2 DC power supplies in parallel/series operation - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 30, 2018, 09:44:10 am
content: I need to pair 2 identical 1 Kw power supplies in parallel for a load of around 1800W.Other than setting both outputs as close to each other in terms of output voltage, what do I need to do to ensure equal load distribution on both units?What are some of the limitations of doing this vs having one 2 Kw power supply instead? Surely efficiency would probably be lower right?Would the same apply for a series connection?

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2018, 01:24:53 pm
content: Make sure the power supplies are designed to be connected in parallel, otherwise there can be problems with oscillation. If they're not designed to current share, then you may need to use diodes, otherwise there can be problems with oscillation. The trouble with diodes, is there's a voltage drop of between 0.4V and 1.4V, depending on the type of device used and die size. To get around this, a MOSFET and an ideal diode controller, such as the LTC4357 or LTC4352 can be used, to give a very low voltage drop.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 30, 2018, 01:34:51 pm
content: Quote from: Hero999 on January 30, 2018, 01:24:53 pmThe power supplies are meanwell so I have a feeling they are capable of current sharing.There is no mention of it in the datasheet though...Will ask my local dealer about it.

author: Richard Head
author_rank: Frequent Contributor
timestamp: January 30, 2018, 01:38:22 pm
content: I'm not sure what your output voltage and current is but you can often use passive sharing but inserting a small series resistance in the positive leads of the two psu's. For high currents a metre or two of copper wire is often enough to get reasonable sharing between psu modules. It does make the voltage droop a bit with load but very often it's acceptable. Allow about a 1-2% volt drop across the leads and ensure that each psu no load voltage is identical before paralleling.

author: Nusa
author_rank: Super Contributor
timestamp: January 30, 2018, 01:44:53 pm
content: Quote from: drummerdimitri on January 30, 2018, 09:44:10 amSeries connection of power supplies means you're increasing the output voltage instead of current capacity. The big consideration is that any supply after the first MUST have the DC ground isolated from the chassis ground, which usually requires some disassembly and modification to the supply. Difficulty will depend on the unit in question.

author: Zero999
author_rank: Super Contributor
timestamp: January 30, 2018, 01:51:57 pm
content: Quote from: Nusa on January 30, 2018, 01:44:53 pmOh I missed the series connection part.At higher voltages, there will be a limit to how many power supplies can be connected in series, due to the strength of insulation, both between the secondary and chassis and mains.Another consideration with series connection, is what happens if one of the power supplies is turned off (say a fuse blows), current limits or shuts down? The other power supply will inject current into its output, in the reverse direction, which could damage it. To protect against this, connect a diode, with a greater current rating, than the power supply's short circuit current, in reverse parallel with each power supply.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 30, 2018, 02:28:01 pm
content: Forget the series question since I only intend on connecting the units in parallel.In case I have failed to mention it, the output voltage will be 12 V and current will be around 140-150A.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 30, 2018, 04:26:29 pm
content: My dealer told me there is no way to connect them in parallel.Does this mean that current sharing is not possible? I don't mind of one pulls twice as much as the other but I do want to use them in parallel after all.

author: C
author_rank: Super Contributor
timestamp: January 30, 2018, 05:04:21 pm
content: Quote from: drummerdimitri on January 30, 2018, 09:44:10 amdrummerdimitri,Read that first line of you post againWill the power supplies really care if1800W = 950W + 850WBoth power supplies are working with in limitations.The problem with your supplies is that you have Voltage control.Voltage control is not Power control which is what you need to share power.When working at a fixed voltage, controlling current gives you power control.With unknown power supplies you have no idea if the Voltage set pot is just setting a voltage or is part of output voltage feedback loop. If part of output voltage feedback loop, you might be able to add a circuit that makes a small change in the feedback to cause the two supplies to current balance.If you remember that a wire has some resistance, just changing how you wire up the supplies can make a difference. Instead of connecting both supplies and then running a wire to load, use two wires with connection at the load. You now have some resistance for each supply.If the two supplies have a remote voltage sense input that is connected to the load, this might be able to be used to get better current balance.First you will need to lie to remote voltage sense input. You could do this by putting a diode between the output+ and remote voltage sense+. You would need a resistor from remote voltage sense+ to output- so diode always conducts a small amount of current. Power supply output is now a diode voltage drop above set voltage. This needs to be at the power supply output.Next a resistor from remote voltage sense+ to load+ is used as current feedback balance adjust.Think of what is happening with this setup.Separate wires from supplies are acting like resistors, Voltage drop changes with current.Diodes in circuit causes you to adj voltage setting lower to get 12V output.Non linear conduction curves of diodes makes a current change through diode change the voltage drop across diode which intern adjusts output voltage to get current balance.Too low or high current through diode will have little effect. You have four resistors that must be adjusted properly for this to function.EDITNote you still have problem that both supplies must be on before load is connected!!!

