SMPS PSU (low noise) - questions  - Page 1
Forum section: electronics / projects

author: ilya
author_rank: Contributor
timestamp: May 08, 2019, 07:17:12 pm
content: I'm quite new to the SMPS design and I'm looking for help in the following qestions.The goal is low noise power for sensitive audio equipment. By low noise I mean at least as good as a typical linear PSU with good filtering. I was looking at the reference TI design (http://www.ti.com/tool/pmp5863) which is basically SMPS with LDO post-regulation. I think this is a way to go in my case as well. Here're the details:The input to the SMPS is 40VDC (mains transformer into diode bridge and filter caps)There are two independent 40VDC rails that can drive two channelsThere are up to 4 different devices that will be powered from the PSU. Each of them will have its own SMPS and LDO sectionsOutput voltages depend on the device that is being powered. I estimate the range of +/-15V to +/-30V, up to 300mA on each railNow, here are the questions that I'm stuck with:A single device that requires +/-30V should probably have two SMPS step-down sections that are fed from separate rails. Do I need to sync SMPS chips?Since there are several devices, do I need to sync SMPS chips between different devices?What SMPS chips are better suited for this task? Where do I start from?I appreciate all the help I can get from people with more experience in the field than I have. Thanks!

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 08, 2019, 07:30:19 pm
content: If I understand your requirements: You will have a pair of main DC busses, unregulated, which will feed several POL regulators.Is this correct?Another question: Do you want to design the individual SMPS regulators yourself, or would you consider purchasing ready-made modules?From these questions, we can start providing with correct feedback.But I can start with something...there are topologies, and modules, which already have both the +/- output voltages.

author: ilya
author_rank: Contributor
timestamp: May 08, 2019, 07:43:57 pm
content: Quote from: schmitt trigger on May 08, 2019, 07:30:19 pmYes, this is correctQuote from: schmitt trigger on May 08, 2019, 07:30:19 pmIdeally I'd like to design the regulators myself. I've browsed through different modules and couldn't find anything suitable. Most of them can't be synced and I have no idea how they will behave together (8 of SMPS modules in a single enclosure, worst case). Besides, most cheap modules are noisy, and decent ones are just too expensive. Another consideration is - I'm planning to enclose the SMPS section in the screen. This is not so easy with modules - they are often too bulky. However, if you think that there are some modules that are worth considering, I'm all ears.I forgot to mention that I don't need isolation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2019, 08:32:29 am
content: What is the motivation behind independent supplies versus one common supply and load switches? Do the loads need to be switched at all?Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 09, 2019, 04:06:59 pm
content: Understand now.The only time I've had to synchronize regulators was for a project with a class-D amp power supply. I used the LM2588 boost regulator.The reason for synchronizing them was that I had intermodulation issues between the supply frequency and the amp frequency.Looking back I realize that I could have had solved the problem another way, that the intermodulation was really caused by less than optimal board layout and/or incorrect filtering.But at the time, I didn't know any better, and besides synchronization appeared like a novelty which I wanted to try out.

author: ilya
author_rank: Contributor
timestamp: May 09, 2019, 08:42:24 pm
content: Quote from: schmitt trigger on May 08, 2019, 07:30:19 pmI took a quick look at the dual output modules that are available from Mouser and most of them have a fixed output voltage with standard values: 15V, 20V, 24V. This menas that I can't feed and LDO with 17V and get a clean 15V after it.So it seems that it's either two modules or complete custom solution.Quote from: schmitt trigger on May 09, 2019, 04:06:59 pmWell, this is very promising. No need to sync chips simplifies the task a lot.Quote from: T3sl4co1l on May 09, 2019, 08:32:29 amThe motivation is very simple. Imagine a system that has several different audio modules powered from a single source. The set of modules may differ from system to system. All these modules have different power requirements. One may need just a +24V rail while another needs +/-15V and still another needs +/-30V. Now, I can feed my +/-40V unregulated DC and get +/-30V rails with reasonable power dissipation. Even 24V may be managable. But to get +/-15V working I have to resort to dropping resistors which is not the best solution.I need to choose the optimal topology for this task. Since I have two rails it may be benefical to use one of them as a positive and another one as a negative. The disadvantage (as I see it) is that I need 2 SMPS chips to do the work. Another solution is to use something like the method described here:http://www.ti.com/lit/an/slva369a/slva369a.pdfBut I doubt I can feed it with 40V and get +/-30V out of it.Another question is what to do with grounds. With 2 chips solutions it seems pretty straightforward. But with a split-rail system - I can't wrap my head around this. Can I establish the common ground point for all modules in the system?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2019, 04:37:11 am
content: Why do the modules take different voltages? How much load current?Tim

author: ilya
author_rank: Contributor
timestamp: May 10, 2019, 08:01:58 am
content: Quote from: T3sl4co1l on May 10, 2019, 04:37:11 amDifferent modules have different voltage requirements. This can't be changed.Load current is in the range of 150-300mA per rail for each module.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2019, 10:14:38 am
content: Can you use common e.g. +/-15, 24, 30V supplies, of up to 1.2A each I guess, and each module connects to only the supplies it needs?I suppose there isn't much savings at that point over four independent supplies, but it's still something.Why do the modules not have POL regulators internal? Can they not be changed to do so?Tim

author: ilya
author_rank: Contributor
timestamp: May 10, 2019, 10:38:13 am
content: Each module has its own POL regulation. They all take common unregulated +/-40VDC and regulate it down to the needed voltage. Now it's all linear. I want to change this to SMPS for better efficiency and less heat dissipation.