Isolated DC/DC converters "unregulated"? - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 02:08:51 pm
content: I'm looking at some isolated power converters so that I can power an isolated coms bus. But all I see on the datasheets is "unregulated". Not much of a deal is made and i fear a burying of the detail. So do these converters simply work at a fixed ratio to the input supply and will follow any error in the supply input. Usually they are specified at +/- 10% input like 4.5V to 5.5V, does this mean that the output would also be in this range for a 5V output model?This converter does not talk of "unregulated", does this mean it is regulated or did they just forget to tell me that?:https://www.xppower.com/portals/0/pdfs/SF_ITB.pdf

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2019, 02:46:31 pm
content: Quote from: Simon on December 25, 2019, 02:08:51 pmIf you look at numbers beyond the headlines, both line and load regulation is specified on page 2. Yes, it sucks.AFAIK, at leastsomeof these regulators include input voltage feedforward, i.e., somelineregulation, because it's easy to implement in their topology; but poorloadregulation, because it would require isolating the feedback signal from the output, which is expensive.The simplest have poor line regulation, though, as well.Actual "proper" isolated DC/DC modules that are properly regulated and protected can be horribly expensive and hard to find.These cheaper modules are often indeed used to supply power to isolated communication transceivers, where the exact voltage, regulation and ripple tend not to matter too much. The efficiency tends to suck, as well.Also note they are built to a price and certain footprint, and often require extra capacitance and possibly an EMI filter.

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 02:48:12 pm
content: Ah another screw you datasheet. Are there any regulated ones? maybe the parametric search on digikey is better, mouser don't even offer a temperature selection.Yes this is for isolated comms.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2019, 02:50:17 pm
content: Quote from: Simon on December 25, 2019, 02:48:12 pmIf regulation is important, it's the cheapest and simplest to choose one with a higher voltage, then use a $0.10 linear regulator after it.The regulated ones are much more expensive so you can sort by price.

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 02:52:41 pm
content: Yea I guess so. It's not much of a problem but I will have to make sure my main 5V converter is regulated or I will have a chain of inaccuracies.

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 03:25:24 pm
content: Quote from: blueskull on December 25, 2019, 03:01:16 pmYea, thes produce regulated ones, basically it integrates a linear regulator, I guess cheaper than jumping feedback over the isolation. After all the voltage drops with load so the heat output would not be terrible at full load as it would be kept down with a low voltage drop.

author: SeanB
author_rank: Super Contributor
timestamp: December 25, 2019, 03:29:23 pm
content: Cheapest would probably be a 5V to -9v one, as that was a common one for coax networks, then add a LC filter to the output of the regulator ( because they typically have a lot of noise on them), and then any precision 5V regulator you want for the desired stability and accuracy. A bonus is that you can also add another regulator for your analogue stuff, using a 6V regulator, giving you a tiny bit of extra headroom for those applications that are running at the full range of input. Same for a separate regulator for the digital logic, reducing jitter in the ADC side.

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 03:35:04 pm
content: Well just for CAN I don't think it's a problem. As suggested I'll just put a zener on the output. but I will need good regulation for my main 5V regulator:https://recom-power.com/pdf/Medline_DC-DC/REM3.5E.pdflook good as they have a wider input range too. So for 24V auto I can take in up to 36V which is nice. None of the unregulated ones will be any good for automotive as the input has to be within 10% and deviations will mess with the output.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 25, 2019, 03:49:52 pm
content: Have you considered ADUMs from analog devices? They are, AFAIK, regulated and you have them in a lot of flavors, including high speed isolated data lines..

author: chris_leyson
author_rank: Super Contributor
timestamp: December 25, 2019, 03:50:35 pm
content: Unregulated DC-DC converters use a simple self oscillating pushpull Royer converter so output voltage will be dependant on Vin. One thing to watch out for is input voltage rise time on power up. Too slow and the oscillator won't startup and will just sit there drawing too much current until the magic smoke gets released. I found this out the hard way with a 1W 5V to 3V3 converter and I think Murata mentioned minimum input risetime in one of their data sheets.

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 04:02:20 pm
content: Quote from: Ice-Tea on December 25, 2019, 03:49:52 pmMy CAN transceiver is isolated. But anything isolated requires secondary power on the isolated side to work.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 25, 2019, 04:14:38 pm
content: Well, maybe consider something like this:https://www.analog.com/en/products/ltm2889.htmlOr if you're married to your current CAN phy:https://www.analog.com/en/parametricsearch/11036#/p165=0

author: tszaboo
author_rank: Super Contributor
timestamp: December 25, 2019, 04:19:03 pm
content: Quote from: Ice-Tea on December 25, 2019, 03:49:52 pmTheir price is bananas.The easiest solution for this is to replace the 0505 converter with the 0509, and place an LM7805 on the output in SO8 package. Not to mention, this is probably the best way to avoid vendor lock-in and price fluctuations.

author: Ice-Tea
author_rank: Super Contributor
timestamp: December 25, 2019, 04:30:39 pm
content: Quote from: NANDBlog on December 25, 2019, 04:19:03 pmFor the "full solution"? I suppose so, might have something to do with the fact it's a LT product. But combining a isolated CAN tranceiver with their cheapest iso power chip sets you back 3.5$, which is not terrible. And I would assume you can do better if you go through a distri and have some volume to show..H

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2019, 04:34:04 pm
content: It's easy to find regulated isolated DC:DC converters on most distributors' websites such as RS Components. Select a wide input voltage range, i.e. 4.5V to 18V, in the search filter, rather than just 4.5V to 5.5V or 12V and the desired output voltage and you'll only get regulated power supplies because it's impossible to have such a wide input range and a non-regulated output. The disadvantage is increased cost. It might be cheaper to go for a higher voltage and add a low drop-out regulator, but that's less efficient.https://uk.rs-online.com/web/c/power-supplies-transformers/dc-dc-converters/isolated-dc-dc-converters/?applied-dimensions=4293624271,4291833029,4294877092

author: coppercone2
author_rank: Super Contributor
timestamp: December 25, 2019, 05:13:17 pm
content: Quote from: Ice-Tea on December 25, 2019, 04:30:39 pmits a small solution

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 05:48:11 pm
content: Well I'm going to leave isolated CAN for now. I'll make this module non isolated along with some other interfaces and then do a dedicated isolated CAN module.

author: Simon
author_rank: Unknown
timestamp: December 25, 2019, 08:38:18 pm
content: This wurth unit will probably be the best solution:https://www.we-online.com/catalog/datasheet/17791063215.pdfpriced at a tenner rather than 25 quid and is regulated.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2019, 06:57:55 am
content: Yeah, typically the tiny ones are literally a two-transistor chopper, a toroid transformer (usually) and rectifier. Don't forget a minimum load, either.How much voltage and current do you really need for a CAN driver? Who cares if it sags a volt or two at full load? There are 3V CAN drivers.Tim

author: Simon
author_rank: Unknown
timestamp: December 27, 2019, 10:04:10 am
content: Well it's supposed to be 5V, I think the 3.3V ones just come in on spec for the thresholds but I'd be wary. With 5V into 120 ohms each end of the line that is a healthy 83mA