Troubleshooting DC-DC Converter Design - Page 1
Forum section: electronics / beginners

author: QWERTYQWQ
author_rank: Contributor
timestamp: December 14, 2024, 12:32:05 am
content: Hello,I'm reaching out for help with a DC-DC converter issue. This is my first DC-DC converter project, and I must admit that I'm not fully knowledgeable about technical details. In fact, I’ve always disliked working with DC-DC converters, but this time I need one due to the efficiency requirements of my board.Below, I’ve shared the circuit schematic of the part in question, which I soldered onto a prototyping board today. Before connecting all the traces and components, I tested them thoroughly, so I am confident that the traces and components are correctly connected. I assembled and soldered the circuit twice today using original and brand-new components each time, but I still couldn’t achieve the desired result. That’s why I’m reaching out to you.The issue I’m facing is as follows: when the circuit starts working, the output voltage never stabilizes at 3.3V as intended but instead reaches a fixed voltage of up to 5V. For example, in my latest attempt on a fresh board, the results were as follows:Vin = 12V, Vout = 3.8V (no load)When I increased the input voltage to 15V using an adjustable power supply, I observed an output voltage of around 5.2V, which increases proportionally with the input voltage. However, this output voltage slowly decreases over time to around 4.4V.Additionally, if I leave the circuit running for an extended period (approximately 5-10 minutes), while input voltage remains at 12V and the output voltage appears stable at 3.8V, suddenly the output voltage spikes and equals the input voltage. At this point, I measure around 0.4V on the FB (feedback) pin.Another strange behavior is observed when I connect the FB pin directly to GND. In this case, the output voltage matches the input voltage, even though the circuit should detect a short circuit condition and stop increasing the output voltage.I’ve tested this issue with three different ICs, and the problem persists with all of them. As I mentioned earlier, I lack expertise in DC-DC converters and am more skilled in software than hardware. That’s why this situation seems foreign to me, and I’ve tried to describe my problem in as much detail as possible.Thank you in advance for your response and support. Best regards,

author: sharow
author_rank: Contributor
timestamp: December 14, 2024, 01:24:47 am
content: Hello.Quote from: QWERTYQWQ on December 14, 2024, 12:32:05 amMost DC-DC converters dislike operating with no load.Try connecting a load of about 47 to 100 ohms.

author: QWERTYQWQ
author_rank: Contributor
timestamp: December 14, 2024, 10:20:26 am
content: Hello, thank you. I have already tested the circuit with various loads ranging from 10 ohms to 330 ohms, but unfortunately, nothing changes.

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 14, 2024, 11:32:32 am
content: Hi!Can we have a photo of your prototyping board?

author: mariush
author_rank: Super Contributor
timestamp: December 14, 2024, 12:10:59 pm
content: Your schematic is confusing ... usually you start with input on the left and output on the rightIt's shown nicely in the datasheet of the chip :https://www.aosmd.com/res/datasheets/AOZ1282CI.pdfThe chip runs at 450kHz ... doing it on a breadboard is kind of a bad idea. The inductor should be as close as possible to the SW pin. Feedback resistors (the 50k and 16k should be as close as possible to the FB pin, and keep the wire/trace going to the 50k resistor away from the inductor.Use a CERAMIC capacitor on the output, at least 10-22 uF (should be rated for at least 2-3x the output voltage or at least 16v, I wouldn't use a voltage rating less than 16v even if the output is just 3.3v), then add a polymer (solid) or electrolytic capacitor in parallel if you want more.A plain 22uF electrolytic capacitor will have high-ish ESR, which the switching regulator won't like.Breadboards have added capacitances and resistances, normally you'd do it on an actual board.Try lowering the feedback resistors , like go down 4.7k-10k for the FB-GND resistor ... formula is in datasheet :Vout = 0.8 x ( 1 + R1/R2) Vout = 0.8 x (R2 + R1 ) / R2 or R2 x Vout = 0.8 x (R1 + R2) => R1 = (R2 x Vout / 0.8 ) - R2The 49.9 and 15.8 sets the output voltage to 0.8 x ( 1+ 49.9/15.= 3.326vSo let's say you go with 4.7k for R2 .... at 3.3v you have : R1 = 4.7 x 3.3 / 0.8 - 4.7 = 14.68Use 15k and 4.7k to get 3.35v and it will be good enough, close enough to 3.3v.

author: QWERTYQWQ
author_rank: Contributor
timestamp: December 14, 2024, 12:38:44 pm
content: Quote from: mariush on December 14, 2024, 12:10:59 pmThank you. I will follow your advice and give it a try. I will share the results with you.QuoteOf course, I will also include updates. After implementing Marius's suggestions, I plan to share some visuals as well.

