Design ideas for a low-count part, low power isolated DC-DC power supply? - Page 1
Forum section: electronics / projects

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2018, 08:46:15 pm
content: As the title suggest, I'm looking for ideas to implement an isolated DC-DC power supply with as few parts as possible. Targetting low input and output voltage (5V max) and low max output power (50 mA max). Efficency would not matter much as long as it's not ridiculously low (thus > 20-30% would be great).Obviously, flyback topologies are popular, but I would like to find alternate ways, avoiding the need of a transformer if possible.Any ideas welcome.Note: a 1:1 converter is fine and even preferred.

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2018, 09:29:12 pm
content: Adum5000 :https://www.analog.com/media/en/technical-documentation/data-sheets/ADuM5000.pdf5v to 5v 100mA digital isolator .. around 33% efficient. So you'd need 150mA at the input to get 50mA at output.Higher dc input voltage? Any switching regulator capable of 150mA output would do, go for higher frequency to get smaller inductors, ceramic capacitors etcfor exampleAOZ1282(max 36v in, max 30.6v out , 600mA, 1 mhz) .. that's around 85-95% efficient.Want AC to DC or something like that?there's regulators likeLM5008which can accept up 95v input voltage. Assuming around 70% efficiency when doing 90v in, 5v @150mA out, you're looking at around 1100mW or 1100 / 90v = ~ 12mATo make sure you don't go over 90v, you could always use a basic linear regulator likeLR8K4for example - takes up to 440v , outputs desired voltage (maximum Vin - 12v) at up to 20mASo you could have AC in - > diode for half wave rectification -> small capacitor to have min 100v at ~ 15mA all the time -> LR8K4 to do 100v+ to 90v 12mA -> LM5008 to do 90v-> 5v 150mA -> adm5000 to isolate.

author: Marco
author_rank: Super Contributor
timestamp: November 06, 2018, 09:41:49 pm
content: Quote from: mariush on November 06, 2018, 09:29:12 pmTechnically still using a transformer, just a tiny integrated one, but then that's hard to avoid.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 06, 2018, 09:54:11 pm
content: Your post title includes the word "isolated".That on its own, suggests a transformer being employed somewhere.The reason flybacks are so extremely popular for low-power, isolated supplies is precisely because of its low parts count, and low cost.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2018, 10:08:53 pm
content: Quote from: Marco on November 06, 2018, 09:41:49 pmYes, I've already used some of those "isoPower" isolators from AD. They work fine and pretty compact.And yes, they use integrated micro-transformers as far as I know.From experience, they tend to have rather high inrush currents at power on and they are pretty noisy (can make EMC testing fail even when you think you have routed them properly - they need extra care). But they do work fine.I'm looking for ideas for a discrete solution. I'd like to avoid transformers and although that may seem pretty difficult at first sight, the challenge is to find other approaches. Maybe capacitive solutions for instance.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 06, 2018, 10:14:48 pm
content: Why not just buy an off-the-shelf DC-DC module ? That easily satisfies "as few parts as possible"

author: nctnico
author_rank: Super Contributor
timestamp: November 06, 2018, 10:17:41 pm
content: Quote from: SiliconWizard on November 06, 2018, 10:08:53 pmCapacitive should be doable with 'safety' capacitors (class X or Y but you have to look up which ones exactly).

author: mariush
author_rank: Super Contributor
timestamp: November 06, 2018, 10:37:07 pm
content: A bit on the ridiculous side, but you could have a bunch of tiny SMD leds on the circuit board and then place a solar cell on top of it... maybe have a layer of glass between them for isolation purposes.Horrible efficiency obviously.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2018, 10:37:25 pm
content: Quote from: nctnico on November 06, 2018, 10:17:41 pmRight... I have something working at least on simulation now.I guess that to get an acceptable isolation voltage this would require rather bulky capacitors. Does that sound right?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2018, 10:39:45 pm
content: Quote from: mariush on November 06, 2018, 10:37:07 pmI haven't though of that actually. Inefficient as hell as is but I wasn't thinking of optronics solutions, so that gives something to investigate!

