighting Company.

 

Future Lighting Solutions; Making LED lighting solutions simple™.

 

For more information, please see www.FutureLightingSolutions.com or call 1-888-LUXEON2 (589-3662).

 

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