How to calculate Heatsink? - Page 1
Forum section: electronics / projects

author: diegoterc3
author_rank: Regular Contributor
timestamp: January 18, 2014, 07:21:11 pm
content: Hello I have been trying to calculate a heatsink (or no heatsink) for a TIP122 TO-220 package.For what I could research and from Dave's EEVBlog #105 I have come to this.To calculate the power for the TIP122 I have measured 1.5A*0.84V=1.26W. Physically it shows 1.5A although simulating it says 1.4A.So from the Datasheet I get:MAX OPERATING JUNCTION TEMPERATURE = 150 CTHERMAL RESISTANCE JUNCTION-CASE = 1.92 C/WTHERMAL RESISTANCE JUNCTION -AMBIENT = 62.5 C/WAssuming I use no heatsink I would have 62.5C/W x 1.26W=78.75 C which according to the datasheet would be ok. I plan to use this circuit for 3 hours straight at different ambient temperatures (ej: 15 C, 25 C, 35 C, 45 C) idk if this would be possible.With heatsink there would be J-C: 1.92C/W x 1.26W = 2.4C. So 150C - 2.4C = 147.6CThermal PAD C-H: 0.5C/W * 1.26W = 0.63. So 147.6-0.63 = 147CCase to Ambient:147-45=102 C /1.26W = 81C/W <---- CASEI would like to know if my calculations are correct. I have attached 2 images circuit and calculations.Thank you in advance

author: grumpydoc
author_rank: Super Contributor
timestamp: January 18, 2014, 07:49:22 pm
content: Not sure what happened to your attached diagrams.Your calculation for the no heatsink case looks OK - but don't forget that this is 78.75 above ambient temperature. Assuming 25oC ambient your junction will be at 103.75, more importantly the case will not be much below that at 101.3oCPersonally I like to avoid anything that will burn my fingers so heatsink/case temps of < 50oC is where I would try to aim - don't just go for what the junction will stand, think about what is reasonable (and, as I said, not buring my fingers sounds reasonable to me). Running the device at a lower temp will always improve reliability as well..So, for a 50oC case temp assuming 25oC ambient you need no more than a 25oC rise.If you are dissipating 1.26W then your total thermal resistance needs to be less than 19.8oC/WAs you note thermal pads are generally about 0.5oC/W.Given a junction to case resistance of 1.92 and a thermal pad of 0.5 your heatsink would need to be 19.8-(1.92+0.5) which is 17.42oC/W or better.Finding a TO-220 heatsink with that sort of thermal resistance should present no problem at all.

author: calexanian
author_rank: Super Contributor
timestamp: January 19, 2014, 07:45:19 am
content: Hahahahaha... Operate device. Touch device. If device is to hot to touch, ADD MORE HEATSINK! Repeat until device is just warm to touch, or your wallet is empty. O0

author: mcinque
author_rank: Unknown
timestamp: January 19, 2014, 11:14:12 am
content: Quote from: calexanian on January 19, 2014, 07:45:19 ammmh... very technical!

author: KJDS
author_rank: Super Contributor
timestamp: January 19, 2014, 11:26:48 am
content: The only thing I'd like to add to grumpydoc's post is that ambient is the air temperature around the device, so if you've got it in a box, then the device dissipation will raise the ambient temperature.

author: lapm
author_rank: Frequent Contributor
timestamp: January 19, 2014, 12:04:06 pm
content: Heat design of device s part of electronic engineering.if you don't do it, you will forget that chip X is running hot and it will bite you when you are doing measurements trying to find out why circuit docent work expected. I have few burns in hands/fingers for this after giving a hand for friends trying to figure out what went wrong in their designs.Besides high device temperature shortens its lifetime. Closer to maximum rating you run it, sooner it will fail. And faster it burns if theres design flaw and you got it way over specs.

author: diegoterc3
author_rank: Regular Contributor
timestamp: January 19, 2014, 05:21:17 pm
content: Quote from: KJDS on January 19, 2014, 11:26:48 amYes, my circuit is in a box but how can I calculate this? As I said it will only work for 3 hours.

