how does blackdog's PSU work? - Page 1
Forum section: electronics / beginners

author: exe
author_rank: Unknown
timestamp: March 18, 2018, 08:54:29 pm
content: Hi!I'm trying to design my own PSU. I took @blackdog's power supply as a reference (schematic attached). But the more I study it, the less I understand how it works.1) The output voltage is defined by current flowing through resistor P2 (20k multi-turn pot).But why voltage drop on P2 should be constant?I don't see a current source to ensure this. I only see a voltage source which is +5V above the positive output.2) what is return path for current flowing through P2?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2018, 09:36:53 pm
content: 1. Consider what the op-amp inputs see, as output voltage varies.2. Through the output node. Note that all the little bias currents from the reference section act to pull the output down.The circuit in general:Oh geez, those aren't fucking JFETs. Who drew this...Anyway: this is theonlyamateur linear power supply design I've seen that is actually reliable, and working, as shown. (Compare to the 30V 3A mega-thread, or whatever it was, which is a living train wreck...)It makes some strange decisions, like using an active rectifier (doesn't really matter; easily substituted with a cheaper, traditional, FWB diode), and schottky diodes alongside shitty old 1N4001s, which can be more coherent.Interestingly, the V/I error amps have low integrator windup. I can't decide if this was by cosmic coincidence, or full top-down intent. It's quite possible that the decision to fix the reference above the output was a direct consequence of this step -- a lot of trouble to go to, but as long as you're going to go that extra kilometer, you get something nicely behaved indeed.I think the only things I really object to are:- The fuse is silly. If fault protection is desired, a current limiting stage can be added, at the input or output.- The "dropout protection" circuit doesn't make sense either; if you're going to specify ~200W worst case dissipation, don't hack it, just put in enough transistors and heatsink to handle it.- A preload (besides the reference bias) on the output would help by biasing the output stage into conduction, improving transient response (the problem is all the nonlinear junctions -- the output transistors and LEDs, aren't fully conducting at the no-load condition).- Get rid of the output capacitors, or use minimal amounts. Also, why bother buying a low-ESR cap, just to put a resistor in series with it? Is compensation really that critical that it needs a 10% stable ESR?Overall, small complaints, and simple tweaks. This is a good design.Tim

author: exe
author_rank: Unknown
timestamp: March 18, 2018, 10:08:01 pm
content: Thanks for answering. I'm still digesting the circuit. I tried to "think as an opamp", still don't get one thing:1) If output voltage goes up, the voltage on P2 also goes up?? Why doesn't swing up to maximum or oscillate? (Unless C24 smooths voltage good-enough so the circuit does not oscillate).2) Can I drive current-setting and voltage-setting resistors with a DAC?3) Is it really necessary to have TL317 and LM329 in front of LT1021? Looks a bit over-engineered to me. Can I just use a 6.2V zener and 2n3904 emitter follower as a pre-regulator?

author: blackdog
author_rank: Frequent Contributor
timestamp: March 18, 2018, 11:51:04 pm
content: Hi,Que? => Oh geez, those aren't fucking JFETs. Who drew this...I have drawn this schematic and extensively tested! :-)About the active rectifier, which is to limit the power loss, that should be clear...If you want to throw away a extra 10-Watts, be my guest, no problemo to use diodes.The pre regulator works, just adjust it for your transformer, and why did i drop this part of the schematic?, because 2 transformers (2x9V) and some relay, works without winding noise of the transformer.I do not understand why over and over again people whine over the reference section, it is to much, overdesigned etc etc.The Architect specifications were LOW NOISE! High stability, at full load, and i mean < 5uV rms, and Yes! i mean uV and not mV.The reference section is not overdesigned, but well designed!If you want to have an average power supply, build somting elseThe currentsource make the power supply extreemly well regulated.Why Low ESR components on the output, because these are almost always, also good quality components.The fuse is only for calamities, probably never blown up, if you want to kick it out, your problemGet rid of the output capacitors, or use minimal amountsNO, NO, NO!These capacitors with the series resistors and the loop time constant determine the dynamic behavior of this power supply.Look at this picture...A 9.5-Ampere Puls generated bij my Dynamic Load, the opamp at that moment was the NE5532A and the output network used is in the picture.The ringing you see is NOT! the power supply, but my Dynamic Load connected to the powersupply under test via twisted 0.5M cables and a high quality 6.8uF capacitor over the Dummy Load connection.If i remove the 6.8uF capacitor, then there is no ringing, just a short puls of about 5mV piek for every 5A change and totale gone in about 25uSec.The residu of the twisted cable induction and the 6.8uF capacitor create's the ringing, this is a difficult test for power supplies and i use it a lot.Kind regardsBram

