DC-DC Converter Stability and performance help  - Page 1
Forum section: electronics / projects

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 24, 2019, 05:51:04 pm
content: Hey guys,So after some more experimentation, I finally got my LTSpice simulation to work out perfectly. The schematic and sim results will be linked below. I plan on making a 4 channel bench supply using the exact schematic that I made in LTSpice. I decided to first make a perf-board prototype of the DC-DC regulator, just to make sure the design would work in the real world and I ran into some weird problems.The switching frequency is only 85Hzwhen it should be around 600Khz. With no load attached the LTC3864 reaches around 65C and I'm able to adjust the voltage using a potentiometer but the output is not stable at all and there is a lot of voltage ripple. The second I attach a load, no matter how small, the voltage drops significantly, the IC gets to around 80C and the frequency drops to 80Hz. I already double checked all of my connections based on the schematic and I'm really not sure what is wrong. Also once a slightly bigger load of over 100mA is connected the output drops to 0v and the LTC3864 almost instantly goes to 110C.Any suggestions would be greatly appreciated!Thanks

author: mvs
author_rank: Frequent Contributor
timestamp: February 25, 2019, 12:10:56 am
content: Quote from: FotatoPotato on February 24, 2019, 05:51:04 pmIt is not switching frequency what you see on scope, but parasitic oscillation.Put C13 from DC2132A eval board schematic back in circuit. It is important component, that closes local feedback loop at AC.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 25, 2019, 01:37:32 am
content: Will that have the same effect, even if I'm not using the same controller?

author: mvs
author_rank: Frequent Contributor
timestamp: February 25, 2019, 01:54:44 pm
content: Quote from: FotatoPotato on February 25, 2019, 01:37:32 amYes, but you may need to adjust its capacity.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 25, 2019, 05:48:32 pm
content: So I was working on trying to fix the regulator last night and I think there was more than just parasitic oscillation. The whole board had problems and even after adding decoupling caps all over the board It still didnt work. I re-built it on a breadboard but this time kept all the component leads short and made the connections as short as physically possible. This seemed to fix most of the issues but there are still some confusing things.First is that the switching frequency is still super low (300 - 600Hz) but as the output load increases the frequency goes up with it, so with a 250mA load the frequency will go up to arround 3kHz but that’s still much lower than the 700ish KHz that I want. I’m assuming that this is part of the burst mode operation but I don’t really know.The other problem is that the with any loads over 100mA the LTC3864 gets super hot, the output dies and it wont re-enable switching unless I turn off the input power and turn it back on. It seems as though the chip is thermal throttling at high loads because when I cool it down with freeze spray it works perfectly fine, even with large 500mA loads. The thermal throttling doesn’t make much sense to me though becuase the IC isn’t handling any of the power so it shouldn’t be getting so hot. I tried adding a 50 ohm resistor to the vin pin on the LTC3864 to limit its current and that seemed to do the trick but its still weird to me.At this point I’m going to make a test PCB so that I can eliminate the parasitic resistance, capacitance and inductance of the breadboard and to add some proper heatsinking to the LT chip. Hopefully that will fix most of the problems but if you have any suggestions I’d be happy to hear them!Thanks

author: Siwastaja
author_rank: Super Contributor
timestamp: February 25, 2019, 05:52:17 pm
content: A photo of your prototype layout would be nice.Breadboard sounds like a bad idea. Usually these are built on bare copper clad for a massive ground plane; even using some multilayer tricks like taping another layer of copper foil where you need another net over the GND.

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 25, 2019, 06:49:43 pm
content: I’m at school now, but I’ll send some pictures of the breadboard layout and of the oscilloscope readings in about 1 hour.QuoteThat is why I want to make a test PCB, so that I can do a proper layout as specified by the datasheet. That will also allow me to add some big and proper ground planes. Once I’m done with the layout (probably today) I’ll send a pic so you can pitch in if you see anything wrong with it.Thanks

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 27, 2019, 12:16:39 am
content: Sorry about the extra wait, I had a lot of schoolwork and forgot to take the pictures, anyway here is a picture of the breadboard prototype, the gate pin of the MOSFET and how it behaves under load. Obviously, the breadboard prototype won't perform as well as a PCB and I'm not expecting it to, the output frequency just confuses me and also there is about 3v of ripple on the output so it will go between 26v and 29v constantly. It is only when there is a load on the output that the voltage settles to around 27v as you can see in the pictures.There are more pictures, but the file sizes are too large, here is a link to download the other pics, sorryhttps://www.mediafire.com/folder/aa30f1dtsxcln/

author: Siwastaja
author_rank: Super Contributor
timestamp: February 27, 2019, 07:15:10 am
content: Good that I asked for photos. I don't think anyone could have guessed you talked aboutsolderless breadboard. This has absolutely zero chances of working.Redo it by soldering. Just take a piece of copper clad and build it on the top in whatever ad-hoc construction style.

author: nick_d
author_rank: Regular Contributor
timestamp: February 27, 2019, 11:19:23 pm
content: There are some good suggestions and plenty to look into, so I am not sure if this advice is particularly valuable, but weird chip behaviour (wrong voltages on pins, wrong frequency etc) can indicate fake chips, so if you bought it off Ebay or Ali then order a Digikey/Mouser one for development and try the Ebay/Ali later.cheers, Nick

author: FotatoPotato
author_rank: Regular Contributor
timestamp: February 28, 2019, 12:57:28 am
content: I have dealt with DOA and fake chips in the past but that was a long time ago, now I only buy from Digikey or Mouser. Nowhere else

author: dfault
author_rank: Newbie
timestamp: February 28, 2019, 07:58:49 am
content: Some general suggestions and thoughts:* Label your schematic with your intentions. Should it be a 5V rail? What's the current limited to? How did you calculate a value?* Consider prototyping a sub circuit before the whole thing. It seems like you may have done that* Things tend to get hot when they oscillate. Also, it seems like you might be over driving the FB on on your schematic.* you can solder less breadboard most converters with care. Pay extra attention to the current sense lines going to the regulator. These are very noise sensitive.* What is the 3632 doing? What is the 3092 doing? Limiting current to the set pins?* the diode on the 3081 is odd. As is the r13, r23, and r24 network. What is the goal?* the lt 308x regulators don't always like capacitance on their set pins. Check the datasheet.