Connecting 2 SMPS in parallel - Page 1
Forum section: electronics / beginners

author: BlackICE
author_rank: Regular Contributor
timestamp: July 09, 2021, 10:33:26 am
content: Can two low cost bench SMPS be connected in parallel to double the output current? I assume in would be impossible to have them set to exactly the same voltage so the one set to the higher voltage will provide all the current until the current draw exceeds what it can supply. Then the voltage would sag the 2nd power supply will come online to provide additional current.Is my understanding correct?

author: esepecesito
author_rank: Regular Contributor
timestamp: July 09, 2021, 11:42:19 am
content: Pretty much correct. But the voltage of the first source will come down as the load increases, until the output voltage of the two are the same, and the the second will also start giving current. If the initial voltage is near enough, this happens in a fraction of maximum rating....So, you can do it, but dont expect to have 2x maximum rating. One of them will be always a little bit more loaded. Depending on how they work, the more loaded will heat up, and if you are unlucky this will shoot the voltage slightly up... in that case at the end one PS will have most of the load...

author: Siwastaja
author_rank: Super Contributor
timestamp: July 09, 2021, 12:31:47 pm
content: Assuming "bench supply" means a lab CC-CV supply, then there is no problem as CC is a mode designed to be used with no time or thermal limitations. Low cost doesn't matter if it works to the specs.Yes, it goes exactly as you described, chances are high that one supply takes all the load until current limit is met, after that they share to some extent. This isn't optimal, but the supplies stay inside intended operating range.If the supply is just a voltage source, say a general purpose power brick, then their current-limit mode (there always issome) is not meant for regular use and is only there to limit capacitive inrush current, or protect against short circuit. Often they implement either a hickup or a latching fault mode, making it useless.Series resistors can help but are a wasteful solution which wastes both efficiency and load regulation. This can be acceptable if you can trim the voltages regularly as close as humanly possible so that resistor values can be small. The same resistors can then be used for current sensing as well to verify acceptable current sharing.

author: esepecesito
author_rank: Regular Contributor
timestamp: July 09, 2021, 02:39:39 pm
content: Quote from: Siwastaja on July 09, 2021, 12:31:47 pmJust know low cost, may matter. I know very expensive PS that work in 4 quadrants, so the one with the lower voltage would force the one with higher voltage by "eating" current. I know that kind of supply is no the typical... but just in case. Being a cheap one is pretty much warranty it will be no problem.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2021, 03:44:35 pm
content: In general, connecting constant-voltage sources (batteries, etc.) in parallel and constant-current sources in series is not a good idea.With lab supplies, the current limiting can be used appropriately for parallel connections of two supplies, but read the manual on how to do it properly.

author: BlackICE
author_rank: Regular Contributor
timestamp: July 09, 2021, 08:28:41 pm
content: It's just one of those low cost CV CC power supplies from China. Yes someone brought up a possible problem where the lower voltage power supply may sink current from the higher voltage power supply. I'm not sure of that and I'm sure it's not written down in the specs. However I did discover hooking up a battery to be charged in reverse polarity with the power supply turned off blew out diodes and capacitors on the power supply. Without any schematics I was lucky was a pretty simple fix with four parts. I'm thinking of using this for powering A zvs induction heater. Which needs more current than either of my power supplies and supply alone.

author: TimFox
author_rank: Super Contributor
timestamp: July 09, 2021, 08:34:56 pm
content: Reversing the current at the output of a "normal" (one-quadrant) power supply is dangerous and should be avoided.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 09, 2021, 08:59:18 pm
content: Those ZVS induction drivers require a stout supply switched on quickly to get them to oscillate and not burn up. Probably not gonna work.When I was spending a lot of time working on rusty cars I thought about building my own bolt/nut heater around one, but never got around to it. Figured 2-3 cheap 12 volt server supplies in series (modified for floating output) and an old Ford starter solenoid was probably the best cheap way to power it, that or used car batteries.

author: BlackICE
author_rank: Regular Contributor
timestamp: July 09, 2021, 09:05:23 pm
content: I read about the startup problem. I was going to leave both power supplies powered up and switch them with an external switch hopefully that solves the problem. If that doesn't work I would add some external capacitors.

author: Zero999
author_rank: Super Contributor
timestamp: July 09, 2021, 09:32:14 pm
content: Quote from: BlackICE on July 09, 2021, 09:05:23 pmJust use a timer relay, set to delayed on for a couple of seconds. If you need more current, use the relay to switch a contactor, or automotive relay.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 10, 2021, 06:47:25 am
content: If it is sold as a lab supply and it dies due to having voltage connected at its output, it's just good riddance, claim for your money back. Lab supplies typically power up large capacitive loads and are turned off from the power switch, for example.There is no way a working lab CC CV supply (cheap or expensive) dies from this use case. If it does, it is already completely broken by design and just a ticking time bomb waiting to go off.A good point above about 4-quadrant (or more likely, 2-quadrant is meant) supply; cheap can be better because cheap ones won't include the current sinking feature. Although if you do have a high end sinking capable supply, chances are you can configure it to turn sinking off, or configure two supplies to share current.

author: Faringdon
author_rank: Super Contributor
timestamp: July 11, 2021, 10:11:42 am
content: You can have your cheap vout reg’ing SMPS’s in parallel and have nice balance of current.So….Add a boost converter to the output of each SMPS.Make the boosters go into the same output.Both boosts are controlled by a single controller.Set the booster output to your required output.Another way is the same essential setup…but this time make one booster regulate the joint vout….and make the other booster copy the input (or output) current of the “master” one.You can also do this with buckboosters or whatever

author: perieanuo
author_rank: Frequent Contributor
timestamp: July 13, 2021, 07:39:22 am
content: for anyone saying it's bad idea, solution is to insert after each ps rectifier diode after V+ terminal, then on diode K you have your outputs100% safe, each ps has his own feedbacki paralleled ps's in labs for decades, never got one fried, just common sense depending on load. if i was unsure, the diode trick (well, big diodes in fact) is guaranteed safe net

author: Siwastaja
author_rank: Super Contributor
timestamp: July 13, 2021, 11:09:27 am
content: Well, the diode only helps if the supplies totally fail on a very common requirement - having external voltage on output.If you have such shitty supply, then the chances are high it will die out being unable to deliver some other common requirement, for example actually having to output the rated current against CC limit.Unlike resistors, the diodes are likely to worsen current sharing and still have the same issue resistors do - worsening load regulation. Series combination of a diode and resistor could be used, of course.So I don't know if "guaranteed safety net" is a good description. On a good quality properly engineered supply, it's indeed nearly guaranteed they won't die, but it's not thanks to the diode, they would work without as well. With a self-destructing disaster power supply, all bets are off and a diode may or may not decrease the odds of blowing up the supply.