dc-dc boost converter layout critique - Page 1
Forum section: electronics / projects

author: iampoor
author_rank: Frequent Contributor
timestamp: September 14, 2014, 11:28:10 am
content: HiI am trying my first dc-dc converter layout. Schematic is a reference design (minus he 220uf caps that I placed to experiment with)How does he layout look? What can I improve? I am a newbie in high speed layout...I only do audio circuits, so all feedback is highly appreciated.UPDATED LAYOUT in Post #4

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 14, 2014, 12:09:36 pm
content: If it's two sided, you might as well fill the back side with copper. Then you can add vias to top side ground in regular positions (and near high current pads) to make things even better.Examples: the trace from D1 to R1 has little ground around it; ground doesn't connect under C6, so current around D3 has to go all the way around those traces to get back to anything (other caps, IC).Also, there's no output pins...Tim

author: 0xfede
author_rank: Regular Contributor
timestamp: September 14, 2014, 01:35:50 pm
content: Go and get dirty, you'll never regret!But for sure C8 will cause it is reversed

author: Alex30
author_rank: Regular Contributor
timestamp: September 14, 2014, 01:46:40 pm
content: Sorry for the lame feedback, but your border on your board is crooked, make sure those lines are straight!

author: iampoor
author_rank: Frequent Contributor
timestamp: September 14, 2014, 07:04:41 pm
content: Quote from: T3sl4co1l on September 14, 2014, 12:09:36 pmHey Tim!Thanks! Yeah that makes complete sense. Thanks for the tip. I know his is a complex question, but with such a small layout at ~2mhz, is here a risk of ground loops using both sides as a ground plane? Obviously I dont expect a definitive answer, I just don wan to stress out thinking about it if its not really an issue. Of course, thats what makes engineering fun, but I want this thing to work first try, dammnit! :pI tried adding a via under C6, but I could see that creating problems with he solder mask so maybe Ill just leave it out....Yeah I haven put those on yet, this is part of a bigger design.Im planning on having this dc-dc module on its own ground plane, and then attaching it to the rest of the design through a .024 trace. Hopefully that will reduce any sor of ground loops or switching noise....Quote from: 0xfede on September 14, 2014, 01:35:50 pmWhat do you mean? I oriented C8 so it would be symmetrical/a straight trace.Quote from: Alex30 on September 14, 2014, 01:46:40 pmHey, when I work on designs at 4am, sometimes I need lame feedback to compensate for my lame mistakes.

author: 0xfede
author_rank: Regular Contributor
timestamp: September 14, 2014, 10:21:13 pm
content: QuoteI mean that you have to swap positive with negative of the electrolytic cap C8.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2014, 02:30:29 am
content: Quote from: iampoor on September 14, 2014, 07:04:41 pmNot so much the frequency as the frequency and impedance. If this were a battery powered application, it wouldn't matter (~mA switching currents). Out of 10uH, I'm guessing it's a little punchier than that (100s mA?). So all the inductance around the traces adds up.It's not that you're using it as a ground plane. Look at it, it's cut up to all hell! Think about the current flow through the negative space, it has to go along the edge, all the way around the traces and components.You do get local ground loops, which is usually a good thing, having a much lower impedance between circuit areas. The problem notoriously associated with ground loops (voltage drops) is improved because your ground is better.Also, a careful and comprehensive approach can deal with ground loop at high frequencies. If there's a difference in ground voltage where the voltage leaves the board (say, if you bring the voltages out with twisted pairs at C7 and C8, respectively), it's not something you can fix with additional CLC filtering (that doesn't filter ground). For that, you need a common mode choke. Which also helps deal with noise picked up on cables, and transmitted from other loads. (Whether that's a concern or problem, depends.)QuoteYeah, that's a short. Try moving C6 up so ground can go under, from D2 to the pour area on the other side.QuoteAh, so ground difference between left and right sides might be a concern. Don't use weak traces, use proper inductors. To keep noise down, bring everything out at one point, and filter at that point (if there's a single point where ground comes together, there's no ground loop pushing noise into your underlying circuit!).Regarding via placement: the ones by C7/C8 and 9V don't do anything -- the common pins already connect to both layers.You can put them just anywhere though, besides the important places.Tim

author: mrkev
author_rank: Regular Contributor
timestamp: September 16, 2014, 12:49:17 am
content: Well, I don't know.. I would flip R3 upside down and move D2 close to it (I don't see why place it on the right side and go back with that trace). You can move c6 more to the right then and forget about that via under C6 (you don't need it anyway - that via under D3 does the job).There's not much that you can do wrong here tho...

author: iampoor
author_rank: Frequent Contributor
timestamp: October 30, 2014, 11:24:31 pm
content: Well, I finally got my boards in and built one.....draws 400ma no load.Im not seeing any obvious flaws, but maybe Im looking way too hard! haha

author: mrkev
author_rank: Regular Contributor
timestamp: October 31, 2014, 12:18:04 am
content: Some kind of short is always a possibility. Do you have right voltage at the output?And I would check if the caps are at right polarity and with right voltage capability, especially electrolytic and tantalum (if you used any). They can work for a short time in opposite direction, drowing much more current than they should. F.e. you have the C8 the wrong way around in your original scheamatic...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 31, 2014, 01:03:07 am
content: Do you have a scope?

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 31, 2014, 01:22:30 am
content: You definitely have C8 reversed on that board (you have the positive side on the negative bus) and judging by the 400ma draw you've already shorted it.Don't power it up any more, that cap is gonna burn out.Put a new one in the other way around.

author: iampoor
author_rank: Frequent Contributor
timestamp: October 31, 2014, 02:01:32 am
content: Quote from: mrkev on October 31, 2014, 12:18:04 amNo, I have 5v +- at the output with 9v in.All the caps are bipolar, and I took the 2 electrolytic filter caps out. I checked all the components for shorts under the pads, and havent found any. I also re soldered everything that may have been suspicious.Quote from: T3sl4co1l on October 31, 2014, 01:03:07 amYes, I am just not sure where to begin! Feedback pin?Quote from: Cyberdragon on October 31, 2014, 01:22:30 amI fixed hat on he producion version and have tried firing it up without c7/c8.I originally had the diodes reversed, so Im wondering if I blew the chip. Only other culprit I can think of would be the inductor? Its just a no name chinesse 10uh

author: iampoor
author_rank: Frequent Contributor
timestamp: November 01, 2014, 02:40:33 am
content: Tried a new chip, same excessive current draw issue. Looking over he PC board I dont see any issues either. Ideas on where to begin with a scope?I can get more hen 2.5v in without the circuit drawing over 500ma.

author: iampoor
author_rank: Frequent Contributor
timestamp: November 01, 2014, 02:46:06 am
content: Here is the schematic.....Im using MLCC's if that matters!?

author: nctnico
author_rank: Super Contributor
timestamp: November 01, 2014, 01:09:42 pm
content: I'd simulate it using switchercad.

author: iampoor
author_rank: Frequent Contributor
timestamp: November 01, 2014, 07:42:57 pm
content: Quote from: nctnico on November 01, 2014, 01:09:42 pmIll try and see if I can replicate the problem. The circuit Im using is just a reference design.

author: iampoor
author_rank: Frequent Contributor
timestamp: November 14, 2014, 07:33:03 am
content: Well it doesnt appear to be the inductor, tried a few different types, same result. Ive taken my scope to the feedback pin on the chip, and got nothing!Interestingly, the power inputs have strange white noise superimposed on them whenever I fire it up!