Advice about working with buck converters - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 25, 2020, 04:04:27 am
content: I've been working on a voltage supply that gives out a positive and negative voltage. Initially, I wanted to use linear regulators, but they became inefficient and needed big beefy heatsinks. I saw EEv's video on building your own PS, and I wanted to avoid the necessity of a big giant heatsink. I decided then to try using buck converters, and my draft of my circuit is below. It took me a lot of trials in simulation to come up with this, and from what I'm running, they seem to work pretty well for the most part (given limitations of SPICE models and just overall annoyance). However, I wanted to get some advice here. From people who have work with buck converters, do you guys see anything wrong with this circuit or if anything can be improved upon? I would like to try my best to avoid the magic smoke that seems to be prevalent.Also, I wanted to get some feedback on my layout. I seen quite a few posts of people not routing buck converters correctly, so I wanted to get some feedback on my first layout of this circuit. I tried to space the components close together, and I tried to keep the feedback loop as small as possible. The traces range from 0.5 mm to 1 mm, and there may be some courtyard overlapping. However, for the sake of the traces, I just showed the top and bottom layers. The board is a 2-layer board with the top layer being a signal layer (so, not assigned to any net), and the bottom is the ground plane. I tried to keep all traces on top so as to not disturb the ground plane.

author: Ribster
author_rank: Frequent Contributor
timestamp: May 25, 2020, 10:37:36 pm
content: Im not sure that negative switcher is going to work.Its a nice effort but you might need a buck-boost topology.It could be that this works, but id suggest researching some jellybean multiconfigurable switcher like the mc34063.That datasheet will tell you how to reconfigure it..

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 26, 2020, 05:32:08 pm
content: Thanks for your reply. I followed this guideline below which showcased how to do it with the LM2673, and based on my simulations, it seemed to work (after getting the model to work). That's why I'm hopeful for it to work. I saw that component, and it seems to be very versatile, but it doesn't provide enough current for what I need. That's why I went with the LM2673.http://www.ti.com/lit/an/snva022e/snva022e.pdf?ts=1590513777269If you don't mind me asking for an opinion, if I were to replace the positive regulator with a premade module like the one in the link below, do you think it will work properly?What I mean is that I build a PCB board with the negative voltage parts, and I replace the positive parts with this module, removing the screw terminals and using headers (and appropriate standoffs) to connect them to my PCB board. This would save me a lot of money given its price.https://www.digikey.com/product-detail/en/dfrobot/DFR0379/1738-1293-ND/7087190

author: Ribster
author_rank: Frequent Contributor
timestamp: May 27, 2020, 12:56:43 pm
content: Yeah the MC34063 is really a jelly bean part. Not very efficient. Dave has made some video's about this also.I think you have taken the good approach. What i do see on your schematics is that you have 22uF MLCC caps, and no 100nF caps.Always a discussion as to what caps and how, but i'd surely add one of those to the input. It's better for the high frequencies.As to the module: i'd say or go custom or go off the shelf.You will see that you will miss something with the off the shelf. Might be only form factor, but also can be electrical.I would stick with your design and make a prototype of it.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 27, 2020, 03:46:04 pm
content: Thanks for your reply. I see your point about the module. For something like what I'm designing, better to go custom rather than 'generic' off the wall for now. The part's cool, but if it introduces issues, then that's a waste whereas, with my design, I can better control it.If you don't mind me asking, what do you think of my first layout? I remodified my post to add images of my layout. I tried following TI's guide for layout by keeping the traces as short as possible, and I'm trying to fix some small issues here and there.http://www.ti.com/lit/an/snva054c/snva054c.pdf?ts=1590526307683

author: madires
author_rank: Super Contributor
timestamp: May 27, 2020, 04:33:16 pm
content: I'd recommend to use large areas for ground, not just traces. The inverting converter should work fine, but the inrush current is higher and the output current lower than for the non-inverting standard circuit, IIRC.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 27, 2020, 06:24:00 pm
content: Thanks for your tips. I'll keep that in mind. My bottom is a ground plane for the circuit. In my images, I didn't fill it in in order to show off the traces a bit better. I was worried about the feedback loops. I've seen posts where someone laid it out with the traces quite far, resulting in a wrong output voltage, so I was trying to avoid those issues.Thanks for your concern about the inverting converter. I read an app note where the output current is smaller, like you said, so I'm trying to rework the switch current tolerances for that. I just couldn't find a good inverting regulator that provided the current I needed, and I thought that this topology using the same parts will help. I'm thinking of switching to the LM2679 to compensate for the reduced output current limit, but what can I do to reduce the inrush current, like you said? Should I put thermisters at the inputs and outputs?

