7W dissipation in linear reg.= 100degC???

awedio · 31st March 2010, 06:18 PM

Hi. I have an LT1084 12V/5A linear regulator dropping about 3V and passing 2.2A that is heatsinked nicely to one of these:

http://media.digikey.com/photos/Aavi...1102B02500.jpg

1.75" tall and rated at 10W/50degC rise

I have measured up to 98degC at the heatsink... dangerously close it would seem to the 105C rating of the TO-220 package. Should I be worried or looking at a bigger heatsink?

Bill_P · 31st March 2010, 07:22 PM

On the Aavid Thermalloy web site 531102B02500G is listed as 10.4 C/W, so you should expect a 73C rise at 7 Watts dissipation. With a room temperature of 25C, you should measure 98C at the heatsink so your results are consistent. Based on the thermal considerations section of the LT1084 datasheet, that puts the junction of the power transistor in the IC at about 117C which really is too hot. It is probably a few degrees higher than that since the thermal resistance of the case to heatsink interface is not zero. I would suggest a bigger heatsink.

Sch3mat1c · 31st March 2010, 07:38 PM

It's rated to 150C (at which point thermal protection should take over, so don't worry too much about exploding it), so it's perfectly okay.

I'd put a bigger heatsink on it, just so I don't burn myself while poking around!

Tim

awedio · 31st March 2010, 08:01 PM

Thanks, guys.

That picture was of a slightly smaller heatsink, the one I am using is rated at 10C/W but hey that's pretty much the same. Bigger heatsink it will have to be, and I should have done the simple calcs first. I just thought that the ratings were "worst case" and I did not expect quite so much heat. Now I have to redesign the pcb to make room for a bigger footprint, I have limited vertical space.

MarcelvdG · 31st March 2010, 08:31 PM

I'd also recommend a larger heatsink; the reliability of the regulator improves a lot when you use a somewhat larger heatsink than necessary to remain just below the maximum temperature rating.

Bill_P · 31st March 2010, 08:46 PM

Quote:

Originally Posted by Sch3mat1c

It's rated to 150C (at which point thermal protection should take over, so don't worry too much about exploding it), so it's perfectly okay.

I'd put a bigger heatsink on it, just so I don't burn myself while poking around!

Tim

It isn't perfectly ok to run anywhere near 150C if you want reliability. Rule of thumb is that failure rate doubles for each 10C increase in temperature. You would have to request a full reliability report for the LT1084 from Linear Tech to see what the projected life time is at 150C. From there back the temperature down as needed to get the component lifetime needed for the application. Conservatively I try to keep junction temps below 100C at the maximum room temperature expected.

peranders · 31st March 2010, 08:58 PM

awedio, curious what are you feeding 2.2A? Strive to 70-80 degrees C tops, that's my advice.

nigelwright7557 · 31st March 2010, 09:40 PM

A bit of heat isnt too much of a problem.
I have seen MOSFETs that will run upto 175C.

peranders · 31st March 2010, 09:48 PM

Do you speak by experience here? This is not a good design practice.

nigelwright7557 · 31st March 2010, 09:51 PM

Quote:

Originally Posted by peranders

Do you speak by experience here? This is not a good design practice.

As I say a bit of heat is OK.
All you have to do is look at some of the cheap amps on the market with high watts and small heatsinks. They seem to get away with it.

If you dont mind paying a small fortune for a heatsink and fans then you can run at 30C all day.

31st March 2010, 06:18 PM	#1
awedio diyAudio Member Join Date: Jul 2005 Location: PA	7W dissipation in linear reg.= 100degC??? Hi. I have an LT1084 12V/5A linear regulator dropping about 3V and passing 2.2A that is heatsinked nicely to one of these: http://media.digikey.com/photos/Aavi...1102B02500.jpg 1.75" tall and rated at 10W/50degC rise I have measured up to 98degC at the heatsink... dangerously close it would seem to the 105C rating of the TO-220 package. Should I be worried or looking at a bigger heatsink? __________________ "Where is my mind? Where is my mind? Wheeere is myyy mind? Waaay out, in the water, see it swimmin'..." Black Francis

31st March 2010, 07:38 PM	#3
Sch3mat1c diyAudio Member Join Date: Jan 2003 Location: Milwaukee, WI	It's rated to 150C (at which point thermal protection should take over, so don't worry too much about exploding it), so it's perfectly okay. I'd put a bigger heatsink on it, just so I don't burn myself while poking around! Tim __________________ See my Electronics webpage -- the home of Vacuum Tube Drag Racing. The key to being a successful Audiophile: "I reject your reality and substitute my own!"

31st March 2010, 08:01 PM	#4
awedio diyAudio Member Join Date: Jul 2005 Location: PA	Thanks, guys. That picture was of a slightly smaller heatsink, the one I am using is rated at 10C/W but hey that's pretty much the same. Bigger heatsink it will have to be, and I should have done the simple calcs first. I just thought that the ratings were "worst case" and I did not expect quite so much heat. Now I have to redesign the pcb to make room for a bigger footprint, I have limited vertical space. __________________ "Where is my mind? Where is my mind? Wheeere is myyy mind? Waaay out, in the water, see it swimmin'..." Black Francis

31st March 2010, 08:58 PM	#7
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	awedio, curious what are you feeding 2.2A? Strive to 70-80 degrees C tops, that's my advice. __________________ /Per-Anders (my first name) or P-A as my friends call me

31st March 2010, 09:40 PM	#8
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	A bit of heat isnt too much of a problem. I have seen MOSFETs that will run upto 175C. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.


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31st March 2010, 07:22 PM	#2
Bill_P diyAudio Member Join Date: Sep 2009 Location: New York, the vampire state	On the Aavid Thermalloy web site 531102B02500G is listed as 10.4 C/W, so you should expect a 73C rise at 7 Watts dissipation. With a room temperature of 25C, you should measure 98C at the heatsink so your results are consistent. Based on the thermal considerations section of the LT1084 datasheet, that puts the junction of the power transistor in the IC at about 117C which really is too hot. It is probably a few degrees higher than that since the thermal resistance of the case to heatsink interface is not zero. I would suggest a bigger heatsink.

31st March 2010, 08:31 PM	#5
MarcelvdG diyAudio Member Join Date: Mar 2003 Location: Haarlem, the Netherlands	I'd also recommend a larger heatsink; the reliability of the regulator improves a lot when you use a somewhat larger heatsink than necessary to remain just below the maximum temperature rating.

31st March 2010, 09:48 PM	#9
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Do you speak by experience here? This is not a good design practice. __________________ /Per-Anders (my first name) or P-A as my friends call me

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