author: mariush
author_rank: Super Contributor
timestamp: January 19, 2014, 05:40:13 pm
content: It depends on the duty of the transistor, how much time it runs producing heat, how much it doesn't... the transistor and the heatsink will be warm and will slowly warm up the air inside the box. The box should have some holes to permit air flow.. remember, these heatsinks are rated with natural convection, basically air moving around the fins. If your box is completely sealed, you have to check the datasheet of the heatsink to see how much to derate it.If you can't easily determine how often the transistor runs, you could put a temperature probe inside the box and run it for a few hours, logging the temperature increase in the air inside the box, then use that temperature as ambient, for safety.Or just assume the ambient temperature much higher than 25c, do the math assuming it's around 40c.Grumpy measured your heatsink at around 17c/w ... those are super simple and small heatsinks, you can find very cheap heatsinks rated for way less than 15 c /w ... for example :http://uk.farnell.com/multicomp/mc33278/heatsink-to220-x-2-7-6-c-w/dp/1710623at 7.6c/whttp://uk.farnell.com/aavid-thermalloy/504222b00000g/heat-sink-to-220/dp/1703170at 6.4c/w

author: IanB
author_rank: Super Contributor
timestamp: January 19, 2014, 06:18:33 pm
content: Quote from: diegoterc3 on January 19, 2014, 05:21:17 pmIf you make the box out of aluminium the inside will remain much closer to the outside temperature than if you make the box out of plastic.Even better may be to thermally couple the device heat sink to the outer (aluminium) box and then let the outer box be part of the heat sink.

author: Tinkerer
author_rank: Frequent Contributor
timestamp: January 20, 2014, 01:04:07 am
content: Quote from: calexanian on January 19, 2014, 07:45:19 amAnyone else think of this in a russian accent?QuoteThis I concur with. Although if something is reaching temperatures that can boil water, I would also suggest a fan if possible.

author: calexanian
author_rank: Super Contributor
timestamp: January 20, 2014, 02:33:08 am
content: Wow... nobody can take a joke here anymore. You guys suck!

author: KJDS
author_rank: Super Contributor
timestamp: January 20, 2014, 09:27:19 am
content: I once added a strip of copper shim to a 7805 by soldering it onto the tab, just using the heat of the device as it was running to melt the solder.

author: Psi
author_rank: Super Contributor
timestamp: January 20, 2014, 09:32:43 am
content: Quote from: IanB on January 19, 2014, 06:18:33 pmYeah, hot laptop powerpacks really scare me, if the outer plastic is too hot to touch what's the inside like...

author: macboy
author_rank: Super Contributor
timestamp: January 21, 2014, 05:30:02 pm
content: Quote from: diegoterc3 on January 18, 2014, 07:21:11 pmI am not following your line of thought in your calculations.To start with the calculations, you need to reduce the number of variables. Either choose a heatsink then calculate how hot the device will get and decide if that's OK, or choose a target temperature and decide what size heatsink (°C/W) that you need. For reliablity, don't operate your device at the "maximum" temperature listed in the datasheet. That is the max temperature for error-free operation, not long-term reliability. 60°C is commonly considered a reliable temperature (and is hot to the touch).So with 45° ambient and 60° temperature, your heatsink needs to rise only 15°. At 1.26W, that means a total (Junction-to-Ambient) thermal resistance of 11.9°C/W. If Junction-to-Case is 1.92 and the thermal pad (Case-to-Heatsink) contributes 0.5, that means the heatsink-to-Air needs to be (15/1.26 - 1.92 - 0.5) = 9.4°C/W or lower.

author: dannyf
author_rank: Super Contributor
timestamp: January 21, 2014, 06:08:58 pm
content: I also follow the "too hot to touch" method,.

author: dannyf
author_rank: Super Contributor
timestamp: January 21, 2014, 08:17:12 pm
content: QuoteWhat's "operating junction temps"?

author: dannyf
author_rank: Super Contributor
timestamp: January 21, 2014, 08:34:02 pm
content: What does that section say? "Maximum ratings"? Do you always work your devices at their maximum ratings?

author: dannyf
author_rank: Super Contributor
timestamp: January 21, 2014, 08:53:31 pm
content: QuoteThat, in the context of what you said, equates to 30 - 60C junction to ambient. For those of us living in more accommodating climate (20 - 35C?), it translates into much more comfortable case temperatures.

author: Psi
author_rank: Super Contributor
timestamp: January 21, 2014, 11:31:13 pm
content: Correct. He was just pointing out that, when developing a product to be sold/used anywhere in the world it's important to assume ambient is much hotter than it really is in your area.And to take into account that ambient inside a case is hotter than the real ambient outside the case.

author: kxenos
author_rank: Frequent Contributor
timestamp: January 22, 2014, 01:59:32 am
content: Here, I made this sheet. Hope it helps

author: diegoterc3
author_rank: Regular Contributor
timestamp: January 27, 2014, 04:37:25 pm
content: Quote from: kxenos on January 22, 2014, 01:59:32 amThank you! This is great

author: KJDS
author_rank: Super Contributor
timestamp: January 27, 2014, 04:51:12 pm
content: Plenty of places are considered to have a max ambient of 70 degrees C. Imagine a radio sat just inside a sunny window in Arabia when it's 50 degrees in the shade.