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 01:07:45 am
content: Quote from: blackdog on March 18, 2018, 11:51:04 pmOk but why JFET symbols? Those are unambiguously JFET symbols, and the devices are unambiguously not JFETs, but MOSFETs. Was this drawn in a package that doesn't have a MOSFET symbol and doesn't allow symbol creation? (If so, please let me know so I can advise everyone to avoid it like the plague...)(Similarly for the schottky diodes, but that's a lesser sin.)QuoteDoesn't bother me, on top of a 10W, 100W or more loss in the pass reg. If you needed efficiency, the whole thing would be a switcher (which, yes, can be as low noise, but -- it also takes more work to get there).If you had a...QuoteAh, so there was a pre-reg or tap switcher or the like, that would improve efficiency notably, allowing one to save on heatsink area.The question is then: whatthermalprotection is there? Easily enough added (or omitted(!)), of course.QuoteWell, let's see. LT1021-5 claims about 60 dB PSRR at 15V and 10kHz; at 8V, it's down about 5dB, so let's say 55dB. That means its supply can be about 35000 nV/rtHz, to only increase the output noise spectrum by 3dB.That's pretty fuckin' noisy!On analysis, this would seem to be overkill.Unless you have measurements showing it both ways (e.g., LM317 alone, with resistor divider; versus with LM329).I didn't notice this point originally, but it's a fair point, and I see no data or argument defeating it.The LT1021 doesn't even seem to be very good; an array of TL431s can do better, and might be cheaper. REF50xx do better, dunno if cheaper. I don't regularly shop for references.Quote??QuoteI'm looking. It looks pretty damning...Quote...Oh.So, wait. You performed a test, on a power supply, thatdoes not measure the power supply's characteristic, but some external network instead?Can you please explain that rationale to me? Because that can't possibly be right.The network should be around 0.3uH + 6.8uF --> 110kHz oscillation. I don't know why the measured pseudofrequency seems to be ~half that. It probably has to do with why the damping is modest, too (maybe the load is not simply an on/off switch and resistor, but has 10uF + 0.05 ohm in parallel with it, perhaps).The voltage ripple should be measured at the power supply output terminals, which is the only possible point the power supply can control, after all (at least without remote sense terminals included). The load step should be nothing more than a resistor and switch, with minimal damping. Its rise time will be limited by cable inductance to about 0.6us, which is fine....Double-wait. Why is the test at9.5Awhen the supply is rated for5A!?Why is it not dropping out during the pulse?This data is inconsistent and thus wholly unusable. Who knows what it was measured on! Are you a liar?You realize you aren't making much of a case for this, right?I guess I might as well retract my claim that it'sgood. It's still better than others as I said before, but this disregard for data and measurement bothers me much more than the otherwise minor inconsistencies in the design.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 01:12:49 am
content: Quote from: exe on March 18, 2018, 10:08:01 pmWell yeah okay, it goes up...But the immediate words that should follow that are:"How much?"If P2 has, say, 5V on it, then the other input should be at 5V (which is also what the output will be at). What does this tell you about the output voltage? What does this tell you about the value of P2, with respect to R32?What if P2 has 10V on it, what must the voltages (and P2 resistance) be?Can we write a relationship (an equation) describing this?QuoteYes, but the DAC better have a wide voltage range!QuoteUnless the author can prove otherwise, I see absolutely, positively no degradation from simply using the '317 as intended, with a resistor divider.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: March 19, 2018, 04:01:33 am
content: Quote from: T3sl4co1l on March 19, 2018, 01:12:49 amOr simply use the LM329 (which is a pretty quiet reference, just as quiet as the LT1021 - 7 uV rms 10Hz-10kHz versus 2.2 uV rms 10Hz-1kHz i.e. essentially the same) in the usual bootstrapped current source/reference amplifier, all fed by an LM317/whatever, as your core reference and derive all your rails from that - sprinkle with op amps and emitter followers to taste.Then junk the £12.28 active rectifier setup for a £0.25 diode bridge and use the money you saved to upgrade your LM329 to an LM399, which will drop-in nicely with no re-design and lower your core reference tempco to 0.3ppm/C.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2018, 10:09:06 am
content: Hi,I will try to explane some things, but bear with me, i am a dyslect and English is not my nativ langguage.And of course... I am from Amsterdam in the Netherlands, that means that I am direct and do not lie , and no bullshit.And again, the reference section...The powerline at my lab is noisy and has a relativ high Ri.Its a powerfull noise generatorThere are several tricks to keep the line noise out of the LT1021 and the LM334.First the extra resistors R3 and R5 in combination with C3 and C4 yes low ESR types.I lifted the TL317 regulator with D6, so the noise of the TL317 would not be amplified 6 or 7 times.R23 and C20 makes another low pass filter, this has a double function, C19 en C20 makes a time constand, so there are no power on abberations.The output of the TL317 is also used for the opamps.The negative power supply for the opamps is just a simple zener regulator, and again C8 makes and R9 makes a slow power on.Replacements for the LT1021-5V, but why... eat one big Mac less and you can affort a LT1021-5V...Ore use one of these: AD587, MAX6350, MAX6126, LTC6655, LT1027 and if you have money to spend, buy a VRE3050 *grin*TimI have explained before, that when I draw this schematic, I could not find the MOSfet in the library of my drawing package.Power dissipationThere are several ways to limit the power loss, the dissipation limiter shown here is one of them.But this needs some attention, for the transformer used.The performance is good enough, but not as high as a switcher.When using say a buck regulator (how often did I have to hear that ...) the efficiency is better.Designing a extremely low noise 200-Watt pre-regulator is difficult.After a number of measurements (test circuits) I decided not to use it. (Most switchers are like a Pulsar sweeping there powerful beam of noise, over sensetive electronics)A transformer with multiple taps, or as I already mentioned, two transformers of 2x9V works well enough for me and saves enough energy.Thermal protectionThis is not the latest schematic, thre are some sensors on the power section and the transformer.Current sourceSorry, i'am talking about the current source that is not in this schematicThere is a current source (about 50mA) at the collector of Q2 or Q4, it alway puls this current out of the power stage.This makes the dynamic behavior better and also ensures that the output voltage decreases rapidly when it is turned down at low load.Power sectionThis is the best I could make, I also tested with smaller power transistors, this was a version with 4 compound stages.I did blow up this version during testing and was worse in terms of Ri than the 2SA1943 / 2SC2911 transistors.Why these transistors, this is to keep the delay of this stage as low as possible, so it would not at to much to the phase delay to the opamps.Dynamic performanceTim explane, why i may not show a picture of the performance, at a peak current of 9.5-Ampere?And do you realy think i'am that stupid, that i measure the performance of this power supply at the dummy load input?This is one of my tests of the power supply, how it would reacts in a practical situation.And of course I also test in a different way, with my Jim Williams Dummy Load, which has a very high bandwidth and is then connected to the power supply to be tested without cables.Search fore AN104 on the LT website, i use te MOSFet version.For real!!!...Double-wait. Why is the test at 9.5A when the supply is rated for 5A!? Why is it not dropping out during the pulse?Of course i am a liar!Why is er no dropping of the line in the middle of the picture at 9,5-Ampere...Because the performance of this power supply is that good !!!There are many measurements and information about this power supply on this page, use google translate is you cant read Dutch.https://www.circuitsonline.net/forum/view/110029Kind regardsBram

