Paralleling power supplies - Page 1
Forum section: electronics / general

author: kc1980
author_rank: Regular Contributor
timestamp: May 10, 2010, 07:03:16 am
content: Is it fairly common (or safe) to parallel several power supply outputs? For example, I have an HP dual output power supply and BK Precision single output power supply. I want to maximize current output to a load, so I set all three outputs to 5V and hook them all up in parallel. All outputs are isolated from each other. Does this work?What other considerations should I keep in mind when doing this? E.g. source impedance of each output, setting same current limit for each output, etc.

author: EEVblog
author_rank: Unknown
timestamp: May 10, 2010, 01:05:40 pm
content: Yes, paralleling PSU outputs is quite common and generally it's quite safe to do, as there is usually sufficient output impedance to handle the load sharing. Sometimes though a supply might not like being powered through it's output terminals when it is off or at a substantially lower voltage, so ensure they are the same before connecting or flicking the load switches.Dave.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 10, 2010, 03:56:26 pm
content: so is that means, both of the following method are safe and can be used?1) two identical ACDC adapters paralleled together to get double maximum Ampere.2) two identical voltage regulators paralleled together to get double maximum Ampere.if paralleling "n" number of devices with "A" ampere maximum for each device, will we get maximum current of (n X A) ampere?

author: kc1980
author_rank: Regular Contributor
timestamp: May 10, 2010, 06:43:20 pm
content: I'm almost certain that scenario number 2 will not work. I don't think the two regulators will split current evenly. One will end up hogging all the current.

author: Simon
author_rank: Unknown
timestamp: May 10, 2010, 07:54:49 pm
content: I once paralleled the two 5V outputs of two computer power supplies and this did not go as planned

author: mkissin
author_rank: Regular Contributor
timestamp: May 10, 2010, 08:51:14 pm
content: It can be a good idea to add series diodes to the outputs of each of the individual supplies, just to stop power flowing backwards into one of them, if they aren't designed to be paralleled. Basically you're looking at paralleling the modules via the "droop" method, and should be aware that it's quite possible for one module to end up attempting to supply all of the load power, at least until the load becomes large.Increasing the current out of an LM317 as you show there in scenario 2 should be done with an external pass transistor, not by paralleling several modules. The datasheet for the device should give an example circuit.

author: eswets
author_rank: Contributor
timestamp: May 10, 2010, 09:36:44 pm
content: Years ago I built a power supply that had the regulators in parallel. I will look for the original schematic (came from a book of tested schematics) but I do believe that it has a diode. I have used it for the past 7years with no issues.

author: saturation
author_rank: Super Contributor
timestamp: May 10, 2010, 09:42:12 pm
content: Yes, I would do the same. The problem with paralleling two voltage regulators is that they will not trackidenticallyalthough they can be close, causing one to take more load than the other. Using a pass transistor is more reliable and is a standard technique to increase the power rating of the LM317.http://www.eleccircuit.com/lm317t-voltage-regulator-with-pass-transistor/To parallel regulators, you need to add more sensing circuitry:http://electronicdesign.com/article/power/double-your-output-current-with-parallel-voltage-r-9270.aspxhttp://www.reuk.co.uk/High-Current-Voltage-Regulation.htmWhich technique to use depends on how much current you'd like to source.Quote from: mkissin on May 10, 2010, 08:51:14 pm

author: ModernRonin
author_rank: Contributor
timestamp: May 10, 2010, 09:45:15 pm
content: > It can be a good idea to add series diodes to the outputs of each of the individual supplies, just to stop power flowing backwards into one of them, if they aren't designed to be paralleled.I thought about this approach myself, but won't the 0.6-0.7V voltage drop caused by a silicon diode mean that you don't get the voltage level you want? Yeah, if you're willing to spend the big buxx, you can get ~0.1V drop germanium diodes, but even that may screw up your voltage significantly if you're running at 3.3V.I figure if you want to parallel linear regs, you need to use adjustable regulators, and set them up so they feed Vcc + (diode drop) into the diode. Then it'll work like you expect. It's tempting to try and sense the voltage/current on the far side of the diode and feed it back to your adjustable reg, but I suspect that's gonna lead to problems with one regulator swamping the other...Edit: And saturation beat me to it while I was writing this comment. Never mind!

author: kc1980
author_rank: Regular Contributor
timestamp: May 10, 2010, 09:56:44 pm
content: Quote from: ModernRonin on May 10, 2010, 09:45:15 pmHow about Shottky diode? Wouldn't that do the trick?

author: ModernRonin
author_rank: Contributor
timestamp: May 10, 2010, 10:00:44 pm
content: > How about Shottky diode? Wouldn't that do the trick?I wish it was that easy. The great thing about the Shottky is that it's a "fast recovery" diode, i.e. it can switch on and off really fast. So you can use it in RF circuits and other fast switching applications.But it still has "voltage drop between approximately 0.15–0.45 volts"Also, "the high reverse leakage current [of the Shottky] presents a problem in this case, as any high-impedance voltage sensing circuit (e.g. monitoring the battery voltage or detecting whether a mains adaptor is present) will see the voltage from the other power source through the diode leakage."-http://en.wikipedia.org/wiki/Schottky_diode

author: ModernRonin
author_rank: Contributor
timestamp: May 11, 2010, 07:32:39 am
content: > i see there is still a limit in current (~4.3A), what if we need more?You can usually get a bigger transistor:http://www.mouser.com/ProductDetail/STMicroelectronics/2N3771/?qs=sGAEpiMZZMuDHCWHhuFg%252baPN204G4fBcX0boRRvcGRo%3dThat'll do 150W, which is 10A @ 15V. The max current it can flow is 30A (@ 5V.)If you need more than 30A (offhand, the only things I can think of would be a *very* fancy audio amp, or a motor driver), you'd be better off using a MOSFET power transistor instead of standard NPN transistors.

author: kc1980
author_rank: Regular Contributor
timestamp: May 11, 2010, 07:36:12 am
content: Quote from: shafri on May 11, 2010, 07:28:27 amOh, i think there's a misunderstanding. It is an HP/Agilent lab power supply -- not for desktop computers. Therefore, the outputs should be isolated. I understand that ATX-type power supplies are NOT isolated.

author: EEVblog
author_rank: Unknown
timestamp: May 11, 2010, 12:40:53 pm
content: Quote from: shafri on May 11, 2010, 07:01:46 amI did say "generally". It's not always applicable as you mention. In many cases the current won't be shared equally, but that can be ok depending upon what load you require and the overhead on the individual supplies.I forgot to mention that in diagram A) it would be standard practice to actually parallel the secondary windings of the transformer for increased current (watch the DOT polarity!).Dave.