Efficient regulated 5V and isolated & reg. 5V supply from unreg 2W 12.7 V - Page 1
Forum section: electronics / projects

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 03:35:29 pm
content: Hi,I am working on a project where I have a 12.7V 2W supply, and from this I need to get one regulated 5V output (to power a 5V AVR MCU and the input side of an isolated RS485 transceiver) and one regulated and isolated 5V output (to power the isolated side of the 5V RS485 transceiver), with similar current requirement. Initially I was considering using DC/DC switching isolation converter such as H1212WS-1WR3, plus LDO regulators, but it will not be efficient to use LDOs because of the 12.7V to 5V conversion. It looks like these DC/DC switching isolation converters need regulation, but their output can go lower than their nominal output voltage, so I assume they will not work well with LDOs unless the switching converter has a nominal output above 5.4 V.So what would be an design/cost efficient manner to achieve what I need with a good power conversion efficiency, given the limited supply from my input? I assume there is a better way than using 2 switching regulators in addition to the switching isolation converter? Are there 12V to 5V isolation switching regulators that are commonly available (i.e. no TI devices back-ordered for the next 2 years)?Thanks!

author: mariush
author_rank: Super Contributor
timestamp: February 12, 2023, 05:01:03 pm
content: 10.8-13.2v in, 5v 1w out :https://www.digikey.com/en/products/detail/mornsun-america-llc/F1205D-1WR3/16348178Or for example 10.8-13.2v in , 2 outputs : -5v and 5v up to 100mA :https://www.digikey.com/en/products/detail/mornsun-america-llc/A1205D-1WR3/16348271Add if needed a voltage doubler IC in inversion mode to get +5v from -5v but being isolated output you could probably just use the negative as +5v and -5v as ground

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 05:41:07 pm
content: Thanks. I had seen these ones, but the output is unregulated at 5V, so it would not work...

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 05:55:53 pm
content: Would there be a way to use a single switching DC regulator to provide both non-isolated 5V and isolated 5V by adding an isolation transformer to the switching output followed by a rectifying diode and capacitor(s)?

author: mariush
author_rank: Super Contributor
timestamp: February 12, 2023, 05:56:05 pm
content: Your AVR won't be super picky about input voltage, it will probably run just as well at 4.5v as it does with 5v. It should also work up to 5.5v or something like that.I guess you could add a very low drop regulator after the output if you want to guarantee less than 5v .. for example LP3985 (5v 150mA) has a maximum voltage drop of 0.1v at 100mA :https://www.digikey.com/en/products/detail/texas-instruments/LP3985IM5X-5-0-NOPB/1872872The only reason why I think you'd need a well regulated 5v would be if you use the 5v input as voltage reference, but otherwise pretty much anything should work well with less than 5v ... you don't need exactly 5v for a digital 1, it will work with less.lots of devices out there are connected directly to usb and work just fine, without caring about voltage drop on the cable, poor connector contact etc etc

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2023, 07:03:49 pm
content: Quote from: kreyszig on February 12, 2023, 05:41:07 pmPick one with a wide input voltage range and the output will be regulated. Here's an example of a data sheet.https://www.xppower.com/portals/0/pdfs/SF_IW.pdf

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 12, 2023, 07:22:21 pm
content: Suggestion for the non-isolated 5V side:https://www.digikey.com/en/products/detail/xp-power/VR05S05/13147728and for the isolated:https://www.digikey.com/en/products/detail/dcomponents/AM1D-1207SZ/12453769I would not go lower than 7V output for the isolated 5V output as the ripple can be fairly high on the unregulated ones. Given the parts shortage, I would not bet on specialized ones with regulated output.Like always, DigiKey parametric search is your friend.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2023, 07:47:15 pm
content: Quote from: dobsonr741 on February 12, 2023, 07:22:21 pmYour non-isolated regulator is a good suggestion, but the isolated converter you've shown there is unregulated. Search for a wide input voltage range i.e. 9V to 18V to get DigiKey to show results for regulated converters. Here's anexample of some search results for regulated converters. Another indication it's not regulated is it's cheap. You won't get isolated regulated converters from any distributor for much under 5$.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 12, 2023, 07:53:54 pm
content: I meant an unregulated 7…9V output isolated converter + an LDO to get the isolated 5V. Is it where you gravitated towards too, given the price?

author: langwadt
author_rank: Super Contributor
timestamp: February 12, 2023, 08:06:50 pm
content: any old buck to make 5V from 12V for the non-isolated side, and then something like:https://jlcpcb.com/partdetail/VPSC-VPS8505/C2986360https://jlcpcb.com/partdetail/Vpsc-VPT85BB01A/C2875071