author: mikerj
author_rank: Super Contributor
timestamp: March 19, 2018, 11:26:19 am
content: Quote from: blackdog on March 19, 2018, 10:09:06 amA PSU with adjustable current limiting between 0-5 Amps should always drop out with a 9.5A load unless the current limiting isn't working?

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2018, 12:13:18 pm
content: Hi mikerj,A PSU with adjustable current limiting between 0-5 Amps should always drop out with a 9.5A load unless the current limiting isn't working?Yes an no, a part of the ennergy is comming from the output capacitors, and this picture is from one of my design stages.And if i want to test at a 9,5-Ampere level or even 15-Amp to see how the output capacitors are behaving during dynamic testing, i will do that.Kind regards,Bram

author: Cliff Matthews
author_rank: Unknown
timestamp: March 19, 2018, 02:32:32 pm
content: Quote from: blackdog on March 19, 2018, 10:09:06 amThanks Bram!I hope you don't mind it all complied into an English PDF?I did a Google translate on the whole thread and pasted it to a 118meg PDF:https://drive.google.com/open?id=1cEGFsj6kk3hI8KUnBXqF0GFBy2YFkUCLThere were 35 linked images that were too small to be usable, they are here:https://drive.google.com/open?id=1iKNivHfgNfMc6xBsww707_LqQOR2TIin

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 03:01:25 pm
content: Quote from: blackdog on March 19, 2018, 12:13:18 pmA part, yes.As you should always ask: "How much?"We can use the capacitor equation to find out.C is about 200uF, and pulse duration is 200us. Therefore:I = C * dV/dt(9.5A) = (200uF) * dV / (200us)9.5V = dVThe waveform shows less than 10mV of dV (100mV if it was 10x probe, I have no idea if that was accounted for or not), so this cannot be.Indeed, since the measured dV is nearly zero, we can say with quite good certainty that nearly zero energy is exchanged with the capacitors! Such is the begrudging truth about capacitors -- we almost always use them fordynamical stability, not for energy storage. Energy storage is only relevant when the change in voltage is large.Quote from: blackdog on March 19, 2018, 10:09:06 amPlease correct me if I am wrong. It doesn't seem to be a translation error; you specified a cable length and distant load capacitance, introducing dynamics other than the supply itself.Tim