author: madires
author_rank: Super Contributor
timestamp: May 27, 2020, 06:31:04 pm
content: How much current does your circuit need on the negative rail?

author: LoveLaika
author_rank: Frequent Contributor
timestamp: May 27, 2020, 08:42:02 pm
content: I'm trying to keep it at a limit of 2.5 amps (worst case, 3 amps). Most likely, my load won't be drawing that much current and won't even reach 2 amps, but I guess I'm overengineering this for the worst case scenario. Going by the guide by TI below, I did a rough calculation and found that due to the inverting configuration, with 15V in and -10V out for 3 amps, the max current through the inductor reaches up to 5.8 amps. I'm reworking the circuit to use the LM2679 for its higher current output in light of this.http://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/196/Working-with-inverting-buck_2D00_boost-converters.pdf

author: Ribster
author_rank: Frequent Contributor
timestamp: June 03, 2020, 11:21:06 pm
content: Quote from: LoveLaika on May 27, 2020, 03:46:04 pmTo give feedback about your layout:I've done 100's of designs in altium, so i have my own approach of things. (I don't have anything against KiCad, i really love the idea and package all together)What i mostly do on my designs is make it more compact. Make the PCB just as big as it needs to be, and i use SMD parts where i can.That being said, what Madires said is correct. Place a polygon pour on the bottom layer as well as on the top layer.Via stich them together to get a solid ground return path.Concerning the width of the traces, if you are going to run 3A through that you should really need to consider the thickness of the traces.https://www.4pcb.com/trace-width-calculator.htmlSomething like this will help you out very nicely.Also, the PCB Saturn program on windows is very handy for such calculations and more.It's good that you kept your loop traces small. The ground is at least as important as is the positive rail. Both need to carry the same current.For the rest of the routing, think about loop stability. Power supplies are susceptible to oscillations. Having the correct decoupling is paramount here.Best to add correct room for the caps you need, while not placing them if you do not need them.For the rest i'd say: build it up and test it out! You will know how it will perform!Also: Put handy text on the silk screen layer for connections, this means + and - annotations etc. Also use the easy connectors that are easy for you. Mounting wires etc is very important.If you need to connect and disconnect something 10 times, make yourself easy and go with the phoenix connect style connectors. Eg. 1843619. Those pluggable connectors can be very handy.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: June 04, 2020, 03:23:41 am
content: Thank you for your reply. I must admit, by the time I got your response, I already submitted my board to fab. Luckily, I took the points that you noted in your response already, putting ground on top and bottom, via stitching (hope I didn't go overboard on that), and thinking about the thickness of my traces. For the output current traces, I went with 50 mils based on some rough calculations I found about traces and current. I had to move some stuff around a bit, but I did my best in keeping the essential components close together.Thanks again for replying. Well, here's hoping that version 1 works as expected. Funny, it seems that almost every time I submit a board, I realized that I didn't put enough silkscreen, or at least I should have added more text via silkscreen. That would've been helpful.

author: Ribster
author_rank: Frequent Contributor
timestamp: June 04, 2020, 09:18:25 am
content: It's good that you are looking at everything in that much detail.Post some pictures of the result. I'm curious how it pans out!

author: IamSynthetiC
author_rank: Regular Contributor
timestamp: June 10, 2020, 05:05:53 am
content: Be aware of the mechanical aspects of the board aswell. If your boards require it, you might need to use a heatsink, or have some airflow coming in from one specific side, and out the other. Position your modules accordingly.You will probably not need a heatsink, or any temprature control.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: June 10, 2020, 02:56:03 pm
content: Thanks for the reply. I don't think I'll be drawing too high currents, so hopefully, a heatsink is not needed. I'm soldering the tab itself to the PCB, so I hope that's adequate enough for my needs. Never really thought much about airflow though. I made sure to keep the modules far away from each other, around 35 mm or so, so I hope that's enough.Thinking about it, I hope that the sharing of grounds won't affect the output. I recently saw a video of how a Bluetooth speaker circuit had noise on the output because two power supplies shared the same ground. I hope that isn't the case here.