How to Properly Understand Thermal Resistance

[96300NAZ]

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.

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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)

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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.

 

[uk_caver]

The thermal resistance of an LED/MCPCB combination just relates to getting heat from the LED to the back of the MCPCB to pass on to whatever the MCPCB is mounted on, and not to dumping heat to the wider environment - it's a rating of it as a heat conductor, not a heatsink.

The thermal resistance to air of a typical MCPCB would be pretty poor, given that there's only a surface area of a few square inches.

 

[96300NAZ]

The thermal resistance I listed was of the LED and not the board. I haven't found specs on the board, however it's aluminum so I would imagine its better than a typical pcb.Still though, you have mentioned, thermal dissipation is not equal to thermal resistance. How can I calculate for an adequate heat sink? Keeping in mind any calculation is not a substitute for practical application; I just need to find a baseline to determine if I can fit within my space requirements.

 

[SemiMan]

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.

Search on Cree or Lumileds or one of the distributor sites for articles on thermal design for LEDs which goes through a whole model. I will see if I have any presentation material I can send you.

Semiman

 

[Steve K]

a web search for "cree led thermal management" pulls up a good Cree application note:
http://www.cree.com/~/media/Files/C... Application Notes/XLampThermalManagement.pdf

21 pages long, with a pretty good explanation of the calculations and various items of concern. Certainly a good review or primer.