Non-Isolated AC buck converter for enclosed projects - Page 1
Forum section: electronics / beginners

author: BlvckBytes
author_rank: Newbie
timestamp: August 10, 2021, 08:37:02 pm
content: Hey there, this is my first post on this forum, happy to be a part of it!I've put this under the beginners subsection, since I only have very limited knowledge of buck converters yet.What I am trying to accomplish is to convert 240V 50Hz AC to 5V 200-300mA DC, non-isolated, on a space as small as possible. Just to get this out of the way: This project will be enclosed in a plastic housing, with no exposed conductors. Whenever I'm tinkering, I'm always using a variac connected to an isolation-transformer to be able to stay safe and have control over the voltage levels.This will be used for projects that need WiFi-Connectivity, where the ESP-07 I'm using seems to consume 200mA@3.3V at peak times. My first try was a capacitive dropper, but it just really has a hard time providing that much current:X_C = U/I = (240 - 3.3)/200*10^(-3) = 1183.5 OhmC = 1/2*pi*f*X_C, f = 50 Hz, C = 2.69 uFI have a 2.2uF MKP laying around, which is already huge, don't want to imagine what the closest to 2.7 uF would be.Next idea was a buck-converter, which is where I'm stuck now. 240V AC RMS is round about 340V Peak2Peak, which is what I'll end up with rectified (bit less with load). I bought a IRFR9220PBF Mosfet (200V, good enough in my book, lol), desoldered an inductor and some capacitors from dead boards I had laying around and got started, with only about 10 VAC to experiment.Learned about how Mosfets use electric fields to create the channel-pathways, and that high frequency on/off will need more current, since I = dQ/dt. I have seen this push-pull transistor circuit, which I extended to be able to control the source-voltage with only the 5V from my arduino. Then, I added a voltage divider for feedback, scrapped some code off a website, altered it a bit, and in the end threw it all together. And yes, it's a steaming pile of you know what, but hey - it worked for up to around 40-50VAC pretty accurately! Higher than this and the pulses get so incredibly short that my puny setup can't handle it anymore and the output voltage rises upwards out of the desired bounds. See the attached schematic, I left out the code and the exact values for now, since it does work, but just not within my requirements and this seems to be more of a general problem with what I'm trying to do here.I guess this is why a transformer would make it easier, since you could use the turn ratio to further step it down? Ugh, this seems to be impossible...I've seen some Chinese IC's which accomplish this, but none of them took 240VAC, still on the lookout for that.Any ideas on how I can accomplish this? I just need to power my ESP, and maybe a few dozens of milliamps headroom for any control logic surrounding it, I have very limited space (no big transformers) and I don't need isolation. All input is appreciated, since I'm still very naive on this topic (as you can already guess by my post). Thanks a lot!

author: langwadt
author_rank: Super Contributor
timestamp: August 10, 2021, 09:12:54 pm
content: probably be hard to make it smaller or cheaper than someting like this:https://www.banggood.com/da/YS-3SWA-AC-86-264V-to-DC3_3V-5V-6V-9V-12V-15V-24V-Switching-Power-Supply-Module-AC-to-DC-Converter-p-1758635.html

author: BlvckBytes
author_rank: Newbie
timestamp: August 10, 2021, 09:24:53 pm
content: You're probably right, yeah... I've just disassembled a very small USB-Charger brick, soldered some pinheaders to the in- and outputs, and now I'm thinking about mounting it on the top side of my PCB, so I still have a lot of space underneath on the other side to route my smd components. This is not the ideal way I was thinking about it, but I think that this project is just a bit too hard for me right now.Thank you for this idea!

author: BlvckBytes
author_rank: Newbie
timestamp: August 11, 2021, 06:34:02 pm
content: I think I've just found a pretty amazing little IC for this:https://www.aliexpress.com/item/1005001965888869.html?spm=a2g0s.9042311.0.0.5d8c4c4daslZ2KIt supports 500mA and 3.3V + 5V voltage selection, pretty cool eh! I ordered the reel of 100, since that seems to be a pretty good price and I'm going to be needing it in a lot of projects down the road anyways. I've attached the datasheet of this IC, since it's only available in Chinese on some weird sites, and I ran it through an online document translator.Going to post again when I received and used it, to make sure it really works as advertised.