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2018, 04:46:14 pm
content: Hi Tim,1eCan you explain to us, why you have such a negetive attitude?2eAnd again you give the impression that I am a liar, 10mV/Div you know for sure that you're not lying ...3eAnd of course I can not do a practical test, because according to your standards, you do not test a power supply in this way.4eSo ... if you do not show better behavior, I will not give you any answer anymore.Kind regards,Bram

author: Kleinstein
author_rank: Super Contributor
timestamp: March 19, 2018, 05:03:59 pm
content: The load test at 9 A is rather short - it's only 200 µs. The current limiting needs some time to respond and in this example it looks like the 200 µs are shorter than the response time of the current regulation.It depends on the application how much delay is good. In some cases a faster reaction might be required, but in many cases a slightly delayed response of the current limit is actually a good thing as the main application is as a voltage source.The LT1021 might be a bit of an overkill for a power-supply, especially if a NE5532 OP is used. For most cases I would consider something like 2xTL431 or an LM329 sufficient.It is possible to set the voltage via an DAC. In this case P2 is replaced by a fixed resistor and the reference voltage is supplied by the DAC. This way of adjustment also has an advantage for the control loop. It makes the control loop independent of the set voltage and thus gives very similar dynamic response at all voltages. In the current version with P2 to set the voltage, the response can be a bit faster at low voltages and slower at high voltage. This mainly effects the low kHz frequency range and might effect the performance for the case of a very large load capacitance. At a very low voltage setting there might be a slight chance to have instability with a lot of low ESR capacitance (e.g. something like 10000 µF of tantalum caps). In the extreme cases with the very fast loop also parasitic inductance of the layout and shunt can have an effect (both positive (especially for current control) and negative).The case of large, low ESR capacitance is kind of the worst case for a voltage regulator and some ringing is normal in that case. One can not expect to reach no ringing in that case - already having it not oscillating with any passive (that is RCL type) load is good. A response with little ringing is only feasible and required for a much more limited load range. One should do tests for both cases: a difficult load (where quite some ringing is acceptable) and the more easy ones that should not show much ringing.

author: Cerebus
author_rank: Super Contributor
timestamp: March 19, 2018, 05:24:02 pm
content: Quote from: blackdog on March 19, 2018, 04:46:14 pmHe doesn't, you just seem to react badly to any kind of criticism. Much of what Tim has had to say about the design is positive, but you choose to ignore that and over-react to the questions and criticisms. Vis:Quote from: T3sl4co1l on March 18, 2018, 09:36:53 pmMy experience of him tells me that Tim is both knowledgable and experienced, and generally quick to help. You can do one of two things, you can take advantage of Tim's interest and possibly improve the design or you can take offence, the choice is yours.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2018, 05:38:25 pm
content: Hi,Here are some pictures of a test with the Jim Williams dynamic Load.The current in this measurement is from 0,5A to 2A.Opamp is the NE5532A.Sensitivity of the scoop is 2mV/Div, no and I am not lying nowThis is the current puls, and yes, it is clean. 0.5 to 2-Ampere.This is the respons on the power supply connector, about +-3mV abberation.One of the the test setups.This is a picture how i make a coax connection to the scoop or measuring amplifier, is has a angel of 90 degrees,The BNC connector has an angle of 90 degrees to keep the coupling with the output wiring as small as possible.Kind regards,Bram

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2018, 06:24:13 pm
content: Hi Cerebus,OK This...My experience of him tells me that Tim is both knowledgable and experienced, and generally quick to help.You can do one of two things, you can take advantage of Tim's interest and possibly improve the design or you can take offence, the choice is yours.So Tim is "mister know it all" (your opinion) and who am i, to reply on thing he says...Uhm... Ooo Wait, I have designed this power supply and measured it extensively for my applications.but you choose to ignore that and over-react to the questions and criticismsOf course, I have not read that, and do you have a crystal ball?The points that Tim touches I have a different opinion, and yes Tim, like me, is entitled to his opinion.And yes, I think that Tim does have good knowledge of electronics, but I am not crazy either.But, let's talk about power supply's designs and their set-up and applicationsKind regards,Bram

author: Cliff Matthews
author_rank: Unknown
timestamp: March 19, 2018, 06:36:22 pm
content: Know-it-all's inject their image even when not wanted. I could make money just copy-pasting an encyclopedia out of Tim's contributions everywhere. I'm stupefied he never gets enough thanks. And.. if he's ever wrong, he's the first to facepalm himself.Now back to your wonderful design good sir..

author: Unknown
author_rank: Unknown
timestamp: March 19, 2018, 06:58:17 pm
content: I have the greatest respect for Tim's knowledge and helpfulness. I always thought he should be donated a "dumpster" scope for his prodigious contributions to this site (I think Tim is actually part electron). Things seem to have gone off the rails when the strong word "liar" was used, which, understandably, Bram (blackdog) became defensive. (look at the trouble you created, exe) Hopefully, the discussion will continue without the heat.

