mixed mode PSU, parallel lin-reg - Page 1
Forum section: electronics / projects

author: liviu.gelea
author_rank: Contributor
timestamp: February 17, 2023, 02:00:13 pm
content: Hello, I'm thinking about building anopen-sourcemixed mode bench top power supply. I just had the idea, so I haven't given it a lot of thought.Please do the same. I just want your gut feeling and experience, not to do all the thinking for me.But the main idea is I want to bring the mains voltage down to about 30V using existing transformer since I don't want to work with anything mains-AC related, the use buck converter to bring voltage to within ~1-2V of target, then drop it further with a set of linear regulators.I know balancing linear regulators is hard but was wondering if any of you have some experience or insights on how that can be achieved.The idea is I would like to avoid spending too much on regulators designed for parallel work such as LT1083 but I cannot find any regulators that output 5A with a reasonable safety margin.I would like to power all my bench top experiments, charge batteries using cc/cv and for that I estimate I need0v - 32v output 0.01v resolution (no idea how to get low voltages with linear regulators. Will look into it. Revisit Dave's video on the matter)0A-5A, 0.001A resolutionLow output noise and ripple, the hole point of mixed mode. Low enough to prevent frying most ICs, I don't need laboratory grade stuffI'm thinking maybe doing PID control, acceleration sensitive separate rotary encoders, Data logging (for graph charging things).Any tips regarding linear regulation?What are some protection features I should consider? I'm thinking charging batteries pose some risks with reverse voltages and also larger voltages pushing current back into an incorrectly configured PS.Is the over current protection feature on some PSUs useful? I can't see a use for it while I have current limiting.Thanks.PS, my experience so far includes designing just a rather noisy buck converter. Please break it to me If you think I am under-qualified for this project.

author: pqass
author_rank: Super Contributor
timestamp: February 17, 2023, 03:12:00 pm
content: You should read this:https://sound-au.com/articles/bench-supply.htm

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2023, 03:29:26 pm
content: Quote from: liviu.gelea on February 17, 2023, 02:00:13 pmDo you *really* need to use linear regulators?For this:Quote from: liviu.gelea on February 17, 2023, 02:00:13 pma regular Chinese DC/DC buck-boost adjustable module, powered by your transformer followed by a (Schottky) diode bridge and a beefy capacitor will work just fine. I made one for my own use precisely for the purposes quoted above, see photos to get a possible form factor idea athttps://www.eevblog.com/forum/testgear/show-the-homemade-equipment-you-are-using-now/msg4668583/#msg4668583(scroll down towards the end of the post).Quote from: liviu.gelea on February 17, 2023, 02:00:13 pmHow low do you need? Let me suggest an alternative approach: implement power filtering, when it is required, in your circuit, instead of relying on the external power supply to provide pure power. In real world they are not perfect: they can be noisy, glitchy, battery voltage is not constant, etc., so you want some sort of power conditioning and input protection in your circuit anyway.If we take my PSU as an example, the noise level is about 100 mV p-p, both loaded and not loaded, which appears as very narrow spikes at 100 kHz. It wouldn't justify the hassle of adding a linear stage for me, being good enough as it is. My scope isn't the best though. But the noise that it doesn't capture isn't going to be a trouble either.Quote from: liviu.gelea on February 17, 2023, 02:00:13 pmNow, you usually don't fry your ICs with noise, unless the noise RMS value is well above their max allowed Vcc-to-GND rating and the noise source is low impedance. I wouldn't be too concerned about that. I never destroyed an IC with mine. But I did destroy a number of small discrete MOSFETs (e.g. 2n7000) when their drain and/or source were connected to the PSU terminals and the gate was connected to a human body. It's clear, why, when you probe the terminals with a scope. But I believe this will happen with the linear PSU just as well, unless, maybe (I didn't verify this), you connect one of the output terminals to mains earth ground. In other words, it's not a PSU's fault: you need to use gate protection diodes.Quote from: liviu.gelea on February 17, 2023, 02:00:13 pmWhich, at least when implemented as a finished product, need their own power regulation and/or filtering.Quote from: liviu.gelea on February 17, 2023, 02:00:13 pmYeah, first of all, proper heat sinking, also considering the air flow conditions. Current sharing may be tricky. Rather than connecting a few LM317s (or whatever) in parallel, it may well be better to use a single powerful pass transistor driven by a voltage follower op amp whose inputs are set by the voltage adjustment potentiometers and the CC limiting circuitry (there should be many reference circuits if you search for them).Avoid placing significant, or, if possible, any capacitors on the output. They will spoil your CC and CV regulation.Quote from: liviu.gelea on February 17, 2023, 02:00:13 pmYes, you will need to analyze the possible flow of current in the situation when the input voltage, the minimum being zero, is less than that across the output terminals. Also analyze what happens when a reverse polarity voltage source is connected to the output terminals when input voltage is zero.

