Power supplies in parallel - Page 1
Forum section: electronics / general

author: bootloader9800
author_rank: Contributor
timestamp: February 19, 2021, 04:02:44 pm
content: Hello All, I have a power supply similar to the one below. It is rated for 30 V, 10 Amps.Can I blindly put 2 of these in parallel to get 20 Amps or is it a big NO - NO to do such a thing with such power supplies?Thanx for the replies.

author: penfold
author_rank: Frequent Contributor
timestamp: February 19, 2021, 04:33:30 pm
content: In general its a no, some power supplies *are* designed to do such a thing, but (in particular) some cheaper models have very sharp transitions between CV/CI modes and you end up with the two supplies oscillating and shifting the current back and forth.You can add external series diodes if they don't have reverse current protection and adding a little resistance (enough to give approx 0.5V drop at max current) can help soften the CV/CI transition. But that obviously degrades the voltage regulation at the load quite heavily.Maybe include the make/model in the subject to see if anyone has experience of those supplies directly.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2021, 05:25:25 pm
content: By definition, you can definitely parallel CC/CV lab supplies, no problem.If the supplies don't work doing that, it can only be because they are completely broken-by-design, and unsuitable for any work requiring CC/CV supply anyway.I say this because cheap supplies in that form factor are notorious for being utter disasters. Not all, but many. Some, for example, will just blow up if voltage is connected to the output of the supply. This can happen if charging a battery, or connecting multiple supplies in parallel, or just having a device with enough capacitance connected and supply turned off. They are all normal use cases for a CC/CV lab supply.Try it. There's nothing to lose. If they, for example, blow up, they would have blown up anyway. You can only hope they don't kill whatever circuits are connected by delivering higher voltage than configured.I also suggest you fix the serious and confusing mislabeling of that unit by scraping off the "GND" text, replace with "PE".

author: drussell
author_rank: Super Contributor
timestamp: February 19, 2021, 05:57:22 pm
content: Quote from: penfold on February 19, 2021, 04:33:30 pmOrdinarily you can easily connect multiple supplies in parallel, there will always be some balancing resistance due to the connection wires and in the case of poorly designed supplies you could always intentionally add a tad bit more, but only the worst possible defective designs should not be able to be paralleled...Do you have any examples where you've had issues with cheap supplies horribly misbehaving or something?Do tell!

author: Siwastaja
author_rank: Super Contributor
timestamp: February 19, 2021, 06:13:04 pm
content: A CC/CV supply does not need any "balancing resistance".For CV supplies, wire resistance is seldom enough to help share current unless they are stupidly accurate.

author: penfold
author_rank: Frequent Contributor
timestamp: February 19, 2021, 06:23:46 pm
content: Quote from: drussell on February 19, 2021, 05:57:22 pmThe ones I'm remembering weren't even particularly cheap ones, Thandar (I think). As far as I ever could tell, they worked perfectly for any normal tasks, but when paralleled, the second that one hit current limit, they would oscillate back and forth. I had this happen with a various other brands also, impossible to predict from datasheet or normal operational behaviour... having discussed this with a few people over the years they all had similar experiences... so I'm pretty gobsmacked you both have not!I absolutely agree it is theoretically something that should be an entirely normal use case, but, from experience I have to disagree, some just don't like it.

author: bootloader9800
author_rank: Contributor
timestamp: February 19, 2021, 07:16:36 pm
content: Quote from: drussell on February 19, 2021, 05:57:22 pmHi, Thanks for the reply. No, I have no experience doing this. I wasn't sure if it ws going to be dangerous in any way. That is why, I first asked. Ty

author: ledtester
author_rank: Super Contributor
timestamp: February 19, 2021, 08:22:01 pm
content: Do you need 30V at 20A?A buck regulator can give you more amps at a lower voltage from one supply.

author: ocset
author_rank: Super Contributor
timestamp: February 20, 2021, 06:09:06 pm
content: It is possible that paralleled power supplies with current clamp limits can end up with each supply's output current going up and down...at the point where one meets its current clamp limit. However, this would not tend to show up as serious vout ripple, so you should be OK with it.The vout and the current clamp are controlled by opamp error amplifiers, so its not as bad as if comparators were snapping on and off. So in fact, its quite a smooth "oscillating" process, if it occurs at all.Some Vicor modules deliberately reduce vout by about 5% over 0-100% load, and as such, they naturally share with each other well. And youre ok as long as you can tolerate this built in load regulation.

author: ram0001
author_rank: Newbie
timestamp: February 22, 2021, 07:40:34 pm
content: Hello Dear People,I recommend that you ask to the manufacturer, you will be sure and the power supply will be safe.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2021, 11:00:58 am
content: You can't find the manufacturer of such low-cost supply or contact them.They are produced in millions by some no-name Chinese low-quality manufacturers, copying each other, then sold in the West after many, many middlemen for surprisingly high cost for what they are. None of the middlemen have any information of the product. Best case, someone in the chain has verified there is no immediate shock hazard, i.e., protective earth properly connected, transformer hi-pot tested.

author: radiolistener
author_rank: Super Contributor
timestamp: February 24, 2021, 01:56:02 pm
content: If you will use diodes to avoid power consumption from one PSU by another, it should work ok. But note, that diode have some voltage drop.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 24, 2021, 02:57:18 pm
content: No, diodes do not help anything with current sharing problems. If anything, they make things worse by having negative Vf temperature coeff: the one supplying more current heats up its diode, which in turn will increase its output voltage due to lowered Vf, supplying even more.Diodes could save a supply that self-destructs when voltage higher than its own output (yet below the maximum rating, obviously) is injected externally. Although, if a certain supply dies from that, it's only matter of time when it happens anyway, for example when turning off the supply with capacitive load connected; better have it die sooner than later so that you can ask for money back and get rid of the broken supply.Resistors can be used to improve current sharing at expense of load regulation, and they may also dampen oscillating control loops, so if you can afford the extra power loss, and loss of regulation, you can always add some resistance if you see problems.