Hobby project - Pixel LED PSU module: Parallel connection of buck converters - Page 1
Forum section: electronics / beginners

author: Skwide
author_rank: Contributor
timestamp: June 28, 2023, 01:40:12 pm
content: Hi, This is a hobby project, not commercial. I'm a keen hobbyist with a science but not electronic engineering degree, so beginner level really for this project.Project: To supply localised 5V for pixel LED strings from a 30V distribution power line to avoid voltage drop issues with 30m of power line draped around a garden.Are there any gotchas to parallel connecting the outputs of a buck converter like this one: ?The most significant problem is overloading one module, due to output voltage variances. But this device has inbuilt current limiting, so in theory the one unit with the higher output would tend to supply up to 5A first.If the load goes up, I anticipate that one throttling and the next module starting to supply current - and so on.- Good or terrible hypothesis?The next problem I see would involve ripple from one module feeding back and disrupting another modules sense circuits as the clocks are not synchronised (in fact this chip uses spread spectrum clock jitter to reduce EMI)- Mitigate with output diodes and/or extra low pass filters to prevent feedback from one module to another?https://www.diodes.com/assets/Datasheets/AP64501.pdfThere is another way - I could common the grounds only and have dedicated string feeds (probably 2 x 50pixel strings per module would be reasonable at 5A)Chose the 5A chip as it's cheap, low external component count and heat issues seem manageable (excellent PCB layout help in the datasheet) and it's as easy for me to make a pile of these than to sweat out working with a higher power design.Many thanks for your comments, even if it's "that's silly, don't..."

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 28, 2023, 01:59:06 pm
content: Quote from: Skwide on June 28, 2023, 01:40:12 pmThat won't happen. Current sharing will only happen if regulator's response to excessive current is to reduce voltage. Most DCDC converters, including this one, do OCP by completely disabling the switching. This will cause the entire load to shift into the other converter, instantly tripping them too.You might achieve sufficient current sharing using droop method through liberal use of ballast resistors, but that might cause other problem too depending on how tight you want your regulation.Quote from: Skwide on June 28, 2023, 01:40:12 pmInterference might actually not that problematic here (you already got much bigger problem, but still..)This converter is peak current mode, most of the feedback is already done internally by sensing switching current.Quote from: Skwide on June 28, 2023, 01:40:12 pmAdding diodes just ensure that you're OR-ing your two converter, ensuring exactly only one is actually supplying the load.This is still safer than straight parallel connection however. Since the diode protects converter that may not react well from being back-driven by the other converter.In any case, you still aren't getting the increased current capacity here.Quote from: Skwide on June 28, 2023, 01:40:12 pmThis sounds infinitely more reasonable. Same effort, much less hassle.

author: Skwide
author_rank: Contributor
timestamp: June 28, 2023, 04:03:25 pm
content: Thank you very much for your thoughts.I had not reckoned on the over current protection tripping out rather than limiting (I assumed it went into a constant current drive mode)Yes - all things said and done, apart from the extra plugs on the control box, separate feeds to each pair of strings is probably the cleanest way to make the system extensible to x strings.4 things in favour of this:2 x 50 Pixel strings is as much as you can drive at 5V without strapping power cables across the strings, in my experience (I tend to drive them to the max), so this is neater with no power cables strapping the strings everywhere.And it should still be possible with a common ground to loop the data line through all strings - BUT using WLED with an ESP32 which I do, I can drive 10 data outputs independently anyway - so for a 12 string setup for a tree, I could just have 6 sockets and 7 (extra one for the ESP32) 5V modules in the control box, which is fairly neat.I'd rather mass produce a small bunch of 5V @ 5A modules I can use all over the place than have lots of custom stuff. I need a small starter project like this to try SMD hot plate reflow for the first timeAs the PCB floods are used for heatsinking, 5A boards are probably a good upper limit and scale up with more boards scales up the heat dissipation too1 thing against - more waterproof plugs, but the ones I like are a few pounds a pair, so not terrible.