author: liviu.gelea
author_rank: Contributor
timestamp: February 17, 2023, 05:22:45 pm
content: thanks. I think I need the linear step to be able to identify noise in other circuiits I plan building. If I can't teust my psu, I will find it hard to identify which other components input noise into a circuit, Since I wrote this topic I found some larger current regulators

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 17, 2023, 09:08:55 pm
content: The same advice I give anyone looking to do this, is simply: why?Noise is a quantifiable thing. How much noise do youreallyneed?For EMI purposes, a few ~mV is typical. For functional purposes, 100s of mV may be acceptable."As low as possible" is not an acceptable answer:verylow noise is possible. Very, very low. Quantum mechanics levels. Break out a cryocooler and some fancy superconducting hardware and you can have nanovolts. You will pay quite a bit for the requisite hardware... (But it IS available, if you really, truly need it!)So we already come to the crux of engineering: how much do you needgiven an assortment of constraints, cost among them?And for such levels, LC filtering is perfectly sufficient. There are precision switching regulators with 1% and better accuracy, which could be paired with a DAC and MCU for "mixed mode" control. (Not sure if this is what you meant; the term "mixed signal" refers to a mix of analog and digital circuitry. SMPS may or may not be digital. Perhaps you meant just in the sense of, the switch is either on or off (true enough), rather than a broader scope such as digital control.) There are regulators operating at over 1MHz (if you're ready to brave the tight layout they require) which are even easier to filter (in the sense of needing smaller inductors and capacitors). Or you can take a different tack and break out a crummy old TL494 or etc. and learn, heh well first of all why it's crummy and old, but also how to improve it (e.g. implement current-mode control), learn all the building blocks, heck, could even make your own from comparators and logic.A linear postreg also does absolutely nothing about common mode noise -- it will not save you from poor layout!Perhaps I minimize the autodidactic value of such a project. It is usually two chained control loops, which is a nice introduction to control concepts, compensation, etc. It might also be a good introduction to transient phenomena, like startup and output-short conditions. (Beware that a short on the output momentarily puts full SMPS output across the linear reg, and design accordingly!)On the other hand, if you just need a handy power supply -- just get one of the cheap Chinese boxes, or modules, and filter it as needed. (At least for the modules -- harder for the whole mains-powered desktop bench supply kinds -- stuffing it in a metal box, or at least on a ground plane, with generous LC filters at input and output, will solve any layout ills within the module.)Ed: another tack is measuring the noise itself: while it's a rather advanced topic, I feel one could craft, essentially a lab course where you build an EMI testing filter (LISN or CDN), put it on ground plane, and use it to test some equipment (like one of those modules). Then add filtering; or if making protos, make a bunch of layout variants and see which ones perform best; or etc. and see how it all goes. It's an introduction to RF concepts, how to use ports, transmission lines, etc. Mechanically, within a certain framework (connectors on ground plane, and bias tees (basically all a LISN/CDN is)), it's just hooking up power and measuring waveforms on the scope (or preferably spectrum analyzer). It has the potential to be the AC version of DC circuit analysis, done IRL. So, covering complex impedance, AC steady state, stuff like that. A signal generator would also be required; quantifying and treating noise sources like SMPS would go towards the end.Obviously, the crafting of that lab "curriculum" is nontrivial, so I'm not implying you should go out and research these topics in their full depth (which, you should, eventually; but it will take time to get there!), but to say, someone (we??) could write such a plan, that would be simple enough for beginners to follow, and useful as far as developing some of that theory. (Or maybe this is already out there? I'm not at all familiar with what can be found in open coursework, let alone subscription platforms.)Tim

