I need a buck converter that can output 3 separate voltages - Page 1
Forum section: electronics / projects

author: J.Xerneas
author_rank: Contributor
timestamp: October 26, 2020, 09:42:36 pm
content: I'm currently working on converting a Smith Corona SD 250 typewriter from 120vac to an internal battery pack. The power coming in goes through a transformer that steps it down to 55vac then it goes into a switching supply that drops it to 3 separate voltages. 5vdc/ground 12vdc/ground 36vdc/ground. I want to run the device off of a set of 6 18650 lithium batteries in series parallel. I need a board that can handle balance charging and protection and a board that can output 3 regulated voltages at the above ratings. I know there is most likely something like this available that's plug and play but I just can't seem to find it. Would it be better just to use 3 different buck converters, 1 for each voltage? Is there a board that can do charging and output in one package? Thanks.

author: Dmeads
author_rank: Regular Contributor
timestamp: October 27, 2020, 04:53:05 am
content: A single buck converter can only supply 1 voltage level, so even if there was a board, it would have 3 buck converters on it. It might have a single IC controlling those converters, but still, 3 separate converters, just on one board.The ones I have seen are low voltage for CPUs and FPGAs and stuff because they have different core voltages and Logic levels (3.3V, 1.8V, 1.2V), which wont work for your project.There are plug and play dual output buck converters on Amazon that can output the voltages you're looking for. I have never seen one with charging integrated into it, you would probably have to make a board for that. (you can have someone make one for you, it wouldn't be cheap)Personally, I would get one of the dual channel ones off amazon, along with a singe channel one. You can find 18650 charging boards there too probably. Then connect it all up in a wire mess. Unfortunately, that is probably the easiest, fastest, and cheapest way to do the project.It sounds like a cool project, I hope it works out

author: sandalcandal
author_rank: Unknown
timestamp: October 27, 2020, 09:16:10 am
content: Like Dmeads said, a single buck converter outputs a single voltage. Three different voltages will require 3 different (or perhaps frankensteined together) buck converters. I note you said 6x18650 li ion cells which will have a nominal output voltage of 21.6V in a 6s1p config which means you'll need a boost converter rather than a buck converter for the 36V rail.Seeing as it already has an in built supply that drops 55Vac to all your required voltages I suggest you just make use of that. Get a single boost converter that gets your battery voltage to what ever the rectified filtered input voltage is then hook it in behind the rectifier (and PFC, but I doubt it has PFC). If you can afford the weight and space an even simpler solution could be to use a 14s (50.4V nominal) battery pack then you might be able to get away with no converters and just a BMS.Sounds like a neat project, good luck!

author: David Hess
author_rank: Super Contributor
timestamp: October 29, 2020, 01:52:32 am
content: Quote from: Dmeads on October 27, 2020, 04:53:05 amSince the voltage across the buck inductor is equal to the output voltage plus voltage drop of the output diode, auxiliary windings can be used to produce other voltages as long as the main output has sufficient load.https://www.edn.com/how-to-reconfigure-a-buck-converter-for-multiple-outputs/Flyback switching regulators are usually more suitable to more outputs but either could work for this application. However in both cases, acquiring a suitable multiple output inductor is likely more difficult than using three separate regulators unless you are used to winding your own inductors.

author: sandalcandal
author_rank: Unknown
timestamp: October 29, 2020, 05:53:36 am
content: Quote from: David Hess on October 29, 2020, 01:52:32 amSeems like getting good regulation of multiple outputs would be a nightmare

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 29, 2020, 04:36:47 pm
content: I like the idea of sandacandal.Just put enough batteries in series to get to 55V and leave the rest original.Battery chargers and management for those kind of voltages are common for "bicycles", electric skateboards and similar vehicles.

author: Benta
author_rank: Super Contributor
timestamp: October 29, 2020, 04:55:10 pm
content: Sounds to me like this is a job for a flyback converter. You can step up and step down as you like, and multiple secondaries for the different voltages is easy.

author: David Hess
author_rank: Super Contributor
timestamp: October 30, 2020, 01:32:28 am
content: Quote from: sandalcandal on October 29, 2020, 05:53:36 amThe cross regulation is not as good but sufficient for many applications.

author: ConKbot
author_rank: Super Contributor
timestamp: October 30, 2020, 02:08:59 pm
content: Maxim is doing some funky stuff with their Single Inductor Multiple Output buck regulators eg MAX17270/MAX17271. H-bridge style buck/boost converter with multiple output transistors so the control logic can multiplex which output gets the pulse from the inductor. Not relevant for OP given their high voltage and power requirements, but the concept is interesting. Especially for systems where a couple ICs may end up wanting an annoying amount of rails.(1v core, 1.2v xyz peripheral, 1.8v IO bank, 3.3v IO bank, etc.)

author: sandalcandal
author_rank: Unknown
timestamp: October 30, 2020, 02:59:14 pm
content: Quote from: ConKbot on October 30, 2020, 02:08:59 pmhttps://www.maximintegrated.com/en/products/power/switching-regulators/MAX17270.htmlThat's incredibly funky