LEDs do something cool!
Today is the 2nd Sunday of Advent. I am preparing for Christmas. Since my childhood I have been really fond of Christmas-trees and candles on the tree. But my father has always been afraid of fire, therefore candles were lit only when my parents were present. It changed with the strings of bulbs. They introduced a new game: when decorating the tree, one had to test the string since the low voltage bulbs were connected in series – the overall voltage drop of the string was equal to the voltage of the mains (230V in Europe). Instead of having a switch installed in the string one of the bulbs was loosen to cut the circuit (simple but sometimes dangerous practice). But after a year, when next time we decorated the Christmas-tree we never knew which bulb was the lose one. With the bulb strings the immediate danger of fire was gone. But still one had to be afraid of possible fire, since the surface of the bulbs was hot and after a couple of days a real tree (my favorite is silver fir) in the warm house gets dry, gets flammable. So, even with a hot bulb touching the fir we easily create work for the firemen. But I can not help liking a silver fir being illuminated all the time between Christmas day and New Years Eve…
This year I wanted to have something cool for Christmas. So I ran into my corner shop (which happens to be an electronics shop) and I just invested 40$ into my long-wished LED lights for Christmas. I immediately opened up the box and added the LED string to the Advent-wreath. Since it is the 2nd Sunday, two candles were lit, plus the LEDs. (Yet two weeks to wait for my favorit silver fir…) And I could not resist to take a photograph which illustrates the entire history of artificial lighting: from candles to LEDs.
Yes, LEDs are really cool. By all means. They can be easily controlled and by using LEDs one can save really a lot of energy. Unfortunately strings of colored LED are all blinking – a bad application of the simple control of LED lights. I simply can not stand this color changing, so I am sticking to warm white (CCT = 3000K) LEDs which very much resemble the real candles. But they are cool. And since they are cool they will not create fire - this year I should not worry putting fire on the Christmas-tree, only the candles of the advent-wreath mean some risk.
Getting rid of the danger of fire at Christmas is not the only reason why we may like cool LEDs. The cooler the LEDs are – the less energy they consume to provide the same amount of light. And not only their energy conversion efficiency increases when their junction temperature is kept low, but their expected life-time becomes also longer. This means that one really has to pay every effort to make sure that LEDs remain cool when they are used in a luminaire. So, good and reliable thermal design of LED packages as well as good design of the thermal management of LED-based lighting solutions is a must.
On one hand we have to make sure that the different thermal interfaces of an MCPCB assembled LED are good: there is no voiding in the LED die attach for example or the LED package does not delaminate from the MCPCB substrate. On the other hand, when such an LED assembly is built into a retrofit LED lamp (nowadays also available in electro-shops), one has assure that the housing of the lamp (the LED luminaire) is capable of transferring all the heat generated in the LED chip to the surrounding air and the air’s natural convection provides sufficient cooling.
Thermal transient testing helps qualify the LED packages, including all the thermal interfaces in the heat-flow path from the LED chip’s PN-junction down to the bottom of the MCPCB substrate. In order to end up with the true thermal resistance of the LED package or the entire LED assembly the usual JEDEC JESD51-1 complient thermal test (static test method) has to be combined with the measurement of the LED’s total emitted light flux. Measurement of the light output metrics of LEDs of course have to be carried out in compliance to the CIE 127:2007 document. If you are interested in details about comprehensive testing of power LEDs, I recommend to have a look at my prior webinar titled LED Thermal Characterization Made Easy which is available as video on demand from Mentor’s web-site, or read an article about this in the SPIE Digital library. General issues of thermal interfaces and thermal interface materials are key to LED reliablity as well, as it has been demonstrated in a recent publication about thermal diagnostics options of LED-based street-lighting luminaires. (In this paper T3Ster and FloTHERM studies on LED street-lighting luminaires and LM80 tests completed with thermal transient testing are described.) Further details on connection between reliability and TIM testing was provided by Andris Vass-Varnai in his very recent webinar. (You my listen to Andris and Ian Clark in person at a tech day in Bangalore on the 7th of December.)
If one is interested in thermal issues of LED-based lighting systems and luminaires, CFD simulation is the right analysis method to use. If you think of the thermal time-constant of a large street-lighting luminaire for example, it will be in the order of magnitude of hours. It is not just question of pure physical testing time that would be required. For the physical testing one needs a prototype of the luminaire. To study different what-if design scenarios by physical testing does not pay off in this case. Virtual prototyping and CFDanalysis would be the right approach in such cases. In the figure above I just show a simple example – an MR16 LED retrofit LED lamp. It has been both measured and simulated with Mentor Graphics’ T3Ster equipment and FloEFD M-CAD embedded CFD simulation tool. The physical testing time in this case required about 1h – measurement took place in a JEDEC standard still-air chamber. In case of larger luminaires different design variants can be studied by the system level designer in his M-CAD system. Various examples for such studies were shown by my colleagues in their webinar titled Demystifying LED Design for Everyday Applications with Concurrent CFD and in another webinar titled Design for Longevity in Your Power LED Products.
I think, our simulation and testing tools are really cool – use them to make sure, your LEDs remain cool. LEDs do something cool together – any time, not only in the Advent period.
Best regards: Andras
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