Thermal Path Connection

Discussion in 'General Electronics Chat' started by philipm, May 12, 2013.

  1. philipm

    Thread Starter Member

    Jun 27, 2012
    I'm working on a small board that uses a CREE XP-E emitter, that features an electrically neutral thermal pad for heatsinking.

    It's quite a compact board, so it would be easier to connect the thermal pad straight to the ground plane. I appreciate that if I make this connection, heat will be conducted to all components that have a ground connection. However, for optimal LED performance, I'm aiming to run the board at no more than 50C. So I don't think this is much of an issue?

    With this consideration in mind, is it still a good design choice? Any other reasons you wouldn't want to do it?
  2. chuckey

    Well-Known Member

    Jun 4, 2007
    From the data sheet P = 1.5 -> 3 W thermal resistance = 9 deg/watt, so tjunc max = 150 Deg. So if your board runs at 50, the junction will run at 50 + 27 = 77 degrees max. Very good, are you sure that your PCB will be running at under 50 degs, when it is dissipating 3W?
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
  4. philipm

    Thread Starter Member

    Jun 27, 2012
    Many thanks, I had missed the PDF from Cree. It's very clear : "The pad is electrically isolated from the anode and cathode of the LED and can be soldered or attached directly to grounded elements on the board or heat sink system."

    In terms of the heat dissipation, I intend to vary the duty cycle to ensure that the board doesn't go above 50C. Indeed, I was actually thinking of using a PID algorithm to target 50C on the board, ensuring we get the most from the LED without overheating.

    Thanks again. Problem solved.