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 09:25:46 pm
content: Thanks. It is the RS485 transceiver that would be on the isolated side, but I guess it might handle the +/-0.5V variation.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 09:26:57 pm
content: Quote from: Zero999 on February 12, 2023, 07:03:49 pmThis one looks quite nice, and there is some availability.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 09:29:01 pm
content: Quote from: dobsonr741 on February 12, 2023, 07:22:21 pmThe non-isolated one looks quite nice. There is no availability for the AM1D-1207SZ though, and no availability for 5.3V - 7V isolated outputs in general.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 09:43:06 pm
content: Quote from: dobsonr741 on February 12, 2023, 07:53:54 pmNot necessarily, it can be ok to pay a bit more for this device to gain in efficiency. $10-15 can be ok. $50 is a bit much maybe however. 9V output would have a poor efficiency with an 5V LDO. It could be nice to find a 5.5V unregulated DC converter, but it does not seem to be available. The XP Power IW1205SA is a bit on the pricier side, but it could work well when paired with a non-isolated regulated switching converter.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 09:53:13 pm
content: Quote from: langwadt on February 12, 2023, 08:06:50 pmThis seems to be an interesting idea. I have a hard time finding these two devices though. So the second one is an isolation transformer that is easy enough to find I guess. I am not quite sure what the first devices does. Is it a push-pull converter? Do you think there is a way to use a similar converter to drive both the isolated side and the non-isolated side?

author: langwadt
author_rank: Super Contributor
timestamp: February 12, 2023, 10:45:42 pm
content: Quote from: kreyszig on February 12, 2023, 09:53:13 pmit is a push-pull converter and matching transformer, commonly used for an isolated supply for e.g. RS485As you can see they are in stock at LCSC and there are many similar parts fot many different manufacturersYou need to feed it a regulated supply, 5V in for 5V out with the right transformersince you also need a non-isolated 5V why not just use a buck-converter, there are thousands to choose from that'll do 5V from ~12V

author: kreyszig
author_rank: Regular Contributor
timestamp: February 12, 2023, 10:54:51 pm
content: Quote from: langwadt on February 12, 2023, 10:45:42 pmSounds like a good idea, so far it seems to be the best combination of devices for what I am trying to achieve. What about something like this guy?https://www.digikey.ca/en/products/detail/rohm-semiconductor/BD7F100EFJ-LBE2/5955713?s=N4IgTCBcDaIAQCEAiB2AYgRgAxYKJoCkBaAGQRAF0BfIA

author: langwadt
author_rank: Super Contributor
timestamp: February 12, 2023, 11:01:52 pm
content: Quote from: kreyszig on February 12, 2023, 10:54:51 pmsure, if you want it expensive and complicated

author: Psi
author_rank: Super Contributor
timestamp: February 12, 2023, 11:04:47 pm
content: Quote from: kreyszig on February 12, 2023, 03:35:29 pmIt's not that bad, many of them just need a minimum load or the voltage can be bit high. So a resistor across the output to draw a couple mA usually all you need.Also some are better than others. There are ones that are totally unregulated and there are ones that are pretty well regulated.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 13, 2023, 12:18:06 am
content: Quote from: langwadt on February 12, 2023, 11:01:52 pmIt seems to be amongst the cheapest isolation capable converters I can find on Digikey. The cheaper ones seem to require more external components:https://www.digikey.ca/en/products/filter/dc-dc-switching-controllers/715?s=N4IgjCBcoEwKxyqAxlAZgQwDYGcCmANCAPZQDaIAzABwCcADACz0gC6RADgC5QgDKXAE4BLAHYBzEAF8iYerWpIQqSEICuhEuSoxKANkqUQRSroN7jVBrQDsEE3BiM6lyo1uUWJ9zZi1X7kyI3tQI-iEIjK5m7tH6jME68TZxBomm%2BpT2OjBg1ClEjHq0YJTpZXZR7CDcvAIiEtKyCv7QyujY%2BESkkBQsrDIgALQwSirqmj0UiAODFm3CACa8Q3KjnDyQIJZcAJ4ceLyLOKhSUkA

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 12:43:26 am
content: If discrete component build is OK, a single buck converter can do additional isolated output(s), too. Not regulated, but an LDO can take care of it. TI has good design guidance:https://www.ti.com/lit/an/snva674c/snva674c.pdfFlip side it needs significant design and validation work.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 13, 2023, 03:27:21 am
content: Quote from: dobsonr741 on February 13, 2023, 12:43:26 amThis TI document is great. This is basically the type of circuit I was hoping for, i.e regulating an isolated and a non-isolated outputs using the same converter. It would be great if I can do this with this converter:https://www.digikey.ca/en/products/detail/texas-instruments/TPS560430YDBVT/9554622

