Trying to make a(nother) +-12v clean(ish) dual power supply - Page 1
Forum section: electronics / beginners

author: ramphands
author_rank: Contributor
timestamp: November 19, 2020, 05:27:55 am
content: Hello all!I wrote on this forum back last November regarding a layout for a +-12v DC power supply I made, using a couple of 15v SMPS wired to create a common ground, then regulating the two rails to get a linear low-noise dual supply.https://www.eevblog.com/forum/beginners/first-ever-pcb-layout-for-smps-to-linear-psu/It ended up working perfectly, with the only real issue being the big heatsinks needed when the power supply starts drawing over 1A. I posted the final thing on my github as a low-cost option for people who are interested in building a modular synthesizer.https://github.com/ramphands/LinearPowerBoardAnyway, now that I have a bit more of an understanding of things, I want to attempt to build an integrated power supply/delivery board with an eye to making it as low-profile (<3CM ideally) as possible, as well as being easily powered. Simply put, I want to do away with the bulky SMPS side and get things down to an on-board solution. At first, I came up with the idea of using a couple of regular external DC bricks and powering the board that way, with the regulators using the aluminium case for the system as a heatsink. I got as far as to put together a layout - you can see them in the attachments.It uses basically the same topology as my original board, save for the ground plane covering the entire bottom of the board and switching to SMD components.However, I then thought it might be better to approach it a different way - perhaps using DC/DC converters to take an initial, say 17v DC beefy power brick, then convert the output into my +-12v, ideally around the 1.5A per rail mark. I am still very much a beginner, but I remember hearing somewhere that a lot of the modern converters are quite finicky in how they are set up, so I wanted to avoid anything too complex.This led me to these by TI:Texas Instruments PTN78000WAH (positive) and PTN78000AAH (negative)https://www.mouser.jp/ProductDetail/595-PTN78000WAHhttps://www.mouser.jp/ProductDetail/595-PTN78000AAH1So I guess my question is this:Would it be possible (and not stupid) to power these in the same circuit using the same DC supply? Or should I explore other paths?I hope there's enough information here to explain my ideas.

author: ramphands
author_rank: Contributor
timestamp: November 19, 2020, 07:05:48 am
content: Thanks for replying!I looked at a design using an LM2577 boost converter to take a 12v input up to 24v, then using the 12v as the ground and the 0v as the -12v.It seemed perfect until I discovered that it's an obsolete part.To be honest, my knowledge of these things is *very* limited - I'm generally the kind of person that learns something purely as a means to complete a particular task in mind, so I'm basically just learning as I'm going. So, to respond to your initial comment, I don't really know how far I'm willing to go - as far as I need to, I suppose! If I can get roughly 1.5A of low-ripple output on each rail, I'll be happy.I looked at just using an AC-AC power brick, but from my rudimentary look at rectification, the brick would have to be pretty damn beefy to provide enough oomph post-rectifier, further compounded by the fact that smoothing caps for such a supply would be similarly beefy, thereby breaking the whole low-profile aspect somewhat.I'm not averse to using some of the QFN ICs, but from what I've read, they're super picky about layout and generally require 4-layer pcbs.

author: mariush
author_rank: Super Contributor
timestamp: November 19, 2020, 07:58:06 am
content: What's your budget? Cause if you can afford to spend 3.5$ on a single IC, then you could use something like LT3581 :https://www.digikey.com/en/products/detail/analog-devices-inc/LT3581EMSE-PBF/2260546Here's datasheet:https://www.analog.com/media/en/technical-documentation/data-sheets/3581fb.pdfIt can do -12v @ ~ 0.85A with 5v input, so it should do a bit more with higher input voltage... not sure if all the way to your desired 1.5A.But you have example circuit and formulas on page 15, so easy to check.

author: ramphands
author_rank: Contributor
timestamp: November 20, 2020, 11:11:10 am
content: Budget is basically not an issue as long as it doesn't get crazy either on the component front or on the PCB front - a lot of the ICs I've looked at seem to demand a 4-layer board with 2oz copper layers which is way out of my price range for something that is just for my own tinkering.The usage of the chip you mentioned in a master-slave configuration looks interesting but terrifyingly daunting from a PCB design perspective. I guess that's why I was drawn to the modules I linked in my original post - they take a lot of the legwork out of that side of things and don't require a 4-layer board. I still am not sure whether they would work though.Thank you for your help - it's very much appreciated!

