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

Discussion in 'General Electronics Chat' started by DickCappels, Jun 24, 2010.

  1. DickCappels

    Thread Starter Moderator

    Aug 21, 2008
    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: Jun 24, 2010
  2. bertus


    Apr 5, 2008

    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.

  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    ambient is simply the localized temperature a device is operating in. Same definition for SMD and leaded parts.
  4. Markd77

    Senior Member

    Sep 7, 2009
    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.
    DickCappels likes this.
  5. DickCappels

    Thread Starter Moderator

    Aug 21, 2008
    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.
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    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.
  7. timrobbins

    Active Member

    Aug 29, 2009
    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
  8. Norfindel

    Active Member

    Mar 6, 2008
    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.