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 04:04:22 am
content: It should be. TI has a lot of design/teaching resources - I like this one:https://youtu.be/WdkazE73MLE

author: kreyszig
author_rank: Regular Contributor
timestamp: February 13, 2023, 02:00:13 pm
content: Quote from: dobsonr741 on February 13, 2023, 04:04:22 amGreat video! There was a lot of useful information there too. When I search for proper transformers though, it looks like isolation flyback transformers with the proper inductance are pretty much impossible to find with 1:1 windings. There are 4-6 listed on the WE website, but they are not listed on DigiKey... Almost all their transformers have at least two output windings. Would it make sense to use a transformer with 2 output windings having the same number of turns and tie the ground of one of the two to the input ground to make a non-isolated output out of it? Otherwise would there be a different design where regulation occurs using one of these two output windings (I assume it would use a buck transformer instead of flyback?). Thanks!

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 02:15:24 pm
content: Take a look what Coilraft has:https://www.coilcraft.com/getmedia/34a4aeef-62e3-4a89-82f8-8a588cd0f865/Doc1393_FlybuckCoupledInductors_TI.pdf

author: kreyszig
author_rank: Regular Contributor
timestamp: February 13, 2023, 02:28:18 pm
content: Quote from: dobsonr741 on February 13, 2023, 02:15:24 pmI think something like this one would be perfect ?:https://www.coilcraft.com/en-us/products/transformers/power-transformers/power-converter-transformers/lpd5030v/lpd5030v-682/I cannot find it in stock anywhere in Canada though, but it looks like Mouser have some on order...Edit: I just saw the samples option on the CoilCraft website, it is great! I would not mind paying for it if they could mail it instead of paying $30 shipping with DHL though.When I look at specifications for a transformer, what should I do about the frequency at which the inductance is evaluated? For example, if the inductance has been evaluated by CoilCraft at 100 kHz, but I use a converter that operates at 1.1 MHz?

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 06:20:16 pm
content: The SRF from the data sheet can give an idea of a red line not to be crossed. Given your ops freq at 1MHz, you'd be already on a <10uH inductor, where SRF already over 30MHz.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 13, 2023, 06:47:06 pm
content: Quote from: dobsonr741 on February 13, 2023, 06:20:16 pmOk thanks, it is a non issue then. The converter I was looking earlier (https://www.digikey.ca/en/products/detail/texas-instruments/TPS560430YDBVT/9554622) won't do finally I guess, since it is not FPWM. It looks like the ones that support FPWM are less commonly available.I have a hard time finding any available TI buck converter that operates with in FPWM mode... The ones mentioned in the CoilCraft document as well as the devices that replace them do not seem to be available anywhere...

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 13, 2023, 07:56:17 pm
content: I would find alternative components with matching layout to the one you desire, even if the feature set is not 100%, life FPWM. Then get the PCB made and test with whatever device is available. IMO if your input voltage is stable and not expected to accommodate a wide range of voltages and wide range of load conditions any buck converter would do.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 14, 2023, 03:12:40 am
content: Quote from: dobsonr741 on February 13, 2023, 07:56:17 pmOk this one seems to check all the boxes to me:https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MPQ4576GQBE-AEC1/document_id/9741/. PWM can be forced, 600 mA output is perfect, efficiency is great, device seems to be widely available and is cheap?

author: kreyszig
author_rank: Regular Contributor
timestamp: February 20, 2023, 01:45:27 pm
content: TI use constant on time synchronous buck converters for their Fly-Buck circuits, and they use their type III ripple feedback circuits for stability (https://www.ti.com/lit/an/snva674c/snva674c.pdf), due to the inverted phase of the current during off time. When using a synchronous converter that is not constant on time, e.g. MPQ4576, is it better to use a type III ripple feedback circuit as well?Attached is the circuit I have based on the MP4576 datasheet andhttps://www.ti.com/lit/an/snva674c/snva674c.pdffor 400 mA non-isolated and 200 mA isolated outputs, with 300 kHz switching frequency, without using any special ripple feedback circuit. Monolithic use many capacitors and a lot of capacitancen their design compared to TI (44uH input/output for 600 mA & 400 kHz vs 1uH input/output for 300 mA & 300 kHz in TI's design).