Power supply design critique - Page 1
Forum section: electronics / projects

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 04:07:10 pm
content: Could a few of you folks that are experienced have a look at my current project and give suggestions for improvements with the board? This is a power supply that can do 0-30V @ 3A. I'd like to have this board made but have never done this kind of thing before. The board and schematic were done in Eagle and for that I apologize but it's the only program I can reasonably use out of the box. I've tried to make the power and ground connections big enough to handle the 3 amps that the power supply is supposed to deliver but am not sure if this will even work as, like I said, I've never done this before. I used apcb trace width calculatoronline to give me an idea of what to design and I came up with a trace width of .07 that would fit within what I had done so far. There are notes in the schematic for the parts that are off board.Edit with latest board and schematic.

author: mjkuwp
author_rank: Unknown
timestamp: March 14, 2014, 05:04:48 pm
content: ME here. (mechanical engineer)you could have a look at a screw of the size you would put in your mounting holes. measure the diameter of the screw head and put a circle that diameter on the silkscreen. Too many times I have seen EE guys put screw holes in boards and not leave room for the head - or barely enough room.Again - from a mechanical perspective I prefer round via shape rather than square. Corners worry me but this is very much a matter of personal opinion. Are the vias big enough? could they be a bit bigger and not have clearance problems?Did you run the design rule checker on the board? I am only looking at the png, not the Eagle files. IN- by J1 looks like it comes very close to IN+ where the D3 connects.I use oshpark for PCB manufacturing. There is a design rule file you can download. Different fabs will have different capability so you have to adjust the clearances to suit.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 05:59:56 pm
content: Quote from: mjkuwp on March 14, 2014, 05:04:48 pmThe standoffs are 4-40 and there should be plenty of room around the head and I had planned on putting nylon washers between the board and the standoffs and screw heads.Quote from: mjkuwp on March 14, 2014, 05:04:48 pmI didn't know how to turn off all the layers except the vias until you said this and I have changed the shape to round as it's my preference as well. I haven't done the part footprints yet but I plan on that as well.Quote from: mjkuwp on March 14, 2014, 05:04:48 pmI used ITead's rule checker and it comes out ok as far as that goes but I did move D3 a bit and centered that particular trace a bit more away from both of the footprints it runs between.I appreciate you looking at it. I'm attaching a larger 600dpi image with the changes so far because at 300dpi online, it's just too hard to see certain things.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 06:05:28 pm
content: You've got your power traces snaking all over the board. Make your power traces as short as possible, and build your analog circuits around those. Use a large ground plane on the back of the PCB for power, and have a separate analog ground for your control circuitry. The analog ground should only be connected to the power ground at one point, I recommend at the input capacitor.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 06:27:55 pm
content: Think of your power supply as a loop. Power travels through the ground to the device under test, then back into your power supply's positive terminal. Your power supply regulates the voltage driving those electrons. The loop needs to be as short and streamlined as possible. Your control circuitry just adjusts your power supply, so that can be off to the sides somewhat, but it needs to be somewhat isolated in terms of ground, or the noise your PSU makes will feedback into the control circuitry.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 06:37:46 pm
content: Quote from: Phaedrus on March 14, 2014, 06:05:28 pmThe problem I had was trying to get all of the components for two separate supplies into the space available, i.e. 10 square centimeters. I started with 0.16 traces as was the default settings in Eagle and then got everything positioned and then widened the power traces and the ground traces. I suppose I could rip up things and start over again but the board size has to stay the same regardless due to space constraints in the case.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 06:46:56 pm
content: I designed my first PSU the same way, and got ripped up for exactly the things I told you. I started over from scratch, and two revisions later I had a workable layout. Trust me, it's worth it to start over using proper design principles. Lay out your power components, traces, and power ground first with the shortest loop length possible, then layout your control circuitry.Every extra cm your power traces go is an extra several ohms impedance, plus that much more noise picked up as an antenna, and your PSU's response will be that much slower.

author: johansen
author_rank: Super Contributor
timestamp: March 14, 2014, 06:55:31 pm
content: can you post a schematic?

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 08:01:18 pm
content: Quote from: Phaedrus on March 14, 2014, 06:46:56 pmI'm going to start over using your suggestions and will post things once I have the changes made. It's probably going to take at least a day. I do appreciate the input. I'll have to rearrange things a bit anyway because C9 and C18 have been changed to a film type and it's a really large package that I'll have to route around to make things work again so maybe this is a good thing.I'm also attaching the schematic as requested plus the original I'm working from. Warning, it's big. I'm only posting the top part because both are identical. The op amps are TLE2141's and the zeners aren't the exact parts so I just put in something to give an equivalent footprint. They are 10V 1W and 5.6V 500mW respectively.

author: free_electron
author_rank: Super Contributor
timestamp: March 14, 2014, 08:16:10 pm
content: why dont you use capacitor footprint like C14 instead of C4.caps like c4 are a pain in the ass. you cant route a trace between themsecondeagles resistor footprints are always too small. the pad pitch should be like on the diodes