author: BravoV
author_rank: Super Contributor
timestamp: January 30, 2018, 05:41:07 pm
content: Quote from: drummerdimitri on January 30, 2018, 01:34:51 pmYou don't use "feeling" when it comes this kind of business, especially working in Kilo Watt thing.Meanwell ? I used it a lot, and most common cheap Meanwell are NOT designed for that. Mark the word "designed".I am NOT claiming to be an expert, its just I know exactly there are good PSU brands that is designed and capable of doing that exactly what you want.So how or what kind of PSU that can be paralleled by design ? For example, look for quality brand like TDK-Lambda and also check it's datasheet first to see if its designed for that.There are many2 other models, just an example ->TDK-Lambda HWS Series (PDF Datasheet)Check out page 10 for parallel topology, especially the "Method (C)" to have an idea how to use properly PSUs in parallel mode. Read carefully what is the disadvantages when using "Method (A)", its clearly written there and the limitations. And yes, usually they're more expensive.

author: C
author_rank: Super Contributor
timestamp: January 30, 2018, 06:30:58 pm
content: For something that sounds so simple it can be a big surprise how complex this gets.TI makes some special chips to make building shared output power supplies easer, also a good read.My post treated the 1800W load almost as a resistor.Even buying same model power supplies, you have a difference.One will respond faster to a load change. Does this result in a high power oscillator?

author: jbb
author_rank: Super Contributor
timestamp: January 30, 2018, 06:44:10 pm
content: Quote from: drummerdimitri on January 30, 2018, 02:28:01 pmIs it possible to simply divide the load? It might be less trouble...In that case, theLTC4370active current sharing device might be workable. This is designed to balance current draw from two slightly unequal supplies by servo control of MOSFET resistance (i.e. to parallel supplies which weren't designed to be paralleled). It's got a lot of stuff inside to make your life easier.If you do decide to try it out, keep the following in mind:There will be alotof dissipation in the pass MOSFETs (e.g. 200mV * 150A = 30W), so you can expect to use larger devices (e.g. TO-220 or TO-247) and a big heatsink.The MOSFETs are used in linear mode, so you can't reliably parallel smaller MOSFETs (because the hottest MOSFET will experience a reduction in gate threshold voltage, draw more current, get hotter, and go into thermal runaway). Also check MOSFET data sheet for DC Safe Operating Area curve. Be prepared to use a higher voltage-rated MOSFET.Layout is important, so a custom PCB will be required. (Datasheet has a suggested layout.)Current sense resistors will dissipate a lot of heat andmustbe non-inductive. Wirewound resistors have inductance and (I am 95% confident) will lead to oscillation.Some kind of over-temperature protection is probably a good idea, because the more the supplies differ the more heat is generated.

author: ajb
author_rank: Super Contributor
timestamp: January 30, 2018, 09:44:24 pm
content: The Meanwell RSP-1000 supplies support parallel connection of up to four units with a 10% derating. There is a specific 'current share' signal pin that must be connected between the supplies that allows them to coordinate load sharing.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: January 31, 2018, 05:41:04 am
content: Quote from: ajb on January 30, 2018, 09:44:24 pmI have the SE model and it doesnt support parallel connection.Will try to trade it for an RSP-3000 since other models are out of stock. 560$ though

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 31, 2018, 05:56:27 am
content: HP server PSUs support load share and are very cheap for what they are.http://colintd.blogspot.com/2016/10/hacking-hp-common-slot-power-supplies.html

author: Cliff Matthews
author_rank: Unknown
timestamp: February 01, 2018, 04:05:59 pm
content: Just a thought.Your Mean-Well supplies come quite close to the limit. Why not add a third supply in series with these two paralleled units to take away the thin margins? I mean something low-tech and beefy, easily supporting all the amperage and only 10-20% of voltage requirements. IMO, that would allow some ballasting of the other two as well.