Hi, First off I'm Marshall. I am new to the high power LED world. My background started with Electronic Engineering but moved on to more computer/electrical (I'm an industrial controls engineer/programmer) so I didn't get into thermal resistance. It seems like easy work calculating the required heat sink, however I think I'm overthinking or I'm missing something. Here are the specs of the unit I purchased from www.led-tech.de.
LED Unit - 3 x CREE XM-L U2 on 34mm Aluminum PCB.
Spec sheet - http://www.led-tech.de/produkt-pdf/cree/XLampXM-L.pdf
Thermal Resistance of XM-L - 2.5*C/W
Supply Current - 2.8A
Forward voltage of LED @ 2.8A - ~3.3v
Power Dissipated - 9.25W (One LED)
Now on to selecting the proper heatsink. According to the lm vs Tj chart, to achieve an 85% (~1000lm) at 2.8A I would need to keep the junction temperature to 100*C. at a power dissipation of 9.25w and Rth j-c, 2.5*C, Tc = 76.9*C. But, if my ambient is, lets just say 30*C, then that would mean my Tj - 53.1*C. Which would mean I wouldn't need a heat sink? All research I've done says that high power LEDs require heat sinking for good heat dissipation however I'm not seeing it with this module. I think I'm missing something or miscalculating somewhere. The specs are for one unit, but the board I have has 3 in series. How does that factor in to heat transfer? Thanks in advance for the help.
LED Unit - 3 x CREE XM-L U2 on 34mm Aluminum PCB.
Spec sheet - http://www.led-tech.de/produkt-pdf/cree/XLampXM-L.pdf
Thermal Resistance of XM-L - 2.5*C/W
Supply Current - 2.8A
Forward voltage of LED @ 2.8A - ~3.3v
Power Dissipated - 9.25W (One LED)
Now on to selecting the proper heatsink. According to the lm vs Tj chart, to achieve an 85% (~1000lm) at 2.8A I would need to keep the junction temperature to 100*C. at a power dissipation of 9.25w and Rth j-c, 2.5*C, Tc = 76.9*C. But, if my ambient is, lets just say 30*C, then that would mean my Tj - 53.1*C. Which would mean I wouldn't need a heat sink? All research I've done says that high power LEDs require heat sinking for good heat dissipation however I'm not seeing it with this module. I think I'm missing something or miscalculating somewhere. The specs are for one unit, but the board I have has 3 in series. How does that factor in to heat transfer? Thanks in advance for the help.