author: Marco
author_rank: Super Contributor
timestamp: November 06, 2018, 10:41:30 pm
content: If you want to keep the coupling capacitance in the pF range you just can't switch fast enough with capacitive coupling, since you get picojoule range energy transfer per cycle.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2018, 10:41:39 pm
content: Quote from: mikeselectricstuff on November 06, 2018, 10:14:48 pmChallenge and cost, mostly. Also, most modules I have seen so far are usually for higher output power and a bit too bulky, but if you know of miniature ones and reasonably priced, that could be an option.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 06, 2018, 10:46:55 pm
content: Quote from: Marco on November 06, 2018, 10:41:30 pmTrue, I'm fine with nF-range values or even around 1 µF, although the question is the isolation voltage you can get while keeping the caps reasonably small.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 06, 2018, 10:52:38 pm
content: Quote from: SiliconWizard on November 06, 2018, 08:46:15 pmYou of course know that you can buy stuff like this in potted modules for just a few €.I tried to make something like this because I needed a version with extra low ripple running from USB.Commercial modules are fairly noisy.https://electronicprojectsforfun.wordpress.com/power-supplies/a-usb-powered-15v-supply/This has 15V output, but the circuit can be easily adapted to 5V if needed.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 12:27:55 am
content: I should definitely have added that I was preferably not looking for an off-the-shelf solution.Anyway, as far as transformerless goes, the capacitive approach seems the best fit so far, although it's not an ideal means of isolation. (Reminds me of Dave's 'Does current flow through a capacitor' topic)I'm currently simulating a simple solution which bears similarities with this:https://www.maximintegrated.com/en/app-notes/index.mvp/id/4553, only I just use 2 diodes instead of a full bridge, which is fine for what I need (and has lower loss), and no specific h-bridge IC. This should do the trick although again it's not ideal isolation-wise, but my main goal here is not about safety, it's mainly for getting a floating ground.This will certainly fit my requirements but obviously the safer approach still seems to use some kind of flyback DC/DC converter. The ADuM isoPower series is nice, although it's not perfect either.In fact it sort of all comes down to transmitting power wirelessly, and there aren't that many options for this. Inductive, capacitive, photovoltaic... sure I could also consider a purely mechanical solution but I don't think that would fit my volume, cost and efficiency requirements - not to mention being mostly silent.

author: Marco
author_rank: Super Contributor
timestamp: November 07, 2018, 12:43:52 am
content: Quote from: SiliconWizard on November 07, 2018, 12:27:55 amSimulate what happens if you ground both sides, I think it goes wonky.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 12:52:23 am
content: Quote from: Marco on November 07, 2018, 12:43:52 amI just tried. It changes efficiency a bit, and I get a slightly higher output voltage (which I would have expected) but still works fine.Will post a schematic shortly.Note: I'm not using an h-bridge topology. If I did, obviously grounding the "isolated ground" would short-circuit one of the H-bridge outputs. It would not like that.

author: Marco
author_rank: Super Contributor
timestamp: November 07, 2018, 12:56:23 am
content: I think it essentially shorts out one of the capacitors, there's still no DC path, but the fact that it changes the output voltage is a bit annoying.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 01:01:58 am
content: Quote from: Marco on November 07, 2018, 12:56:23 amI don't mind. And this should be a degenerate case, as you'd obviously lose the isolation - I only expect the system not to be destroyed by it.You nailed an important factor though, as if using an H-bridge as suggested in Maxim's note, this would lead to a short-circuit of one of its outputs. If it's protected against short-circuits, fine, but if not...

author: Marco
author_rank: Super Contributor
timestamp: November 07, 2018, 01:30:31 am
content: It doesn't really short it ... it just makes the MOSFET burn all the energy put into one of the coupling the capacitor each cycle, which creates a limited amount of power, but still too high for comfort.

