A temperature *rise* of x degrees Kelvin and x degrees Celsius is the same. So a thermal resistance of 400 degK/W = 400 degC/W. ThetaJA is the semiconductor junction to ambient thermal resistance. Thus, if your room is at a cosy 21 degC and you dissipate 0.2W in your LED, then the LED’s junction temperature rise above ambient will be 400 x 0.2 = 80 degrees (C or K, it doesn’t matter). and the actual absolute junction temperature will be 80 (the rise above ambient) + 21 degC (ambient) = 101 degC. You need to consider the conditions under which the ThetaJA is specified, i.e. for a SMT LED I would expect it to be quoted for the LED soldered to a PCB with a typical pad size given as opposed to floating in free air! You will find all sorts of graphs in the data sheet for this. The aim is to ensure the LED’s junction temperature remains within it’s absolute maximum ratings, and some safety margin is always sensible. You also need to consider the environment ambient to the LED. If it’s in an enclosure where the temperature may rise to, say 45 degC, then this becomes your ambient temperature. The 0.2W example above would be untypically high for a small SMT LED. A more typical example would be driving a PCB mounted SMT LED having a forward voltage of around 3.5V at, say, 10mA with an ambient enclosure temperature of, say 50 degC maximum. Let’s use your figure of ThetaJA of 400 degC/W (which is quite typical). The power dissipation of the LED is: 3.5V x 10mA = 35mW The LED’s junction temperature rise would be: 35mW x 400 degC/W = 14 degC The operating junction temperature would be: 50 degC + 14 degC = 65 degC A typical maximum junction temperature for such a device would be 100 to 125 degC so the above example is working well within it’s maximum rating. I’m glad we didn’t need a heatsink or, worse still, forced air cooling. Things get a little more complicated… But not too much. Hope this helps. T
----- Original Message ----- From: McInturff, Gary Sent: 10/09/12 09:28 PM To: [email protected] Subject: [PSES] thermal resistance - K versus C Thought I knew what I was doing, obviously I don’t I have a device with a thermal resistance ThetaJA (J-P in this documentation). It’s for SMT CHIP LED the specified value is 400C/W. for a plastic package that seems about right. I am looking at another LED but it lists ThetaJA as 400K/W. (Kelvin/Watt) Seems pretty good as it would convert to 126C/W – the lower the better. But then I converted the ThetaJS (I think its junction – solder point), and that value was 180K/W. When converted its -93C. Does the change in sign just indicate the direction heat is flowing? ThetaJA is positive since the heat is leaving the die through the package, and the ThetaJS is negative because it is heating entering the die through the solder process? Gary McInturff Reliability/Compliance Engineer Esterline Interface Technologies Featuring ADVANCED INPUT, MEMTRON, and LRE MEDICAL products 600 W. Wilbur Avenue Coeur d’Alene, ID 83815-9496 Office:208-635-8306 Cell: 509 868 2279 Toll Free: 800-444-5923 X 1238 [email protected] www.esterline.com/interfacetechnologies http://www.esterline.com/advancedinput Technology, Innovation, Performance... - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to < [email protected] > All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas < [email protected] > Mike Cantwell < [email protected] > For policy questions, send mail to: Jim Bacher < [email protected] > David Heald < [email protected] > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
