Q: Advantages of using DC-DC isolated converters - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 18, 2016, 03:38:57 pm
content: Requirement: Five isolated regulated 24V power supplies (4 amps max)Consider the following two approaches:A. The simple (and less expensive) approach:[AC -> DC 24V/4A] x 5, result: 5 isolated 24V/4A suppliesvs:B. The more complex approach involving DC-DC isolated converters, which can get expensive when we start talking 4A current requirements:[AC -> DC 24V/20A] PS ===> [24V/4A] x 5 DC-DC isolated converters providing 5 isolated 24V/4A suppliesWhat would you say are the advantages of going with approach #2. I see disadvantages:a. All of the isolated converters bear down on the master 20A supply.i. Load spikes on one of the converters can cause noise on the others, by impacting the master 20A supply.ii. If on the of the isolated converters faults it can bring down the master PS or impact it in a very negative way.b. If the master 20A supply fails, the whole thing fails (i.e., no redundancy)But, there have to be some advantages to approach (B). Please offer advice.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 18, 2016, 03:52:39 pm
content: If you need 5 isolated supplies you'd only need 4 converters.Possible benefits of #2 :Depending on PSU choice, a single big PSU may have better efficiency, though losses in the DC/DCs may offset this.PSU+DC/DCs may be smaller, if that matters.Less mains wiring, probably lower inrush & earth leakage.There may be benefits depending on physical layout, e.g. if you needed some distance at low voltage, maybe a 48V supply plus seperate DC/DCs might have an efficiency/cable size advantage.It will depend a lot on the overall requirements, including cost, size, efficiency, weight, approvals etc.

author: nctnico
author_rank: Super Contributor
timestamp: October 18, 2016, 04:09:08 pm
content: Depending on the number of units it could be economic to build a multi-output converter (for example a forward converter) either fed by mains directly or from a power supply. But is also depends on the required isolation voltage and saftey requirements.

author: ajb
author_rank: Super Contributor
timestamp: October 18, 2016, 04:18:06 pm
content: Option 2 may permit a more preferable mechanical design and simplify final assembly. Saves a fair bit of final wiring if you're able to slap all of the DC-DC converters onto one big PCB with your other electronics (you could do this with board mount AC-DC converters as well, but those aren't as common at the power levels you're talking about, plus it means handling mains voltage on your PCB which you may prefer to avoid).Another concern with option 2 is that putting five big DC-DC converters on the output of one switching AC-DC converter may cause them to interact poorly without additional input filtering.

author: SharpEars
author_rank: Unknown
timestamp: October 18, 2016, 05:37:09 pm
content: Quote from: ajb on October 18, 2016, 04:18:06 pmI am not sure I understand what you mean by this. Why would additional input filtering be necessary in this case?

author: SharpEars
author_rank: Unknown
timestamp: October 18, 2016, 05:38:37 pm
content: I should add to this that space is not really an issue and there is no "single PCB" requirement. Option B will be less efficient, because you have power loss of the primary converter (80-90% efficiency) and then again by the DC-DC converters (80-90% efficiency).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 18, 2016, 05:40:00 pm
content: And of course if this is high volume you'd get a custom PSU made with teh required outputs.For lower volumes if cost is not a big deal, you could use a modular PSU with five 24v output modules

author: Zero999
author_rank: Super Contributor
timestamp: October 18, 2016, 05:48:30 pm
content: Beware if you opt for separate mains power supplies.Quite often cheap mains PSUs with a three pin plug (earth + phase & neutral) are not isolated: the earth is connected to the negative rail. If you go for mains power supplies with no earth, then the leakage current between the mains and secondary will be too high, if the secondary sides of more than one power supply get connected together for whatever reason.I'd recommend a mains power supply with a three pin plug, but make sure the secondary is not connected to the earth and the earth leakage current will be low enough to satisfy the safety regulations if five of them are connected to the same mains plug.Another option is to go for the non-earthed mains PSUs and connect one of the secondary conductors (it doesn't mater whether it's the +V or -V) of each to earth, via a 100nF 250VAC X capacitor, so any leakage current is bypassed safely to earth.

author: SharpEars
author_rank: Unknown
timestamp: October 18, 2016, 06:14:47 pm
content: Quote from: Hero999 on October 18, 2016, 05:48:30 pmI certainly would like the ability to mix and match the five 4A PSUs (i.e., connect them in series and parallel with each other to form other voltage/current ratings), so this info is important!

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 18, 2016, 06:30:38 pm
content: Quote from: Hero999 on October 18, 2016, 05:48:30 pmThe only time I've ever seen -ve connected to earth is PC PSUs - never seen a generic PSU do that.

author: Zero999
author_rank: Super Contributor
timestamp: October 18, 2016, 10:44:06 pm
content: Quote from: mikeselectricstuff on October 18, 2016, 06:30:38 pmWhat sort of generic PSU are you talking about? Yes, bench top and most industrial power supply modules will be floating. I was referring to the type in black boxes with an IEC connector on the back: if it has three pins is almost certain the negative rail will be earthed.

author: nctnico
author_rank: Super Contributor
timestamp: October 18, 2016, 10:49:04 pm
content: I think Mike is referring to open/closed frame PSUs you built into equipment. I have the same experience with the lump-in-cord power supplies though. I actually kinda expect the negative to be connected to earth.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: October 19, 2016, 08:27:31 am
content: Quote from: nctnico on October 18, 2016, 10:49:04 pmYes - the OP is talking about PSUs in the 100W range, which you don't generally find in lump-in-cord form.

author: Zero999
author_rank: Super Contributor
timestamp: October 19, 2016, 10:55:50 pm
content: Quote from: mikeselectricstuff on October 19, 2016, 08:27:31 amThey're quite common nowadays. RS sell them:http://uk.rs-online.com/web/p/desktop-power-supply/1229837/But what the orignal poster needs is this:http://uk.rs-online.com/web/p/embedded-switch-mode-power-supplies-smps/6802802/

author: wine+dine
author_rank: Contributor
timestamp: October 20, 2016, 07:08:22 am
content: Quote from: Hero999 on October 18, 2016, 10:44:06 pmYou haven't taken many of these apart, have you?Typically, the PE connection is there for the input filter, and the output is not connected to it except through a small Y capacitor. Measure AC from the LV output to mains earth with a high impedance meter, you get roughly 1/2 mains voltage.

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2016, 08:25:14 am
content: Quote from: wine+dine on October 20, 2016, 07:08:22 amYes.QuoteOn a desktop PSU, the output is normally connected directly to PE. I don't need to dismantle it to confirm this: a simple continuity check suffices.On an industrial PSU, it's built as you've described above: the PE connection is connected to the secondary side via a small Y capacitor.