Cheap Power Supplies in Parallel for Redundancy? - Page 1
Forum section: electronics / beginners

author: petreza
author_rank: Contributor
timestamp: March 02, 2016, 01:31:02 am
content: I bought this 24V power supply:http://www.amazon.com/gp/product/B01688AMGY?psc=1&redirect=true&ref_=oh_aui_detailpage_o08_s00to run a water cooling pump.I want to have some power redundancy so I am thinking of getting a second one and hooking them in parallel. Is this safe?Thanks!

author: ivan747
author_rank: Super Contributor
timestamp: March 02, 2016, 01:48:06 am
content: Yes, do it this way:http://www.proflow.ca/wp-content/uploads/2013/05/diode_protection_for_redundant_power_supplies.pdfUsing ORing diodes you will have no problems running them in parallel *for redundancy*.What happens is that whatever supply has the highest voltage output (even by a little) will end up supplying most if not all of the current. That's bat if you want the power supplies to share current but if you want redundancy it would be okay, I guess.

author: petreza
author_rank: Contributor
timestamp: March 02, 2016, 10:46:08 pm
content: Thanks for the advice!So it is recommended to choose diodes that are rated at least 1.5x for the maximum of both voltage and amperage and a Schottky diode is better because it has lower voltage drop and heat generation.In my case the maximum load is 110W @ 24V = 4.58A24V * 1.5 = 36V4.58A * 1.5 = 6.87AWill this do the job:http://www.digikey.com/product-detail/en/vishay-semiconductor-diodes-division/SB15H45-E3%2F73/SB15H45-E3%2F73GICT-ND/4498398Would it be OK to shrink wrap the diode inline with the wire - by my calculations it should be generating about 2-2.5W of heat.Could you confirm that I should (must) put the diodes only on one of the (positive) outputs of the power supplies. (In fact putting diodes on all 4 wires might defeat the purpose, right?)Thanks again for your help!

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2016, 10:52:56 pm
content: Some of the more recent designs have current sharing capability that hasto be configured. Basically a connection between two power supplies tomonitor each others load current. Take a look at its datasheet.The other two lines are generally speaking Kelvin sensing lines to removecable drop as errors in regulation, so no, no diodes would be used there.They are inputs to error amp. Use power schottky diodes to minimize wastedpower.Regards, Dana.

author: biffster
author_rank: Contributor
timestamp: March 02, 2016, 10:57:10 pm
content: You need one diode on the positive of each PSU as you don't know which will have the higher voltage.You have to be careful sometimes with Schottkys as they tend to have higher reverse leakage current. You can see from the data sheet (Fig 3) that at higher temperatures the reverse leakagecouldbe around 10 mA but I don't know if this will cause a problem.I'd also try not to heat shrink your diode if you can help it as it will act as insulation and make it heat up more.

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2016, 11:39:37 pm
content: Schottkys also have lower reverse breakdown capability, so take this intoaccount when the design is considered. They also generally have a much fasterreverse switching time, in most applications a preferred characteristic.Regards, Dana.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: March 03, 2016, 01:39:28 am
content: You can add ballast resistors to share the load.As a startup procedure, you want to run the pump only on supply A, then only on supply B, and then A+B in parallel.Since you mention heat-shrink, through-hole components do not belong in wire harnesses to be tugged at...

author: rx8pilot
author_rank: Super Contributor
timestamp: March 03, 2016, 01:43:27 am
content: Not sure what your skill level is, but an 'ideal diode' circuit will dissipate very little heat. Most traditional diodes will need a substantial heat sink for that much power.

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2016, 08:44:40 am
content: Quote from: danadak on March 02, 2016, 10:52:56 pmThe only issue with that is the original poster wants to use two power supplies for redundancy. What happens if one of the capacitors on the output stage of one of the power supplies fails short circuit or the overvoltage crowbar is activated? It would affect the other power supply.Using ORing diodes means if one power supply develops a fault, then the other one will carry on working unaffected.

author: danadak
author_rank: Super Contributor
timestamp: March 03, 2016, 02:51:48 pm
content: Depends on the way the active sharing is designed, this ap note discussesfault tolerance and some basic methods.http://www.switchpwr.com/parallel.pdfRegards, Dana.

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2016, 08:16:36 pm
content: Quote from: danadak on March 03, 2016, 02:51:48 pmHow much do the power supplies discussed in that article cost? Probably a lot more than 21$.

author: danadak
author_rank: Super Contributor
timestamp: March 03, 2016, 11:25:17 pm
content: Not sure how to answer you, re cost, as thats driven by many considerations, mostwe do not know. He is running a cooling pump, is it cooling a nuclear reactor ora humingbird feeder elevator motor......cost in one case clearly no object in the scaleof things.Our role is to offer solutions, ideas, and technology alternatives for the designer to choose from.Regards, Dana.

author: mongo
author_rank: Contributor
timestamp: March 04, 2016, 09:27:42 am
content: If you can read the chip numbers this solution is very cheap, although at that price I would be tempted to just buy it.http://www.mini-box.com/Y-PWR-Hot-Swap-Load-Sharing-Controller

author: rx8pilot
author_rank: Super Contributor
timestamp: March 04, 2016, 05:43:45 pm
content: Quote from: mongo on March 04, 2016, 09:27:42 amI like that one.

author: albert22
author_rank: Regular Contributor
timestamp: March 04, 2016, 09:58:57 pm
content: QuoteThere is a schematic in that page apparently uses 4 PMOSFETs and a LTC4416.

author: jboard146
author_rank: Contributor
timestamp: March 04, 2016, 10:07:00 pm
content: FYI - Here is the schematic.

author: jboard146
author_rank: Contributor
timestamp: March 04, 2016, 10:14:56 pm
content: There is actually some interesting reading in the LTC4416 datasheet.Its not exactly a cheep chip at ~$5 in qty 1 (digikey)

author: rx8pilot
author_rank: Super Contributor
timestamp: March 05, 2016, 06:02:02 pm
content: That is a solid controller for the application. Skip DigiKey and buy 1000+ from Arrow and the cost is reasonable.Sent from my horrible mobile....

author: pmbrunelle
author_rank: Regular Contributor
timestamp: March 05, 2016, 08:11:39 pm
content: By using one controller chip, you piss away whatever reliability you had from having two supplies diode ORed.Now there's only one chip to go wrong, with no backup.

author: biffster
author_rank: Contributor
timestamp: March 05, 2016, 09:18:22 pm
content: Quote from: pmbrunelle on March 05, 2016, 08:11:39 pmThat's only true if the controller chip and the power supply mean time between failure (MTBF) are similar. If the chip is much less likely to fail than the PSUs then there might not be any point in having two controller chips.