PCB as heatsink - Page 1
Forum section: electronics / beginners

author: JWoodrell
author_rank: Contributor
timestamp: January 21, 2014, 04:41:08 am
content: hey guys I am finalizing my first for profit project, and was looking to save a few buck and use the PCB as a heat sink rather than use a separate mountable one (to-220 package) I can get the IC in a package that lays flat on its back to use the PCB as a heat sink. so that got me playing with the layout. I can devote about 4 in^2 (in both the top and bottom layer uninterrupted) to the heat sink copper but the device should dissipate 5-8 watts based on the charts in the datasheet. it isn't square, more of a rectangle shape.I don't know if this will be enough cooling for it or if I should forget it and just go with the separate HS... unfortunately I don't have time to run a test PCB and see if it would work, so if i went with it I would be flying blind and hoping it was fine. the test PCB i have proofed out already had a separate HS and it only got kinda warm to the touch, not hot at all. but the HS was 3.7C/W so it was kinda beefy which is the other thing i want to get away from, it gets in the way during assembly.the chip is running on a single rail 24V supply which should be equivalent to +-12V on this chart which is the only one in the datasheet.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 21, 2014, 09:20:01 am
content: Generally you can't get more than 2-4 watts from a PCB area as a heatsink. Beyond that you need a real heatsink. There is a point of diminishing returns where adding more board area does not decrease the DegC/W anymore.You said you are using a TO-220 package. If you can, you should use the D2Pak, it's designed for this. The TO-220 is not on the chart below but it's worse than the D2PakHere's a chart from an ST Micro application note:AN1703You've got about 1700 sq mm of copper for your thermal area, but according to ST Microelectronics testing, it won't help you to have that much because beyond 600 sq. mm. there is no further improvement. So you can cut your board size back to give you just 600sq. mm. of copper heatsink and you will still get about 34 degC/W Rthj-pcb( see chart below)If you are expecting 5-8W Pd , that's a rise above ambient of 170-273 deg C (!) ... you can't do it.Computed another way, if you allow the junction to operate at 170C, and ambient is 25C (it won't be if you are already in an enclosure, ambient will be higher, maybe 35C, buts lets say 25C in free air), then delta T = 170C - 25C = 145C, and 145C / (34C/W) = 4.2 Watts maximum Pd allowed for the device.So, you're going to need a heatsink. You can't beat physics.My suggestion would be to either reduce the board size fully and save money on the board, and put that into a heatsink, or use a combination of both, knowing that you can only safely use the PCB for about 4W into a 600 sq mm area, and use a heatsink for the other 4W . Those are your options.

author: Rerouter
author_rank: Super Contributor
timestamp: January 21, 2014, 10:10:00 am
content: also note that is for 2 oz copper, its worse with generic 1 oz most cheap manufacturers offer by default,

author: dr.diesel
author_rank: Super Contributor
timestamp: January 21, 2014, 11:03:22 am
content: Dissipating 8 watts is really unacceptable IMO, even if properly cooled via a heat sink. You don't mention your application, if you must have a linear it would be far better to use a switcher upstream, lower the input voltage etc.

author: lewis
author_rank: Frequent Contributor
timestamp: January 21, 2014, 12:21:26 pm
content: It looks like an audio amplifier application, possibly a TDA2050/TDA2030.Dissipation of 5-8W implies an output power of about 0-2W according to that graph. For such a low power output, that dissipation is very high. For much better efficiency, and therefore lower heat dissipation, you'd be much better going for one of the many class D amplifier chips littering the market. Here are a few:http://www.ti.com/lsds/ti/audio-ic/mid-high-power-amplifier-product.page#p776=Analog&p2614=ClosedThey're a bit more complicated, and you need to get the layout absolutely right to minimise EMI, but they allow you to make the design much smaller, cooler, more efficient and potentially cheaper.

author: JWoodrell
author_rank: Contributor
timestamp: January 21, 2014, 03:30:05 pm
content: it is an audio amplifier, I am using the LM1875, which is a to-220 but is has the beefy thermal pad/tab for power dissipation. from what I can see it should be equivalent to a d2pack in thermal transfer, because it has to dissipate 20W when using max voltage (+- 25V) into an 8 ohm load.the heatsink i am currently using is a chunky aluminum fin beast. at about $1.50 a piece, then the cost of assembly handling and thermal grease and other whatnot...I am using a single 24 volt rail because the system it is installed on runs from essentially 2 series car batteries (24V system) it works well enough for what it is, boosting the amp output doesn't gain me all that much in volume so it wasn't worth it doing a dc/dc boster to take the 24v up to 50-60v or so.

author: codeboy2k
author_rank: Super Contributor
timestamp: January 22, 2014, 01:15:59 am
content: That picture of a heatsink is indeed a picture of a heatsink suitable for 8W-10W or so.Not the case in your posting, but we often encounter newbies on the forum who want to dissipate 10-15W using a linear regulator and don't realize just how much heatsink they are going to need.4, 8, 10 Watts... seems like a small number, amiright ?? lolIn your case, I don't know how you can reduce the size of your heatsink without changing to a more efficient design. Often times, the metal case of a project enclosure can provide suitable area for heatsinking, so you might want to consider that instead of the large finned heatsink with it's assembly costs. An extruded aluminum case makes an excellent heatsink too (but do the math !)cheers!

author: scientist
author_rank: Frequent Contributor
timestamp: January 22, 2014, 04:04:52 am
content: It should be noted that the emissivity of the soldermask or board affects how much heat you can dissipate. For instance, the emissivity of human skin is about .95 (with 1.0 being a perfect blackbody), a sheet of polished copper board is about .15, glossy soldermask is about .45, matte black soldermask is about .85. So the darker and matte-er you can get your board's surface, the more heat you can dissipate.