author: Cerebus
author_rank: Super Contributor
timestamp: March 19, 2018, 07:03:56 pm
content: Quote from: blackdog on March 19, 2018, 06:24:13 pmIf you wanted to prove that you react badly to even the most lightly implied criticism then you have succeeded.Quote from: blackdog on March 19, 2018, 06:24:13 pmAgain, nobody said that you were crazy. You just choose to be prickly and take offence at things that aren't said or implied.I also think I catch a whiff of Dunning–Kruger effect here. You've clearly learned a bit, but you act as if you've learned it all, because you act as if you're too good to listen to anyone else.Quote from: blackdog on March 19, 2018, 06:24:13 pmYou've designed and tested one power supply and you imply that is the ultimate in qualifications.Thatis being a "know it all" on scant evidence.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 07:11:56 pm
content: Now that's much more reasonable, 1.5A step and a simple blip.I don't get why it looks so ragged, if that's a noise thing or a trigger thing or what. Or why the trigger is off at 239ms, maybe that has something to do with it? Dunno.But this doesn't resolve the inconsistency of the earlier screenshot. If I had never seen that at all, I'd have no reason to doubt this! But with? I don't know.Quote from: Kleinstein on March 19, 2018, 05:03:59 pmShouldn't be a problem here -- offhand, if the current error amp has to slew 5V (it should really only be about 2V), it can do that, in and of itself, in less than a microsecond; and with about 100pF effective integration capacitance and a few kohms connected thereto, it'll only take about a microsecond.So I'm still perplexed how the circuit (if it was, in fact, built and measured as shown) could do that.Quote from: blackdog on March 19, 2018, 06:24:13 pmPlease understand:I don't give a shit about opinions. My opinion doesn't matter. Nor do other's. Brains conflate or fabricate all sorts of things. Brains filter everything through their own experiences. Everyone's opinion is so different as to be useless noise. And that is why, as they say: "talk is cheap". Because it's not worth anything.What I'm after is hard data. Numbers. Measurements. Facts.Instruments don't lie. They might be connected wrong, they might be set wrong, but once all of those factors are determined, there can be no lie.I mean no offense by my frankness. I just don't find pleasantries useful. Circuits certainly don't care; you can stick them with a burning hot soldering iron and they beg for more.HTH,Tim

author: BravoV
author_rank: Super Contributor
timestamp: March 19, 2018, 07:12:47 pm
content: Quote from: blackdog on March 19, 2018, 10:09:06 amFor you actually its never about electronics, its all about attitude, and pretty bad one.

author: BravoV
author_rank: Super Contributor
timestamp: March 19, 2018, 07:17:13 pm
content: Quote from: T3sl4co1l on March 19, 2018, 07:11:56 pmThis, which my self and also believe a lot of others here looking at their responses to you, don't mind at all, and I have learned a lot from you here in this forum, thank you.

author: exe
author_rank: Unknown
timestamp: March 19, 2018, 07:36:36 pm
content: Guys, please be polite and respectful to each other.Concerning PSU, there are tiny bits that could be done differently, but who cares? Final performance is that matters. Also, as I understand, each building block was comprehensively evaluated. So, I guess, if I start substituting components it's not granted I'll get the same performance (quite likely the opposite). No offence, but I thinknot everyone knows/understands/appreciates how much effort was put into design of this PSU(I know this because I read that circuitsonline.net thread, as well many others to better understand this circuit). Because it's so popular, people like to challenge this circuit, but not always with enough respect, imho.It's that I'm defending blackdog (although I would), it's my personal opinion based on my personal experience in designing power supplies (I designed 4 or 5 of them, all garbage, to be honest). That's why I'm looking for a good not-too-overcomplicated PSU that I could "enhance" with an MCU.Aaanyway, it's time for silly questionsIs a separate isolated power supply needed?Can I just use a charge pump to generate some voltage above the output? Let's say I can sacrifice some noise performance (my PSU enclosure is not shielded (plastic), so I'm not going to get microvolts of noise even if I try hard).

author: Cerebus
author_rank: Super Contributor
timestamp: March 19, 2018, 07:44:40 pm
content: While we're on the subject of testing PSUs with step loads: A while back I was comparing specs of a bunch of bench power supplies on the typical 50% to 100% current load step recovery time test. The reason for this was to develop a set of target test specifications for a PSU I was designing. What was conspicuous by its absence in those specs were specified slew rates for the waveform of the applied test load.What I'd found testing my own design in simulation was that I could get as stable as I liked but if I went to too fast an edge for the test load then it would (kind of obviously) all go to hell in a handbasket. So I wanted to know what slew rate was reasonable to test against. (Obviously in simulation I was able to apply physically improbable/impossible edge speeds.)So, the question is: What is a reasonable slew rate for an applied step load when testing PSU regulation recovery times? Are there industry standards? (I couldn't find evidence of any.) Or do people just test with whatever slew rate the electronic loads available to them support?