author: nctnico
author_rank: Super Contributor
timestamp: November 07, 2018, 01:55:17 am
content: Another wild idea: how about using coils for wireless charging? These are small and self adhesive. Stick one on the top and one on the bottom of your board and you'll have your transformer with a hefty isolation barrier. Using resonant mode for energy transfer is probably a good idea.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 02:10:15 am
content: Quote from: nctnico on November 07, 2018, 01:55:17 amObviously a bit larger than I would like as far as area goes, although for a few tens of mW we could probably get away with pretty small coils. Regarding height, it's hard to beat though!I like the idea. Will also work on that.Edit: given the very short distance (a board's thickness) between the coupled coils and the resonant mode, we may even get enough power with coils routed on the PCB directly with a limited number of turns. That gets interesting! Noise coupled to surrounding parts of the PCB may be a concern, though.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 02:18:57 am
content: Quote from: Marco on November 07, 2018, 01:30:31 amRight... they use a full diode bridge so that wouldn't be a short. I was thinking of my approach which uses an half bridge basically, and thus you'd get a short-circuit. With an H-bridge driver, a full diode bridge prevents that problem, so it's preferable. The H-bridge approach gets you a x2 factor (minus the Vf and losses), so I took an half-bridge approach which was simpler and would do the trick in my case.

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 07, 2018, 07:47:05 am
content: Quote from: SiliconWizard on November 06, 2018, 08:46:15 pmwhat about just an high frequency oscillator and a transformator with small toroid core?a 555 or even a buffered cmos logic oscillator or (I do not know if there is a low voltage part) just a self oscillating mosfet driveras far as I remember there are various ANs on the subject on the net

author: capt bullshot
author_rank: Super Contributor
timestamp: November 07, 2018, 08:01:41 am
content: Lowest part count on such DC/DC usually is kind of a self-oscillating Royer converter.Many of the small (1W) DC/DC modules are built internally this way. One needs a transformer with a center tapped primary, another feedback primary and a secondary winding, two transistors and two resistors on the primary side, a rectifier and a capacitor on the secondary. With a bit more effort, the converter can also be done with only one center tapped primary.Often, if there's a buck regulator in your circuit anyway, one can couple a small transformer to that regulator and have an isolated output with very few components involved. Anyway, any half-decent solution with low part count involves a transformer.Edit: There are smallish driver chips (e.g. the TI SN6505B) that ease the job of building such converters. Beware, this chip might cost more than an optimized converter built from discrete components.

author: Giaime
author_rank: Regular Contributor
timestamp: November 07, 2018, 08:04:08 am
content: I used these in the past:http://www.ti.com/isolation/transformer-driver/products.htmlWith those transformers:https://katalog.we-online.de/en/icref/SN6501-Rev-G_Push-Pull_4?sid=3832d0653a

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 07, 2018, 03:35:13 pm
content: Thanks for bringing the Royer converter.Indeed, it cannot be any simpler, component wise, than this converter.Although a caveat, if I recall correctly, is that the transformer core should employ a “square loop” material for optimal efficiency.But for the low power level involved, perhaps any power ferrite may be sufficient.

author: ezalys
author_rank: Frequent Contributor
timestamp: November 07, 2018, 04:30:05 pm
content: You might consider a resonant royer... think royer but with the primary shunted with a capacitor.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 07, 2018, 04:32:21 pm
content: Hi,a square hysteresis loop material is no requirement. A normal transformer is absolutely sufficient at small power levels.I used old ISDN transformers for my low-noise converters and some energy harversting experiments, also employing Royerconverters:electronicprojectsforfun.wordpress.com/energy-harvesting/

author: ezalys
author_rank: Frequent Contributor
timestamp: November 07, 2018, 04:33:30 pm
content: Also, how many layers are you targeting? You might just stick the transformer inside the PCB... then the inductor is zero parts. Then you do resonant royer or forward or something.