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 08:16:51 pm
content: I assume your 28VAC is coming from a transformer from mains? I'd recommend making sure you have a couple X capacitors, Y capacitors, a common mode choke, a fuse, MOV, and optionally a TVS diode. This will reduce your input noise, protect the PSU from surges, and reduce your back-EMI.EDIT: I've attached a basic EMI filter schematic for you

author: Thor-Arne
author_rank: Unknown
timestamp: March 14, 2014, 08:19:32 pm
content: This looks like an early version ofthis.Read the forum there, it has a lot of issues resolved.Also, the v.3 schematic is simplified quite a lot since the original design was posted.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 08:46:39 pm
content: Quote from: free_electron on March 14, 2014, 08:16:10 pmThe small cap footprints are for the ones I have already but I suppose I could use a larger footprint as the leads are long enough. I had thought about changing the resistor footprints to be a little longer to help in routing.Quote from: Phaedrus on March 14, 2014, 08:16:51 pmI've got fuses on the transformers coming in, one for each transformer at 6A. I wouldn't even know where to begin with the choke, MOV and TVS. I have no idea what X and Y caps are.Quote from: Thor-Arne on March 14, 2014, 08:19:32 pmActually, that one is a modified version of what I'm working on. I've actually seen an argument between the guy that did this version and the guy that did the version you posted. Both authors differ on the direction to take but I'm sticking with mine for now based on that very argument. There are reports that it works fine for the folks that have built this version and I'm actually pretty far along in the project so changing too much is out of the question.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 08:50:27 pm
content: Buy or salvage a computer power supply, open it up, and rip out everything before the bridge rectifier, and put that in your circuit before your transformer. Basically. Also, you can use the bridge rectifier, since they're usually decent ones rated for >4A.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 09:00:42 pm
content: Quote from: Phaedrus on March 14, 2014, 08:50:27 pmI already havethesefor my rectifiers. Can you point me to a circuit that would have these things in it? I may have the stuff needed laying around already but I certainly don't have any old computer power supplies around to look at. I'm also wondering about what you mean by X and Y caps. I've never heard these terms before but that's probably my limited experience showing.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 09:08:46 pm
content: Look at the attachment. This is from a Corsair 430W power supply.X capacitors are common mode fail-safe filter capacitors that can be used for filtering EMI at mains voltage and above. Y capacitors are also fail-safe filter capacitors, but are meant to be used in differential mode (two in series with the connection between them grounded, as in the schematic on the previous page). The advantage vs. electrolytics is that they can't catch on fire, and won't fail shorted.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 14, 2014, 09:34:42 pm
content: Quote from: Phaedrus on March 14, 2014, 09:08:46 pmI have one of those X caps that I pulled off of the plug I salvaged for the case, XG-VS @ 0.47uF. I've seen them used across the mains plug but never knew what they were really for. The plug I salvaged had this cap and a small resistor accross the hot and neutral. I'll have a look at the board that came out of the case and see if I can find the other parts as well.

author: sync
author_rank: Frequent Contributor
timestamp: March 14, 2014, 09:54:44 pm
content: Quote from: Phaedrus on March 14, 2014, 08:16:51 pmThe schematic is wrong. Shorting the mains directly with the fuse is a bad idea.And keep in mind this is a simple linear power supply. No need for this amount of input filtering. Attached the AC input of the well known E3610A.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 14, 2014, 10:11:55 pm
content: Quote from: sync on March 14, 2014, 09:54:44 pmWhoops, sorry, didn't pay attention to the fuse.

author: Thor-Arne
author_rank: Unknown
timestamp: March 15, 2014, 12:08:09 am
content: Quote from: liquibyte on March 14, 2014, 08:46:39 pmOk, keep us posted on your findings.I looked on of the other designs, but it stalled due to too many revisions.Was planning on taking it up again some day.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 15, 2014, 03:24:22 pm
content: Quote from: sync on March 14, 2014, 09:54:44 pmThis is how I have things hooked up now except the switch is a SPST push button on the hot side before the fuse.Quote from: Thor-Arne on March 15, 2014, 12:08:09 amI've run into an issue where the ouput only goes to 10V or so but I'm sure it's my fault with a bad soldering job or dyslexic mistake. It's kind of why I wanted to have a board made. Perfboard can bite you if you aren't careful putting it together.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 15, 2014, 08:10:41 pm
content: OK, I attempted a redo on the board. I have new found respect for you guys that do this for a living, what PITA. I know this isn't going to be right by a long stretch but I'm trying. I've tried to keep all of the power traces as short as possible and have moved the positive side of things to the top and the ground portions to be completely on the bottom. I've changed the resistor footprints to be longer and changed some of the caps to the sizes of the one's I'll actually be using. I also changed the footprints of the zeners to match the size of the ones I have and I think they are just a generic model. I hope I'm moving in the right direction.

author: liquibyte
author_rank: Frequent Contributor
timestamp: March 17, 2014, 03:12:20 am
content: Started from scratch again trying to use big traces throughout the board. Fitting two o f these in a 10cm square is tough.