author: liviu.gelea
author_rank: Contributor
timestamp: February 17, 2023, 09:53:07 pm
content: First of all appologies are in order. I'm wasting a lot of your time on this and I really applreciate it as well as feel guilty for having given such comprehensive advice without having put a lot of effort into it myselfI think I should give you some details as to where I'm at so perhaps you will understand my wishes.I have a cheap ZHAOXIN dc power supply which outputs 15v @ 3 amps with a decent 100mV noise.This PSU has let me down in many separate ocasions:* I was unable to power a 24v fan i designed into a fume extractor without using a boost converter* While designing a buck converter I found it difficult to determine which noise came from poor layout and which came from dirty input.* While charging a battery pack with 8 cells (2s4p) my PSU started to lose some of its magic smoke. It still works, but I don't know if is has much magic smoke left in it.* It gets quite hot and fan is noisy* Current resolution is just 0.01A which is useless when trying to drive low power circuitry* displayed voltage is off by 50-100mV* has some interna relays which switch depending on used range. Sometime they oscilate like crazy when I'm near theSo I figured I'd buy a better PS. I ordered one and it was a switching PS costing 150$. It had a constant ark-like buzz when in current limiting mode, was very slow to update because of enormous output capacitance and noise was 500mV @ 2v, 0.6A on a 3.3ohm 100w resistor.I know I didn't do the best job at measuring it but I use same methodology on both power supplies. One is 500mV and the older one is 100mVI looked online for another model but all I found either had:* single potentiometer for managing both current and voltage* costed 250$+* costed 500$ + if I had heard about the manufacturer.Now I would gladly pay a lot more in components and time to build my own with data logging, separate knobs but I can't find myself paying 250$ for what basically is a noisy electrical heater with 2 7-line LED display.So If I'm building one, I'm surely adding some cool features.

author: shapirus
author_rank: Super Contributor
timestamp: February 17, 2023, 10:54:35 pm
content: It sounds very much like you bought a crappy PSU twice. Unless your immediate aim is to learn how to build a good PSU, which is not exactly a trivial thing, especially when you reach the point of putting it all together in an enclosure, I'd suggest doing some research on what kind of decent PSU can be had for reasonable money. Even without searching I am certain that it has been discussed multiple times on this very forum. There were also some reviews on the EEVblog YT channel which are worth viewing.From all I read above, except for actually measuring the noise, it sounds like you just need a decent PSU which does not necessarily have to be linear (though the magic smoke story makes me want to suggest that you also need to make sure that your power calculations are correct and you do not overload the PSU).I can't readily imagine a practical situation where a linear supply will have a decisive advantage over a switching one, provided that it is of a reasonable quality, and for the circuits which are actually sensitive enough to the quality of supply power, you will have to add their own filtering and, sometimes, shielding, anyway.As for the noise measurements, as was mentioned above, not only you need to have proper tools for the job, but also understand how to do it properly.

author: liviu.gelea
author_rank: Contributor
timestamp: February 18, 2023, 10:00:00 am
content: Thanks everyone. You've convinced me. I have far to many unfinished projects anyway. I'll look into a decent psu under 300$ US.