author: blackdog
author_rank: Frequent Contributor
timestamp: November 07, 2018, 04:53:50 pm
content: HiLast year i needed a low noise low power power supply with 3 to 5 outputs.So i designed something that was not switching and it uses a small audio power IC in an Wienbridge configuration.This is a topic on a Duchs forum om mine, use Google translate if you wish.https://www.circuitsonline.net/forum/view/138876/1#highlight=voedingAnd this is the main schematic witch shows only two seperate outputs.Just for some inspiration...Kind regards,Bram

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 07:35:29 pm
content: Thanks for the additional ideas and the Royer converter.@ezalys: as nctnico suggested, using just flat coils on both sides of the PCB instead of a true transformer is much simpler and should work well enough with resonant coupling. Would not require a minimal PCB thickness.If coils directly as PCB traces, as I further suggested, give a decent efficiency, it would really be a low-cost and low part count solution. I like the idea.One additional benefit of using resonant coupling is that you can get waveforms close to sine waves if your Q factor is decent, which yields way better results in terms of EMI than flyback converters.This is also what you get with blackdog's solution, but the quiescent current would certainly be much higher.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2018, 08:02:17 pm
content: The smallest/cheapest DC-DC converters are just multivibrators with a transformer hanging off it. No protection, no control, only raw AC. They tend not to go into this aspect in much detail, in the datasheets, but it is apparent from the characteristics they must provide (e.g., output regulation is sloppy and PSRR is nil).You'll find circuits can in fact be simpler, but even more compromises must be made.This is the basic plan of the blocking oscillator. The transistor turns on hard, due to positive feedback, then turns off due to rising inductor current and/or falling base current (due to the relatively small value of Cbb, which discharges through Rb); in either case, hFE rising until it comes out of saturation. When it switches off, the voltage flies back, where you employ a clamp diode on the collector (for a boost or SEPIC converter) or secondary (for an isolated flyback supply).Like the Royer, a feedback winding is needed, which itself may be more expensive (i.e., custom magnetics) than throwing a few more transistors at it (especially transistors with nearly zero incremental cost, i.e., on an IC).Discrete circuits are entertaining, but do not fool yourself into thinking they are in any way size- or cost-effective except in very rare cases today.(I'd say, the case of making a million qty run is rare enough to count.)Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 07, 2018, 08:58:39 pm
content: Quote from: Wolfgang on November 07, 2018, 04:32:21 pmVery interesting comparison! Thanks for sharing.You comment that a low drop rectifier is mandatory, is absolutely correct to extract the last millivolt.Do you think synchronous rectification would significantly help here? I understand that the circuit's complexity would increase, and thus this suggestion may be only a brain-fart.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 07, 2018, 10:25:38 pm
content: Quote from: schmitt trigger on November 07, 2018, 08:58:39 pmSync rectification could help, thats correct. There was some IC that did it very elegantly LT4320 but this does it to 600Hz only.Maybe there is a faster part just as simple ?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 10:37:46 pm
content: Quote from: Wolfgang on November 07, 2018, 10:25:38 pmhttps://www.infineon.com/cms/en/product/power/ac-dc-power-conversion/ac-dc-pwm-pfc-controller/synchronous-rectification-ics/(Minimum working voltage will not fit all applications though, but it's a lot lower than for the LT4320.)Some interesting stuff:https://www.monolithicpower.com/en/products/ac-dc-power-conversion/synchronous-rectifiers.html

author: IDEngineer
author_rank: Super Contributor
timestamp: November 07, 2018, 10:38:14 pm
content: Quote from: SiliconWizard on November 06, 2018, 10:08:53 pm"Pretty noisy" doesn't begin to describe those nasty littletransmitters. I wasted a couple of weeks once trying to quiet them down, both radiated and conducted. In some cases the PRIMARY is noisier than the secondary, corrupting the incoming supply. TI devotes many pages in their appnotes trying to put RFI/EMI lipstick on these pigs... I tried all of their "solutions" and none was even barely acceptable. I'd be embarrassed to have my name associated with something that noisy.As I've noted elsewhere in these forums, also watch out for isolation capacitance. Think of it as a qualitative measurement of the coupling across the supposed "isolation" barrier. It's generally ghastly, with medical-rated modules being modestly better for horrific increases in price. Note that being expressed in terms of capacitance means the coupling is frequency dependent... anything high frequency on one side is cheerfully conveyed right through the module to the other side.I'll never touch those modules again if I can help it. I have lots of samples from many suppliers here, in various voltages and configurations, and hope I never have to work with them again. My recommendation: Stay far, far away unless you're building something like Widlar's Hassler.