author: Cerebus
author_rank: Super Contributor
timestamp: March 19, 2018, 08:02:49 pm
content: Quote from: exe on March 19, 2018, 07:36:36 pmStrictly it needs to befloatingwith respect to the output 'ground' as opposed tofully isolatedfrom all the other supplies, i.e. it needs to move with the final PSU output voltage. Obviously you need galvanic isolation from the primary side, but you'll still get that if you steal a few 10s of mA from the bulk supply. The current demand is modest, but not trivial, so all the usual charge pumping tricks (voltage doublers, IC charge pumps and the like) are going to be a bit marginal. You need current consumption figures for the actual control circuity under worst case conditions to know if you'd be able to get away with it.A dirty little trick is, if you are using a toroidal transformer for your 'bulk' supply, you can add a couple of minor windings yourself to get your secondary supply. If you've got a 200VA transformer, it's not going to care about a couple of 2VA windings added on the outside. Obviously you've got to use the right materials, right insulation etcetera, etc. Bone up on transformer construction techniques before even thinking about attempting this, so you know how to do it safely. I say toroidal transformer because it's easy to add windings. Obviously youcoulddo the same with an EI transformer or similar, but it would be so much physical work that you'd be better off ordering a custom transformer from the start.

author: alm
author_rank: Super Contributor
timestamp: March 19, 2018, 08:18:07 pm
content: Quote from: Cerebus on March 19, 2018, 07:44:40 pmThis Keysight application notespecifies that the load should have a rise time at least five times faster than that of the power supply. Obviously this is not particularly helpful if you are trying to establish the latter. I guess that means increasing the slew rate until you observe no change in power supply behavior?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 08:45:18 pm
content: A charge pump would be workable, but probably too noisy (even if you don't need as low noise as the original design was aimed at!).If the floating negative supply can be eliminated (say by using a RRIO opamp, and omitting the LEDs, using schottky diodes instead?), then only the positive would be left, and a voltage doubler could be used to make an extra-high supply, which powers a CCS which powers the floating rail.RRIO amps don't tend to be very low noise, but that's alright.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: March 19, 2018, 09:03:37 pm
content: Quote from: alm on March 19, 2018, 08:18:07 pmThat's helpful, thanks.The minimum possible rise time of the PSU is pretty much limited by the ftof the pass transistors and is relatively easy to calculate/measure as is the actual rise time once the whole control loop is taken into account.My problem was just understanding what was an acceptable load profile for measuring transient response. Obviously the response overshoot/undershoot (and any ringing) of a PSU gets worse with faster edges on the test load, and gets better with slower edges. I suppose that my question reallyoughtto have been "What's the slowest slew rate that is regarded as an acceptable test?". If one sets the criteria as 5x faster than the supply rise time then that seems reasonable to me as you're basically saying "faster than the supply can respond". Kind of obvious really, but experience tells me that I'm more likely to miss the obvious than the subtle.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2018, 09:14:05 pm
content: Directly applicable example: I was testing a new DC-DC module against my bench charger/discharger supply. The former has loop bandwidth in the mid 10s of kHz, while the latter has a rise time of a few milliseconds. I was unable to detect any perturbation on the module output, while switching the discharger on and off. In contrast, a MOSFET + resistor + function generator easily shows the step response of the system, which in this case was limited by the module's output filtering network (amazingly, no changes needed on the compensation network!).Tim

author: blackdog
author_rank: Frequent Contributor
timestamp: March 19, 2018, 10:32:50 pm
content: Hi EXE,If its only a "normal" hobby supply, you can trow a few extra windings on the power transformer.But wat is your problem with a extra small transformer for the reference section?Its also possible to build de Circuits Online power supply, maybe with my modifications?https://www.circuitsonline.net/forum/view/131554Google translation time againThis is a link for a more up to date schematic, it is not a complete schematic, i removed the "difficult" components.But to make Tim more happy with me, (i mean this friendly) i changed al the diodes who are of a Schottky type and the last MOSfet i dit forget.Analog Devices has a new opamp, interesting for fast lineair power supply's.Its the ADA4625-1, minimum 5V en Max. 36V power supply.Fast, 48V/uSec, low noise, low bias current and relativ good DC specs.But what is really special, is the phase margin, which is 88 degrees, normaly it is around 65 Degrees for opamps.If its realy this good (i have to do some test) than i can choose for a lower value electrolitic capacitor on the output or better dynamic performance.We shall see what the tests will tell me.Kind regards,BramPSI started with a TL431 Reference for this power supply, look a the link below, the pictures tells enough if you cant read Dutch.https://www.circuitsonline.net/forum/view/116156

author: Jwillis
author_rank: Super Contributor
timestamp: March 20, 2018, 02:52:57 am
content: Most definitely an interesting design.I will have to give this one a try sometime.I'd like to shake your hand for all the hard work you put into it and would very much like to see the finished design.Bravo!

