+-12V and 5V rails out of single 15V supply - Page 1
Forum section: electronics / beginners

author: newbie666
author_rank: Regular Contributor
timestamp: September 06, 2019, 08:24:46 am
content: Hello,This is my first ever attempt at building switched mode power supply and I'm a complete noob so please be patientRequirements:Input:single supply of 15-20VOutputs:+12V / 1A-12V / 1A5V / 2A50mV ripple on all of the rails is acceptable.I was thinking about using TI's LM2596 for +5 rail as it seems to be quite popular IC.For split rails, should I be looking at split rail converters? The ones I found have rather small output current, even this application note shows examples of rather small output currents:https://www.analog.com/media/en/technical-documentation/application-notes/AN-1106.pdfWould it be easier to use another step down converter and then an inverting one after it to generate negative rail?Again, I have zero experience with this stuff so I'm looking for tips on "best engineering practice" for those requirements.Bonus questions: what are your favourite converter ICs that could suit this application?Thanks for help!

author: Calvin
author_rank: Regular Contributor
timestamp: September 07, 2019, 03:42:02 pm
content: Hi,the LT8582 of Analog (formerly known as LinearTec) might do the job of generating the +-12V.The Datasheet features several design samples and all the formuals that help to create Your design.The 5V require only a stepdown converter/regulator or a linear voltage reg.The easier way should be to look for complete dc-dc converter modules which may require only a few external parts for filtering.regardsCalvin

author: MarkL
author_rank: Unknown
timestamp: September 07, 2019, 04:06:22 pm
content: Unless you're rolling your own SMPS to learn about them, I would also use modules.The Recom R-78B-2.0 series meets your requirements for Vin and all Vout and Iout. The -12V is provided by using the negative output wiring option. The ripple is a little higher @ 75mV, but you could provide some additional filtering.https://recom-power.com/pdf/Innoline/R-78B-2.0.pdfThere are other manufactures that make similar products.Beware of dual output modules if you're expecting good regulation on both outputs at the same time (e.g., +12V and -12V). Most modules do not regulate both outputs separately, so loading one output will affect the other.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 07, 2019, 05:39:39 pm
content: What I'd probably do:- 12V synchronous buck regulator. This produces +12V and is rated for about 3A alone.- The buck inductor is dual winding, and the secondary is rectified with the correct phase to get -12V. This works for any combination of load currents, as long as the buck converter is always active (synchronous). (If a non-synchronous regulator is used, the load on +12 must always be greater than the load on -12, otherwise the -12 catch diode takes over and regulation is lost.)- 5V from a separate regulator, which may be the same type, or a different type; cascaded (powered from +12V) or not. Cascading has the advantage of automatic supply sequencing: 12V comes up first, then 5V shortly (or jointly) thereafter. (Sequencing can also be done through the soft-start pin on suitable regulators.) If not cascaded, then the +12V regulator only needs to be rated 2A.- Maybe a single LC at the input and outputs, and laying out the supply section so the inputs and outputs are all to one side. This avoids crossing ground loops, and facilitates good filtering.- If 15V is a little too tight for any regulator, or the input minimum really is lower than this still (say it needs to continue operating, even for momentary or sustained dips to 10V or less), I would choose a SEPIC configuration instead; this can (almost trivially) use secondaries to generate as many voltages as you like, so +/-12V is easy, and +5V is harder (you need a custom 5/12 turns ratio), but that can either be solved by following it up with an LDO to make nice clean 5V, or tweaking the tolerances on everything so both supplies are marginal but it works out (i.e., 5.5V is 10% high, while 11V is 8% low, but this may be acceptable after all -- supplies are largely arbitrary, and rarely need to be well regulated).That's an overview of the production-grade solution, but that may not be all that helpful for a newbie; for that, I would suggest:- Individual buck modules or regulators, as you or others have noted. If board-level, follow the application information and recommended values. 12V 1A should be pretty close to a standard example, or use a design tool.It'll probably work, and not need weird tweaking like compensation done.Or for modules, plop it in, use whatever capacitors are recommended (if any), and there you go. If it's freaking out and you can't figure out why (and maybe you'd need a scope to figure it out?), can always try another module. (Alas, the pinouts may not be compatible. The three-terminal modules are usually the same, so that's nice!)- For -12V, you may try a bootstrap mode regulator. This is shown in the application section, maybe not on LM2596, but a bunch of the SimpleSwitchers show how it's done. Basically the output gets tied to GND, and what used to be GND, gets pushed down and becomes your -12V rail. The input sees the sum, or (15-20V) - (-12V), so you need a reg with a relatively high input voltage rating (>= 40V). It also sees the difference of currents, so you need a 2A regulator for a 1A output.Alternately, use a 12V 1A DC-DC isolator module. Over the top, definitely more cost; but you can put that output wherever the hell you want, and it'll always have 12V (at up to 1A..) across its pins!- Would still recommend input and output filtering. Connect to the inside bulk cap, an inductor of maybe 1uH or less. Put a ceramic cap maybe 1uF on the other side (the external facing side) to ground, and again, arrange the input and outputs at the edge of the area so switching ground loops are avoided. (There's always an internal bulk cap, so overall, this gets you a CLC filter on each terminal.)Easy, this way, to get lots of attenuation at switching frequencies (~100kHz) and modest harmonics (into the 10s MHz); good layout (or even shielding) is required for good attenuation into the 100s MHz.Tim

author: newbie666
author_rank: Regular Contributor
timestamp: September 16, 2019, 03:06:13 pm
content: Thank you all for replies, especially T3sl4co1l for detailed writeup. I spent some time going through a couple of application notes to learn about buck inductors using dual winding inductors to generate bipolar supplies. This one was especially helpful (for posterity):http://www.ti.com/lit/an/slva369a/slva369a.pdfI have a couple of follow up questions. Another application note from Maxim (https://pdfserv.maximintegrated.com/en/an/AN3740.pdf) shows a SEPIC converter in a following configurationApplication note lists following advantages of this circuit:1. Quasiregulated output2. 'Clean' inductor current waveform; less noise generation3. Ripple reduction due to coupled inductors4. Single magnetic component (off-the-shelf 1:1 transformer)- Will ripple be better behaved in SEPIC converter with split supplies than buck configuration?- I can't figure out how this circuit will behave without any load. Does it require minimum load as it is the case for buck configuration with split supplies?Thanks for help!Michal

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 17, 2019, 12:59:52 am
content: Has the same |I_buck| > |I_neg| limitation I think.The coupling cap improves regulation by shunting leakage, which is nice.Oh also, that's not SEPIC, but zeta -- which is really the same thing, just swapping the switch and diode. Since it's taking the primary output as well, it's a hybrid buck-zeta really.All currents (input, buck output and flyback output) have discontinuities, so adequate filtering is needed. The buck output may have nonzero current ripple, but it isn't continuous because the inductor current is stolen by the flyback side I think. Hence the mutual minimum load restriction.I think I have the right idea, with regards to operation, just from looking at it; but I would be more confident drawing it out. I recommend the same, or even better, SPICE it and confirm operation.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 17, 2019, 06:35:29 am
content: If your project’s purpose is to learn something new and flex your brain “muscles “, then by all means design and build your own SMPS.If the purpose is an ancillary function of a larger project, then as others have recommended purchase some ready made modules. It will be faster and cheaper.