heat dissipation graph on soic regulator confusion - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: February 20, 2014, 03:34:54 pm
content: I'm looking at:http://www.onsemi.com/pub_link/Collateral/MC78L00A-D.PDFFigure 8 seems to suggest that I can get more dissipation out of the part if I use a certain PCB layout, so is this graph actually saying that I can take my 0.5W reg and now dissipate up to 2.5W ? naturally they state 2oz copper whereas standard is 1oz.Should I just go for a DPAK type reg ?

author: Rufus
author_rank: Super Contributor
timestamp: February 20, 2014, 04:49:38 pm
content: Quote from: Simon on February 20, 2014, 03:34:54 pmYes it can dissipate 2.5W if the ambient is less than 50C and you put two 4cm x 4cm 2oz copper heat lands on it.A D-PAK isn't much better because physically it isn't much more of a heatsink than an SO8.You can just about dissipate a couple of W from a small component on a PCB if you can spread it across a larger area with plenty of copper usually on both sides and internal planes if you have them. Otherwise you need to heat sink the component or the board around it.

author: Simon
author_rank: Unknown
timestamp: February 20, 2014, 06:41:29 pm
content: Well this is a little power circuit to take 20-30V down to 8V @ 30mA so I'm not in a hurry to heat sink stuff, I'd assume that 1oz copper will be able to carry out 1/2 the heat 2oz can so might as well not bother going above 1/2 the copper surface area for dissipation, what about solder resist, I suppose I need to make sure the copper is left uncovered.I could have copper coming out from under the plastic case out of the ends so getting more copper cross section area to carry heat out and regain the loss of it being 1oz and not 2oz.How about vias ? i guess they won't carry much heat away to the bottom layer ?

author: Rufus
author_rank: Super Contributor
timestamp: February 20, 2014, 07:07:13 pm
content: Quote from: Simon on February 20, 2014, 06:41:29 pmIt is common to place multiple vias close to the heat source to transfer some to the other side of the board.Resist isn't going to make a lot of difference.

author: Simon
author_rank: Unknown
timestamp: February 20, 2014, 07:19:44 pm
content: Well that sounds like a plan even if the board will be bolted to the back of the sensor it powers but the shape of it allows a 1.3mm gap between most of the sensor back and the board

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 21, 2014, 02:44:27 am
content: So-8 won't be able to dissipate anywhere near as much as dpak or d2pak unless it has a thermal pad (which I don't think it does) because so8 depends on its leads but dpak d2pak have a big solder pad to efficiently transfer heat to the board.

author: Simon
author_rank: Unknown
timestamp: February 21, 2014, 04:35:51 pm
content: Well I did my board with a nice big ground plane and put the regulator in the middle of the board with everything else at either side so that there is maximum area closer to the regulator to carry heat away.I've put a resistor in series with the regulator so that at the minimum expected voltage I have just the 2V overhead required by the regulator across it so that at maximum voltage (+10V) I have 0.7W so should cope ok and should never see 30V anyway, more like 28max