Monthly Archives: February 2015

How to affect the lifetime of a LED module

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:

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

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.

LED Module, the point in the middle with the marking "TC" is the TC-point

LED Module, the point in the middle with the marking “TC” is the point where you measure the TC-temperature.

  • TJ 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.

  • RTH = thermal resistance mentioned earlier. TJ = TC + RTH*PD, where PD 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).

Lifetime prediction for aLED Module at 700mA

Lifetime prediction for aLED Module at 700mA

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 TC 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 TJ values, you should use the formula given earlier in the text to find out corresponding TC temperature.

When you find out the maximum TC 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(TA), 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.

A table from aLED modules' datasheet. The typical current is 700mA, but you can drive it with as much as 1190mA. You can also drive it with much lower to save its lifetime.

A table from aLED modules’ datasheet. The typical current is 700mA, but you can drive it with as much as 1190mA. You can also drive it with much lower current (minimum in this case 70mA) to make its lifetime longer.

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.

What is COB LED

COB LEDs are very popular nowadays in LED lighting business. We talk and write about COBs, and our customers use COBs in their luminaires, but what exactly is COB? First of all, the abbreviation COB comes from words Chip-on-Board.

Citizen COB frontside, The yellow substance is phosphor, which turns the blue light of the chip white

Citizen Electronics’ COB frontside, The yellow substance is phosphor, which turns the blue light of the chip white

In COB packages many LED chips are usually attached to substrate with non-conductive adhesive. LED chips are wire bonded together to make different LED setups. The amount of single LED chips, inside a one COB LED package, can vary from few pieces up to several hundred pieces. Substrate is located on base material. Base material of COB LED is usually MCPCB or ceramic PCB. COBs often have blue diodes and use yellow phosphor layer to convert light to desired color temperature.

 

General drawing of a COB LED.

General drawing of a COB LED.

In early years of 21st century there were few SMD LED packages, which could be considered almost as COB packages due to their construction. Generally COB LEDs became available and popular in LED lighting market around year 2007. At first there were first quite a lot doubts towards COB LEDs in the market . Mainly because this package construction enabled LED manufacturers to put high powers in small package. Over then, this “high power” meant over 10W,

Also there was very little experience of COB LEDs, so these LEDs had a lot to prove.  Although now several lifetime tests have shown that COB packages are very reliable, if heat management is done properly.

When thinking about heat management, one important feature is thermal resistance. But it is worth noticing, that you can’t define which COB LED is better to conduct heat just from thermal resistance value. You should actually test LEDs in your own application.

Today COB LEDs are available from few hundred lumens up to 30,000 lumens. This means that

Citizen COB backside, aluminium PCB, which will conduct the heat to the heat sink effectivily. However it won't be enough to cool the LED

Citizen Electronics’ COB backside, aluminium PCB, which will conduct the heat to the heat sink effectively. However it won’t be enough to cool the LED

almost every light source can be replaced with COB LED. So available powers go from few watts up to almost 200 watts. The most powerful COBs require exessive heat sinks because they generate a lot of heat.

COBs offer great variety due to possibility to have many different LED setups even inside one COB package. Usually LED manufacturers have different lumens packages available in same size COB, so lighting manufacturers is able to use e.g. same connectors and optics in different solutions. Also it is good to remember that usually you can underdrive or overdrive COB LEDs and those might have quite wide driving range. This allows you to drive LEDs with very high efficacy, you can make balanced solution or you can make economical lumens.

COB LEDs are generally used in luminaires where lamp or single spot light sources have been used. So basically COB LEDs are used in almost every kind of luminaire. Although COB LEDs are used less often in linear lights or  panel lights, some solutions of that kind have been made with COBs. Luminaires which have really high luminous flux are usually made with more than one COB LED to distribute heat flux and to ease design of heat sink.

Sochi Olympic cauldron, Lights made using Citizen COB

Sochi Olympic cauldron, Lights made using Citizen Electronics’ COB (Copyright: Zers Pride LLC)

COBs offer very high luminous fluxes from small size packages and thus allow flexible design of luminaire. This gives you more freedom, when designing a luminaire. COB packages usually have excellent uniformity of light from light emitting surface, this is important e.g. if you want to avoid multiple shadow effect, which might occur with SMD based solution. It is also very easy to test COB LEDs. You only need COB LED, constant current driver and e.g. piece of aluminium, which can be used as heat sink.

LED Shadows: On the left, a shadow from a COB. On the right, a shadow from a LED with multiple light sources

LED Shadows: On the left, a shadow from a COB. On the right, a shadow from a LED with multiple light sources

We can see that COB LEDs have been very important development step in LED lighting business. This great package design has allowed to increase powers used inside one LED package tremendously. Also one good indicator of success of COBs is that today every major LED manufacturer and package maker has COB LEDs in their selection.

If you wish to receive updates on our  products in monthly basis, you can subscribe to our newsletter by clicking here