author: QWERTYQWQ
author_rank: Contributor
timestamp: December 14, 2024, 01:30:11 pm
content: Hello, I finally managed to get the circuit working. The output voltage remains stable at 3.3V and works under load (10Ω). However, after running for an extended period, the output voltage shuts off on its own, and it only returns to 3.3V when I press the FB pin with my finger. When the output shuts off, I measure 1.6V at the output and the FB pin voltage stays fixed at 0.4V.I believe, as Marius mentioned, this is likely a layout issue. Additionally, I replaced the FB resistors as Marius suggested, but I chose different values. Specifically, instead of 49.9k and 15.8k, I used 31.2k and 10k. The old resistors were 0603 package, but I replaced them with 1206 package resistors. Since I didn’t have a 31.2k resistor in my inventory, I substituted it with a series combination of 20k + 10k + 1k + 200Ω. This way, I achieved the 3.3V output voltage.Additionally, I placed a 37μF ceramic capacitor at the output as well. Below, I’m sharing photos of the circuit and oscilloscope readings.Please review them. What can I do to further improve the system?Circuit Photo:https://ibb.co/LtphVRQhttps://ibb.co/GQ8k6YdWith no load:https://ibb.co/GvH5G9vWith load:https://ibb.co/qsD57VVhttps://ibb.co/wz7t39b

author: mariush
author_rank: Super Contributor
timestamp: December 14, 2024, 02:24:21 pm
content: Yeah, it's an improvement, but there's room for a lot of improvement.To give you an idea about how close the other parts are supposed to be, have a look at page 15 in this datasheet:https://www.diodes.com/assets/Datasheets/AP63200-AP63201-AP63203-AP63205.pdfYours is just a bit more complicated due to the extra diode that needs to go to ground, but other than that, switching frequencies are similar (AP63200 runs at 500 kHz)Note how close the inductor is to the switch pin, and how the ground fill goes under the chip and connects the input and output ceramic capacitors together at the ground point.

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 14, 2024, 04:58:55 pm
content: Try to connect a small capacitor between Pin 3 (FB) and Pin 2 (GND) as on the picture.

author: QWERTYQWQ
author_rank: Contributor
timestamp: December 14, 2024, 07:06:23 pm
content: Quote from: Harry_22 on December 14, 2024, 04:58:55 pmHello, thank you for the drawing. Earlier, I tried placing 1nF and 20pF capacitors sequentially between pins 3 and 2. However, this capacitor placement caused a reduction in the output voltage. For example, with the 1nF capacitor, the output voltage dropped to 1.6V, and with the 20pF capacitor, it dropped to 2.56V. I was unable to increase the output voltage under any circumstances. During this time, the FB pin voltage was around 0.56V. As a last resort, I removed the capacitor between pins 2 and 3.My main question is this: I plan to power an STM32 with this buck converter. As you know, the MCU tolerance is up to 3.6V, which is why I have significant concerns. I've seen how easily such a buck converter can cause issues. Similarly, the importance of its placement has made me a bit apprehensive, as I want this circuit board to work reliably for a long time and far from my reach.For this reason, could you recommend a more reliable DC-DC buck converter circuit for beginners like me? It will be used to power a 3.3V STM32 and some EEPROM chips. The circuit diagram and MCU code are actually ready, but this DC-DC converter issue is holding me back.As I mentioned before, I usually use linear regulators in my systems, but this time efficiency is a very important criterion for me. Please help.

author: rteodor
author_rank: Frequent Contributor
timestamp: December 14, 2024, 09:04:08 pm
content: How does the noise on the input side looks like on the scope ? I have a feeling that you do not have proper decoupling as close as possible to the chip. Where is C34?.Quote from: QWERTYQWQ on December 14, 2024, 07:06:23 pmYou could try an older and slower chip like MC34063 as target practice or you could iterate a few times on the current switcher. You seem to be able to make it work eventually.

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 15, 2024, 01:54:28 am
content: Quote from: QWERTYQWQ on December 14, 2024, 07:06:23 pmUnfortunately, I doubt that you did this, as I do not see any traces of soldering at these places.Quote from: rteodor on December 14, 2024, 09:04:08 pmThis is probably the most useful advice for you.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 15, 2024, 03:17:40 am
content: Quote from: QWERTYQWQ on December 14, 2024, 07:06:23 pmIf time efficiency is your concern, why not purchase a ready made buck regulator module from eBay/Amazon?https://www.ebay.com.au/itm/126435521799?mkcid=16&mkevt=1&mkrid=705-154756-20017-0&ssspo=OZ9adRIZTXO&sssrc=2047675&ssuid=&widget_ver=artemis&media=COPY

author: QWERTYQWQ
author_rank: Contributor
timestamp: December 16, 2024, 09:55:39 am
content: Hello,Thank you for your responses. I’d like to try my luck and continue with the current chip. I’ve designed a board and would like to hear your thoughts on the placement of the mentioned DC-DC converter before sending it for production.To give some details about the board, it consists of 4 layers:The first layer is the signal layer.The second layer is the +3.3V and Vcc (12V) layer.The third layer is the GND layer.The fourth layer is another signal layer.I’ve aimed to include as many vias as possible.Let me know your thoughts!

author: Harry_22
author_rank: Frequent Contributor
timestamp: December 16, 2024, 06:21:05 pm
content: This is a completely different matter!This should work.

author: QWERTYQWQ
author_rank: Contributor
timestamp: January 08, 2025, 07:13:04 am
content: Hello everyone,Yesterday, I finally received the circuit boards and had the opportunity to test them in the evening. I left the circuit exactly as shown in the .png images I shared, and it actually works!The input voltage range is between 5V and 30V DC, and I was able to achieve a stable 3.35V output voltage even under load. In the case of a short circuit, it draws a low current of around 20mA. Under load, the output voltage remains almost unchanged.If anyone is looking for a 3.3V DC-DC converter circuit, you can achieve a stable output by using the chip with the component layout I shared in the images.Thank you all for your support!