Understanding Power LED Lifetime Ratings

The first article of this series discussed the optical and electrical properties of power LEDs. This article will dive deeper into the lifetime of a power LED. 

 

Power LEDs

The light output from power LEDs is highest when they are new, but it declines gradually over time. It is common to consider a lifetime of power LEDs the working time required to bring the LED to its 70% light output (called L70).

There are several factors that can influence LED lifetime. Some of them are:

• chemical make-up of semiconductor and optical system
• structure of led die
• implementation of phosphor conversion
• mechanical structure of the device
• materials used
• device’s thermal performance
• consistency and quality of the LED manufacturing process.

 

The combination of such factors can result in significant differences in LED lifetime. Typical lifetimes are from 30,000 hours to 100,000 hours, so a real test of LEDs for such long times is not possible to manufacturers, it would take years of testing.

In 2008 the Illuminating Engineering Society of North America (IESNA) defined a standard to measure the reduction of light output from LEDs in a short time (IESNA LM-80) and a mathematical model to estimate the expected lifetime from the measured data (IESNA TM-21).

 

Fig.7 – LM-80 data and TM-21 projection graph for led Lumileds LUXEON 3030 2D

 

With LM-80 the light output is monitored for at least 6000hrs at maximum intervals of 1000hrs, and the measure is repeated at three different temperatures. The collected data are manipulated with TM-21 models to make the lifetime extrapolation that can be presented as a plot.  

 

Luminous Flux: Measuring Light Output 

In simple words, the total quantity of visible light flowing from a light source in any one second is called luminous flux and its unit is lumen.

The LED is a device where the output light is a function of the drive current. The power LEDs manufacturers usually indicate in the datasheet the luminous flux for a rated current, and to provide a graph to show how the luminous flux changes as a function of the current.

 

Fig.8 – Flux bins table and normalized luminous flux graph for led OSRAM DURIS® E 2835

 

Typically, for each led the manufacturer provides some selections of luminous flux, called flux BINs, so when you order a led you can specify the code related to the luminous flux you want. Also provided is a graph where the luminous flux is normalized to the one indicated in the flux BINs table and it is plotted how the flux changes as a function of the LED current.

In this way, the designer can easily calculate the luminous flux flowing out from a led driven with a specific current and selected with a specific flux BIN. The values provided for the flux BINs and the graph flux vs current are provided for Tj=25°C. 

Since the luminous flux is also a function of the temperature, manufacturers provide a graph to show how the relative luminous flux decreases with temperature:

 

Fig.9 – Normalized luminous flux vs junction temperature for led OSRAM DURIS® E 2835

 

Using the graph above the designer can apply a correction factor to the flux calculated for Tj=25°C in its application, in order to calculate the luminous flux for the temperature really expected in the application.

 

Luminous Efficacy: Electrical to Optical Conversion

The LED can be considered as a device that essentially is able to convert electrical energy to optical energy.

In the process of this conversion, there is also some loss due to the heating, so only a portion of the electrical energy is converted to light emission.

 

Fig.10 – Representation of electrical-to-optical energy conversion in the LED

 

The parameter that represents the efficiency in the input-to-output conversion is called luminous efficacy.

It can be calculated as:

$$Eff = \frac{\Phi _{LED}}{P_{LED}} \left [ \frac{lm}{W} \right ]$$

 

where:

• Φ_LED is the output luminous flux calculated for the LED
• P_LED = V_LED • I_LED is the input power provided to the LED

 

The luminous efficacy is considered as an indicator of power LEDs performance, so LEDs manufacturers work continuously to increase the luminous efficacy.

Today, the best-in-class power LEDs have a luminous efficacy around 200lm/W.