author: Wolfgang
author_rank: Super Contributor
timestamp: November 07, 2018, 10:48:38 pm
content: The rant is always the same; People want chips in a microscopic package (meaning low dissipation capabilities). In the sake of energy efficiency, switching times are reduced as much as they can, creating a UWB noise level all around the circuit. Then, some cosmetic RFI measures try to amend things until it is tolerable.If you want quiet circuits, stay away from fast components. Efficiency will suffer somewhat, but RFI problems are a lot less.There are some Jim Williams appnotes where switching speed was deliberately reduced to keep RFI problems at bay.

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 07, 2018, 11:12:52 pm
content: Quote from: IDEngineer on November 07, 2018, 10:38:14 pmYep, that partly explains why I'm looking for alternatives, even if they have to be made of discrete parts.Apart from the noise issues, the startup can be a huge problem as well especially if you're powering them from switching regulators. Had a few headaches. Lastly, I also noticed that they tended to run very warm even at minimal load.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: November 08, 2018, 12:00:59 am
content: https://power.murata.com/data/power/ncl/kdc_nxe1.pdf

author: Marco
author_rank: Super Contributor
timestamp: November 08, 2018, 07:18:35 am
content: Why do they all use these ridiculous poor regulation, minimum load architectures instead of something nice with pulse skipping and feedback?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2018, 08:16:42 am
content: Quote from: Marco on November 08, 2018, 07:18:35 amThey do.The ones that do, cost twice as much.Tim

author: LukeW
author_rank: Frequent Contributor
timestamp: November 08, 2018, 08:23:58 am
content: "isolated DC-DC power supply""avoiding the need of a transformer if possible"Use a big light bulb and a solar cell.Or a motor-generator.

author: Marco
author_rank: Super Contributor
timestamp: November 08, 2018, 11:37:44 pm
content: Quote from: T3sl4co1l on November 08, 2018, 08:16:42 amWhy though? Does Mean Well put a nugget of gold in each and every regulated module? Efficient market hypothesis is failing here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2018, 01:24:47 pm
content: It's all about market. Correct.As is the case with proof-by-contradiction, we can examine our premises and determine which ones are failing.A nugget of gold, no, that would be uneconomical by any stretch.More silicon? Likely. A controller/regulator is needed, and cheap though they are in quantity, they're not quite as cheap as a pair of jellybean BJTs.What does that leave? A somewhat higher price leads to lower production numbers, which leads to much higher production costs per part.Likely, the cost difference at equal quantity is only, say, 20 or 40%, but the quantity difference magnifies it.Furthermore: since it's impossible for a regulated converter to compete in the same space as an unregulated one, they might as well move to an upscale market, targeting reinforced isolation for example, or reduced EMI. RECOM's products are notoriously expensive ($20 for a 5W converter?), but equally high in quality and capability. So, that would account for the last stage of cost difference.Tim

author: Marco
author_rank: Super Contributor
timestamp: November 09, 2018, 07:33:07 pm
content: Quote from: T3sl4co1l on November 09, 2018, 01:24:47 pmIf you can't make an ASIC just use a microcontroller on both sides. The microcontroller can have a regulator, function as a controller and function as a digital isolator with a little air core transformer on the PCB. It won't cost much more than a regulator or controller, packaging dominates at that scale. So basically the extra cost is the micro on the secondary side, big whoop. Also some extra engineering cost, but look at the Murata NXE1 ... compared to the investments for embedding a toroid in the PCB stack I don't think it's all that relevant.Their highest tech module as far as physical design goes has a stone age controller, without any real need for that given component costs. It's a crying shame.