Buck converters sharing ? - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 30, 2020, 04:05:14 pm
content: This is something I've thought of trying, but don't know if it's possible. With buck converters, can they be put in parallel for increased overall current output ? Being there set to the same voltage output for each converter. And if it is possible, would I need to put each buck converters output through a bridge rectifier so they don't see each other from there individual outputs. Or perhaps a power resistor would be needed for each converter. I'm not sure about any of this, but would appreciate if you have any advice on this theory.Thanks for reading, any thoughts appreciated.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 30, 2020, 04:48:32 pm
content: As with any parallel power supply there are two ways to go. The simple but crude method is current sharing resistors. You will lose power in the resistors and current sharing will be uneven due to mismatch in the separate voltage references. The size of the resistor needed depends on the output voltage tolerances and how evenly you need the current split to be.The "correct" way is to use a single error amplifier for the output voltage that feeds back to both regulators to request approxately equal current output from both.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2020, 07:03:17 pm
content: Yup. In particular, if you use an array of constant-current channels, their clocks can be phase-shifted to reduce the summed ripple currents at the input and output, which lets you save on capacitance. The lower peak current per channel also makes layout easier, even if you don't take advantage of the phase shift.This is basically impossible for voltage-mode controllers, but easily done with current-mode controllers: yet another advantage to them.Tim

author: wraper
author_rank: Unknown
timestamp: December 30, 2020, 07:16:50 pm
content: You should not do this with separate PWM controllers. It's normally done with PWM controller which controls multiple phases. Here is examplehttps://static5.arrow.com/pdfs2/2020/10/3/2/51/49/287289/ons_/manual/ncp4200-d.pdf

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 30, 2020, 08:02:48 pm
content: Thanks for your replies, clocks and phase shift ? I'm not sure what you mean to be honest. These buck converters are low voltage, typically DC input not exceeding around 40 Volts. I would intend on putting about 28 Volts DC input to achieve 12 - 14 Volts output. The buck converters are the ones in the image below, I know these are switching devises to drop the voltage. But for clocks and phase shift I've no idea. If it's possible I would try to get four converter modules to share a single output. Would replacing the voltage potentiometers to a 1% Resistor divider in two resistors be worth considering. Just for a closer tolerance for output sharing.Thanks again for the help.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 30, 2020, 08:36:14 pm
content: Oh ok, frequency of the switching so there closer matched to each other. At least I think that's what clocks and phase shift is with converters. How do I go about this, I'm guessing I'd need to change some SMD component values or something like that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 30, 2020, 11:23:30 pm
content: Oh, modules. Not a design question then. Yeah, best you can do is just a. get the proper rated module to begin with, and failing that, b. wire them together with sharing resistors or whatever, and deal with the reduced regulation.Tim

author: Hiemal
author_rank: Regular Contributor
timestamp: December 31, 2020, 12:15:30 am
content: If those modules are like the one I have (looks identical) then they're using ahttps://www.ti.com/lit/ds/symlink/lm25116.pdf?ts=1609373560090&ref_url=https%253A%252F%252Fwww.google.com%252Fcontroller. Mine doesn't have the IC sanded.This controller does have a SYNC pin. I'm not sure if that's relevant or useful at all, so I'll defer that to T3sl4co1l.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 31, 2020, 12:30:26 am
content: Quote from: wraper on December 30, 2020, 07:16:50 pmWhy? If the power supply has stabilization on the secondary side, or it is not an isolated power supply, there is no problem.It is common practice to put 2 PWM or 2 LDO in parallel - this is often indicated by manufacturers in the datasheet.PWM does not require any decoupling by the diode (it is already in its rectifier) and does not require alignment by resistors. We get a dual capacitor that will be pumped by two friends.In any case, the phase will be shifted spontaneously because the generators cannot be synchronized both in phase and frequency during autonomous operation.

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 31, 2020, 12:51:25 am
content: No it wasn't a design question, i posted it here as i thought it to be a technical question. I wasn't sure if buck converters could be used in parallel or not. Looking round the web for info did throw up adjusting frequency of such converters, and mentioned phase shift. I'm a novice, so way beyond my capability i expect, and I've only got an old Haymeg 60MHz oscilloscope, not a storage scope. I wanted to see if 4 x buck converters would take a linear DC input, as share the power output between them. About 20 Amps to 25 Amps. Not continuous, about 50% duty cycle from the output. Some interesting reading though. Guess i'm going to have to taste it and see. i can monitor voltage current and temperature, but unfortunately not the voltage wave form or frequency.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 31, 2020, 01:18:37 am
content: Yeah. If you know what the controller/regulator is, and if it meets certain requirements, it may be possible to link them together in the ways described -- but if this information is unavailable, you'll have to settle for the simpler way out.Also, if they have trimmers for voltage setpoint: try to set them as close to equal as possible. Can save on ballast resistors that way.Tim

author: davelectronic
author_rank: Frequent Contributor
timestamp: December 31, 2020, 02:33:56 pm
content: Yes I will look up the devise the converter is using, try and interpret as much as I can first. Its not a mission critical application, just thought it's worth a try. Thanks all for your help. Happy new year.