What do surface mount LED manufactures mean by "ambient temperaute"?

Thread Starter

DickCappels

Joined Aug 21, 2008
10,152
I have been staring at datasheets for surface mount LEDs and other surface mount components, and it appears to me that LED manufacturers use the term "ambient temperature" to mean the temperature of the mounting surface rather than using the term in the traditional sense for other, especially leaded, components, which in that sense refers to ambient air temperature.

Unfortunately, I have not been able to find a reference, such as the test method or a picture of a test fixture. And the manufacturer, while trying to be helpful, either does not understand the question or cannot provide an answer that seems sensible. So, I am looking toward forum members who have a firm understanding of what "ambient" means in the context of a surface mount light emitting diodes to er...shed some light on this mystery (sorry! ;))
 
Last edited:

bertus

Joined Apr 5, 2008
22,270
Hello,

The ambient temperature is basicaly the temperature of the surroundings of the part in question.
The heat produced will raise the temperature above this ambient temperature.
So if the ambient temperature is higher the total temperature will be higher.

Bertus
 

mcgyvr

Joined Oct 15, 2009
5,394
ambient is simply the localized temperature a device is operating in. Same definition for SMD and leaded parts.
 

Markd77

Joined Sep 7, 2009
2,806
Smt parts dissipate a lot of their heat through the pcb traces so board temperature is probably more important than air temperature. They are likely to be pretty similar though.
 

Thread Starter

DickCappels

Joined Aug 21, 2008
10,152
Thank you for the comments. They have helped my thought process. After a couple of hours of experimentation today, I am starting to think that the manufacturer is referring to the temperature of the substrate upon which the LED is mounted.

Regards to all.
 

retched

Joined Dec 5, 2009
5,207
I would use board temperature as ambient in SMD cases. If you are in a 20deg room but the board operates at 35deg, then 35 is the local temperature for the component.
 

timrobbins

Joined Aug 29, 2009
318
Tamb should be as suggested, but that dictates the need for standard fixtures or configurations (http://focus.ti.com.cn/cn/lit/an/slma002e/slma002e.pdf).

Imho, it certainly makes more sense to reference thermal performance to something more boundless, such as a mounting block of copper of such size that its temperature (ambient) does not change during a specification test. Effectively Rth=0C/W for the mounting block (or is at least theoretically deducable), which leaves Tjc and the interface resistance, which should be significantly higher. One can then introduce the user controllable thermal resistance elements (eg. the pcb and cooling methods) to allow relatively sinple thermal design to be applied.

Ciao, Tim
 

Norfindel

Joined Mar 6, 2008
326
Manufacturers have the most twisted ways of showing thermal data. All you really need is the maximum junction temperature, the thermal resistance junction-to-case, and sometimes the thermal resistance junction-to-air (if there's a remote possibility it will work without any heatsink). Then you can calculate the heatsink you need in the conditions you want it to work.
 
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