author: mariush
author_rank: Super Contributor
timestamp: November 20, 2020, 02:50:54 pm
content: You don't HAVE to use two of them in master-slave configuration, in fact I was suggesting using just ONE of them for -12v, as it's a chip that has the current capability, it can do 1A or more, and will output a relatively CLEAN -12v.For regular 12v, there are plenty high frequency switching regulators which can smooth out 12v+ to 12v and use very small inductors and components, and which are quite cheap.I wanted to also post to give some feedback on the linear supply you made, because I just checked out that github page.I would have used something else for the +12v output, like LM1085 (3A linear regulator). the downside is that its tab is Vout, not ground, so if the heatsink is shared with the other regulator, you would need an insulator between the regulator and the heatsink (and some plastic insulator between the screw and the regulator.Those two big heatsinks are a bit of a mechanical problem. I would seriously consider a square heatsink with lots of fins or pins, and using some standoffs to have the heatsink a few mm above the circuit board, leaving room for whatever resistors and/or ceramic capacitors may be required to be close to the chips.I assume that the heatsinks are around 40mm wide and the whole board is around 80x80mm .. so you could have used for example heatsinks like these :https://www.digikey.com/en/products/detail/advanced-thermal-solutions-inc/ATS-CPX070070025-136-C2-R0/4754008Drill two holes for the regulators, screw them to the heatsink with insulator pads if the tabs are not both ground , then bend the leads of the regulators at 90 degrees and you can use 4 standard motherboard spacers / standoffs to keep the heatsink half a cm above the board.This also means you'd need to move the capacitors or thick resistors outside the heatsink area .. alternatively drill round holes for the capacitor in the heatsink footprint. For resistors, you could switch to surface mount resistors, 0805 and 1210 resistors are easy to solder.For connectors, yeah i suppose those connectors you used are fine.I think a good alternative would be those molex hdd connectors, which can use awg18 or even awg16 wires so you have low voltage drop, and are keyed and have 4 wires : 5v (red) , ground , ground , 12v ... instead of 5v you can have your -12v on that.So basically you can take advantage of mass produced connectors and ready made cables and the contacts and housings are available for purchase, everything is easy to crimp, the connectors are low profile and have good friction fit and so on.

author: ramphands
author_rank: Contributor
timestamp: November 20, 2020, 04:17:47 pm
content: This is gold, thank you! I welcome any and all feedback.I totally misread the part of your original post regarding -12v; I saw the advice on the LT3581 and focused purely on the dual output configuration.Also thank you for the feedback on the github design. Regarding my design with the big heatsinks -you're right that the original power board is pretty much exactly 80x80. When I put it together, size wasn't really a major consideration due to the enclosure size. I was more concerned that I could pull as much power out of the regulators as possible without them bursting into flames. I went with the 350/337 purely because I originally referenced the standard 317/337 combo, but then found that the 350 has a higher output current and better ripple rejection.I've had a look through the datasheet of the LM1085 and think I'm right in that you're suggesting it's better because of the lower dropout voltage (1.5 vs 3) which would result in less heat to manage, or is there another reason I'm not seeing?Between the time I made that original PCB and now, I've done a lot of SMD soldering. As a result, I'm happy to hand solder down to 0402 although I'm much more comfortable with 0603. In fact, I prefer SMD stuff to through-hole now. It's quite relaxing! Anyway, returning to your post, those heatsinks look much, much better than the ones I have - I really never even considered that type of heatsink. Baiscally, I just crunched the numbers, found that I needed 3.2degC/W to keep the 350 in check at maximum power, used Mouser's search system to whittle down my options and I went with the one that had the best natural convection performance. I would definitely have gone with something like the one you linked had it appeared, as I could have mounted it on the rear panel with the fins perpendicular to the ground and the cooling would be far superior to what I have.

author: mariush
author_rank: Super Contributor
timestamp: November 20, 2020, 05:24:10 pm
content: Re why LM1085 ... basically it's because of higher current limit, so it would allow for a higher current peak / burst without going into overcurrent protection.A linear regulator dissipates the difference between input voltage and output voltage as heat.Yeah, the LM1085 has a lower voltage drop of around 1..1.5v, but that's only a benefit if your input voltage will be lower.If you power a LM350 and a LM1085 with 15v DC, BOTH will dissipate the same amount of power : ( Vin - Vout) x CurrentAs for the heatsink ... those tall heatsinks are problem because they can cause fatigue on the leads of the to-220 components, can cause solder cracks in the solder joints over long time ... it's a problem if the device gets mechanic shocks (gets dropped for example) or subjected to vibrationsWith those flat heatsinks with fins or pins, the mechanical shocks and everything is spread in each corner of the board in those 4 screws/standoffs and much less through the leads of the components. If you insulate the tabs of the to-220 chips from the heatsink, the heatsink can then be screwed to pcb with metal standoffs and screws so some of the heat from the heatsink could also "bleed" through the standoffs into the ground plane of the circuit board (or whatever you decide to attach to the holes)Another hidden benefit is that you could easily add a 40...80mm fan on top... maybe bigger if you make your circuit board bigger. you can add a simple resistor to limit the current going to the fan to get some low rpm fan improving the heatsinks.