LM317 KCT vs KCS (thermal performance of KCS is better but not sure why/how) - Page 1
Forum section: electronics / projects

author: aiq25
author_rank: Regular Contributor
timestamp: April 21, 2019, 08:33:49 pm
content: I was recently looking at the thermal performance of LM317KCT and I was looking at the RthetaJC. It looks like the KCT version is 4.2 degC/W while the KCS is 0.1 degC/W. I do not understand the difference because they are both TO-220. I have some samples of the KCT version. Is the KCS really only 0.1 degC/W? Also the datasheet doesn't mention (from what I can) there difference between KCT and KCS versions.I guess I just don't understand why the thermal performance is different if the specs and packaging is the same. If anyone has any information and can share I would really appreciate it.

author: magic
author_rank: Super Contributor
timestamp: April 21, 2019, 08:59:40 pm
content: There are a few variations of TO220, differing in thermal plate thickness and other details. The relevant datasheet(s) should show drawings and exact dimensions.0.1C/W is suspiciously low for TO220 and may be a typo.

author: aiq25
author_rank: Regular Contributor
timestamp: April 21, 2019, 10:13:03 pm
content: Quote from: blueskull on April 21, 2019, 09:08:05 pmQuote from: blueskull on April 21, 2019, 09:08:05 pmQuote from: blueskull on April 21, 2019, 09:08:05 pmThanks. This is something I'm just experiementing with. I understand the 0.1 number is not realistic but just for comparsion it is a lot higher than the 4.2. I think I might try these two IC's out under the same conditions and see the thermal performance. I'm making just a simple power supply with the LM317 and there is a switch mode supply before hand as a tracking regulator.

author: FriedLogic
author_rank: Regular Contributor
timestamp: April 22, 2019, 07:26:45 am
content: I would have thought that a value of 0.1°C/W at least is a likely a typo, but there are other interesting changes in the data sheets. The previous version from 2015 had 3°C/W for the KCS, but also had 1.84°C/W for the KTT, which is down to 1.1°C/W in the 2016 data sheet. The junction to ambient numbers are also lower.The TI LM317A data sheet also has much lower values than the old NS data sheet or the 2018 ST data sheet.How much of this is to do with differences in the parts, and how much to do with the testing is another question.

author: magic
author_rank: Super Contributor
timestamp: April 22, 2019, 12:58:34 pm
content: 1.1°C/W I could perhaps believe, it's in line with TO220 power trannies I have seen. Some FETs get it down to 0.2~0.3, but we are talking parts which cost 8 dollars more than LM317, not 8 centsI still think it's a typo.

author: duak
author_rank: Super Contributor
timestamp: April 22, 2019, 05:27:23 pm
content: I'm assuming this is the TI part. I would bet it's a typo too. I remember reading datasheets from shady companies written to promise 200% more performance than you'll ever get. Maybe TI has developed a material with negative thermal resistance.Building on Blueskull's replies, it looks like the LM317 dice could be packaged in either of two leadframes - look at the differences between the two options on the mechanical drawing pages. One is TI's and the other is National's?I ran across an app note from TI on defining and measuring the thermal chc's:http://www.ti.com/lit/pdf/spra953No surprises here.

author: aiq25
author_rank: Regular Contributor
timestamp: April 22, 2019, 07:25:33 pm
content: Quote from: duak on April 22, 2019, 05:27:23 pmYes, this is the TI part. Thanks for the app note, will look at it.