author: exe
author_rank: Unknown
timestamp: March 20, 2018, 10:39:22 am
content: Quote from: Cerebus on March 19, 2018, 07:44:40 pmWhat I often see in specs is something like "50us recovery time". As I understand, this reads as "in 50us or less the output level gets back to normal (or settles within 5-10%) under any step load". But 50us is waay to slow, imho.So, what is a good recovery time? My benchmark is LT3080 which I consider a fast regulator (there are some faster, but not as versatile). It has recovery time around a few us (the datasheet has a few pictures). I also check overshooting and, uhm, undershooting at step load. If it's below 100mV (at 3.3V) for step load 0.1A => 1A, then it's a good power supply. (Of course, slew rate of step load affects results)So, everything close-enough to LT3080 I consider a good PSU. I assembled my variation of blackdog's PSU, it was good. If I find oscilloscope screenshots, I'll share.BTW, there are many things affect performance. Like, pass transistor. My 2STA1943 (from ST) showed better performance than 2TA1943 from Toshiba. But I measure the transistor alone, not in the circuit. So, I don't know what would be real impact. Also dropout voltage affects performance a lot. BJTs don't like to work near saturation voltage (imho).My two eurocents, I'm no expert

author: Cerebus
author_rank: Super Contributor
timestamp: March 20, 2018, 11:42:49 am
content: Quote from: exe on March 20, 2018, 10:39:22 amYou misunderstand, what we're talking about is the slew rate of the current step waveform of the load that isused to testthe PSU load effect recovery time. We're not talking about the load effect recovery time itself.As it is, we already have an answer, one that convinces me anyway, which is 5 times faster than the rise time of the supply.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 20, 2018, 12:17:28 pm
content: Hi EXE,Measuring the performance of a fast linear power supply is not easy.And if you are say above the 2-Ampere output current, you will also have to take the EM field of the all wires with these current into account.I already showd a picture of my setup and the BNC connector has a 90 degree angle to the current wiring.Even de sense wires i try to give them a 90 degrees angel.The measuring point for performance measurements are the points where the sense wires soldered to the current wires.That is on the back of the 4mm banana sockets, every mm extra wire give a extra L and R.I want to measure the performance of the electronics ande not the wiring during testingAnd i explaned already that i also do testing with one of my Dummy Loads for more practical testing.If you connect your device under test with 1M cables to te Power supply, and most users do not twist the cables, all those nice specs are out of the windowMy standard measuring puls from one of my generators used with my Dummy Loads.This is a picture of how my 200-Watt Dummy Load is performing in one of my tests of this dummy load afther building him.To see how a power supply is reacting on realy fast pulses, i use my Jim Williams dummy load.Kind regards,Bram

author: Kleinstein
author_rank: Super Contributor
timestamp: March 20, 2018, 05:24:14 pm
content: There is a limited advantage of a super fast regulation for a lab supply: Once you add 1 m of cable the response at the other end of the cable will not be very fast anymore and one would likely see ringing from the cable. So if one has a critical load, there should be local decoupling caps as part of the test circuit. For the lab supply there is limited need to be faster than the cable - it may be even an advantage to show some damping to the cable instead of the virtual short.This is a little different with the LT3080: this is a voltage regulator that can be placed close to a possibly sensitive load. On the down side the LT3080 will not show such good DC specs - in parts due to not having extra sense lines down to the terminals. This not so good DC performance helps with AC stability and thus allows the fast response. Also the LT3080 is missing the adjustable current limit and the very fast recovery is likely with a small output capacitance, while low drop on fast transients is with more output capacitance.There are different requirements for a voltage regulator compared to a lab supply:The voltage regulator needs to get good response with a well behaved load with suitable load capacitance. Performance with a poorly chosen cap can be poor, up to oscillating. Some DC drop is usually acceptable.A lab supply should react reasonable well with most loads. And should not oscillate with any load (with less than 90 degree phase shift). This often requires the capacitance with ESR at the output that with a well behaved load mainly slows down the response. So optimization is not for the best cast tested with a voltage regulator, but more for the difficult cases. Usually DC load drop should be very low, which may require some compromises in the medium frequency range.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 20, 2018, 06:26:26 pm
content: I enjoyed reading thatWhat's a good view on the necessity of both sinking and sourcing on a bench supply vs lab-grade?

