How to affect the lifetime of a LED module

Lifetime of a LED module? What it means? Does it mean that LED module does not function anymore after the defined lifetime value? How to determine lifetime for the LED module? This post tries to explain what different lifetime estimation curves mean and how to interpret them.

Few terms have to be defined so that we can understand the lifetime for a LED module. First of all, the LED component itself, a tiny or a little bit bigger one, defines the lifetime of a LED module. Usually in normal conditions, all the other components last longer than the LED.

Some of the key parameters to evaluate LED module lifetime are listed below:

• T_C temperature = the temperature that can be measured from the LED module’s T_C

This is usually placed so that it is as near to one of the diodes in the center part of the module as possible. This means as near LED component’s soldering pad on the PCB as possible.

• T_J temperature = the temperature of the PN-junction inside the LED diode.

This usually can’t be measured, but it can be calculated when the LED diode’s thermal resistance and the LED’s power consumption are known.

• R_TH = thermal resistance mentioned earlier. T_J = T_C + R_TH*P_D, where P_D is the power consumption of the LED diode.

Thermal resistance describes how well heat is transported out of the diode junction to soldering pad on the PCB. The smaller the thermal resistance value is, the better heat is transported away from the diode’s PN-junction.

• LM or L = Lumen Maintenance.

LM value tells that how many percents of luminous output is still left from the original.

• L80Bxx – the lumen maintenance lifetime

Bxx is a value at which xx% (e.g. 50%) of the products lumen output falls below 80% of the nominal initial value. If xx% is 50%, then it is expressed L80B50.

• L80Fxx – the electrical failure time

Fxx is the value at which yy% (e.g. 10%) of the light source population has experienced conventional lights-out failure. If xx% is 10%, then 10% have experienced the catastrophic LED failure, other 90% of the LEDs continue lighting but at reduced lumen output level, was it then below or above 80% of the nominal initial value.

NB: It is very important to look at LED manufacturers’ lifetime estimates as some give the estimates as B values (for example B50) and some give the estimate as F values (for example F50).

How to design the LED module to have certain lumen output after certain operation time

There are few aspects that you should take into account when you want that your LED module operates more than expected operation time at the light output level that you, your customer or target application determines. If you for example require that after 50 000 operation hours of your LED module lumen output should be around 80% of the original lumen output value, you should consult your LED supplier. They will inform you the maximum T_C temperature allowed for your LED component that this target will be reached. Or if your LED supplier gives lumen output curves versus operation time for certain T_J values, you should use the formula given earlier in the text to find out corresponding T_C temperature.

When you find out the maximum T_C temperature, there might still be some cases that you don’t reach the target lifetime. Then you should take some actions.

In the following there are some actions that you should think about if target hours are not reached:

• Try using PCB with better heat conductivity.

Which PCB you have used? Is its thermal conductivity good enough to transport heat away from LED components that affect lifetime of the whole module? If you have used FR-4, why not to change it to Aluminum PCB.

• Try adding larger heat sink or some other heat conducting elements.

Do you have effective heat sink under the PCB to conduct heat away from the PCB itself, not only from the LED components? Is there good thermal path from PCB to heat sink and have you used for example thermally conductive paste or tape?

• Is there a way you could transfer the heat out from the inside of the luminaire?

Is your luminaire closed? Is there any way to transport heat away from the inner parts of the luminaire?

• Think about separating light source and the power source.

The power supply also creates heat. Especially, if the luminaire is closed. Then it forms a closed system with two heat generating elements, the LED module and the LED driver. They both share the same heat load inside the luminaire, thus affecting each other.

Is there any means to divide the light source and the power supply, so that they are not in close contact with each other? Many times, if there is some kind of a metal profile into which the LED module is attached, the LED driver can be placed on the other side of the profile in order to avoid direct heat transfer between these two elements.

• Can you leverage existing cooling solutions.

Ambient temperature(T_A), affects the heat management of the luminaire.  Especially, if the luminaire is designed for application in some larger building, it can be possible to use for example building’s air conditioning system, to transfer heat away from the luminaire.

• Can you change the driving current?

Of course, if you think the LED module itself as a single element and not as a part of a luminaire, also the driving current matters. Could you meet your lumen output requirements with smaller current during first years of operation? And when your light source’s lumen output starts to decrease in the course of years, is there a way to increase the driving current a little bit to reach the needed lumen output level after certain number of operating hours.

Those actions are some examples that you should consider to give your LED module required operation hours at certain lumen output level. Of course you should take the “big picture” into account: which kind of luminaire structure you have, which kind of driver you use and which kind of environment your luminaire is placed in.

Proper heat conduction and management are essential for long lifetime of your LED module! LED does not like heat.

If you have any comments or questions, you can post them in the comments.