author: David Hess
author_rank: Super Contributor
timestamp: March 20, 2018, 07:05:05 pm
content: Quote from: exe on March 18, 2018, 08:54:29 pmThe inverting input of operational amplifier IC1a is tied to the output. The non-inverting input follows the inverting input due to negative feedback. Usually this is described the other way but the result is the same. So the top of P2 is equal to the output voltage. Since the reference produces an output 5 volts higher than the output, R32 which produces the reference current has a constant voltage across it.QuoteThe reference produces a voltage which is 5 volt higher than the output voltage so the return path for current through P2 is the same as the current through the output.The above is just a repetition of what T3sl4co1l said but may be more understandable if said in a different way.I like the floating design which allows the use of low voltage operational amplifiers and easy high side current sensing. If you are going to build a power supply at this power level, then the cost of an extra but small transformer is well worth it.I like that the output voltage does not rely on modifying the closed loop gain which compromises frequency compensation although not usually to a degree which matters.The displayed load transient response is poor; power supplies should have a very tame transient response to handle difficult loads. In production designs, I slightly overcompensate for the *worse* case of component and load variation.The controlled ESR bulk output capacitance is just a bad idea and indicates a problem with the frequency compensation. I have seen designs with comparable output voltage and current which were much faster and better behaved that used less than 1 microfarad of output capacitance in series with like 10 ohms although I question whether this level is performance is actually ever needed in such a high power supply unless it is part of a source meter. Someone else can run the numbers but this looks like dominant pole compensation on the output which largely defeats the purpose of using fast transistors and operational amplifiers.Why use a Sziklai pair instead of a Darlington pair when not required? This is especially odd since the output power transistor is a PNP instead of NPN although the difference in price probably does not matter anymore; PNP power transistors used to be much more expensive than NPN power transistors. I suspect the lack of local feedback around the Sziklai pair is causing problems. Designs like this using much slower transistors often include local feedback. Even fast Darlington designs often use local feedback.I would unload the outputs of the operational amplifiers with emitter followers or even FETs. Heavy loading compromises precision defeating the purpose of using a good reference and remote sensing. The ultimate performance of the suggested ADA4077 precision operational amplifier is completely wasted here.The not shown input clamp diodes on the operational amplifiers may be screwing up the performance during transitions between voltage and current mode. Also, the ADA4077 datasheet is horrible and I would never recommend or use this part because of it.I am suspicious that the ADA4077 and NE5532 displayed any difference in AC performance with everything else going on.I think the output TVS should have been an SCR crowbar circuit. I would expect it to behave like one once and only once if it was actually needed. I hope it was not needed for handling overshoot.The preregulator is clever but I am surprised switching artifacts do not show up in the output. Old designs used phase controlled SCRs and a big inductor, SCRs and transformer taps, or a buck switching regulator.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 20, 2018, 10:12:24 pm
content: Hi David,Can you explane this?The displayed load transient response is poor; power supplies should have a very tame transient response to handle difficult loads. In production designs, I slightly overcompensate for the *worse* case of component and load variation.David did you read everything?Wat is bad about this test see the two pictures below, do you find this horrible?This is the current puls, 0.5 to 2-Ampere.This is the respons on the power supply connector, about +-3mV abberation, sorry for the noise, the scoop is only 2mV/Div.Ooo...Why use a Sziklai pair instead of a Darlington pair when not required? This is especially odd since the output power transistor is a PNP instead of NPN although the difference in price probably does not matter anymore; PNP power transistors used to be much more expensive than NPN power transistors. I suspect the lack of local feedback around the Sziklai pair is causing problems. Designs like this using much slower transistors often include local feedback. Even fast Darlington designs often use local feedback.My measurements tell me different.The rightmost line (F) with the text Double Compound transistor indicates the output impedance of this power supply.And it is measured with the NE5532A as used opamp, can you point my to a powersupply that has a lower Ri over this frequency range?Ooo again...The ultimate performance of the suggested ADA4077 precision operational amplifier is completely wasted here...Nop, The DC stability is fine with this opamp in use, but the hf performance of the ADA4077 is ofcoure less then with a faster opamp.Some DC measurements: With the NE5532A at 15V output the DC drift was < 0.2mV, thats less than 0.0015% within a time of 6 hours.I am suspicious that the ADA4077 and NE5532 displayed any difference in AC performance with everything else going on.Your susspicion isnotcorrect :-)SwitcherI had already explained that I do not want a switcher in a low noise power supply.And that I finally switched to a transformer with tabs and three relays to limit the power loss.Some attentionThe only thing I think about is the opamp of the U control loop, in difficult situations there is quite a lot of energy going to the + input of this opamp.This can probably be solved with two diodes that are anti-parallel from the + output to the + input of the opamp.Current limitingSome pictures about the performance of this power supply and this time the I loop performance.This is measured over the current sense resistor.I used one of my dummy loads for a hi current peak.And this is the result without the transistor for the peak limiting, Within 10usec back to 5-Ampere, fast enough for you?But I was not satisfied with that yet, so I applied Q6 to further limit the peak current and now its fine by me.And again, during the development I sometimes test with other conditions than for a 30V and 5-Ampere power supply, which was the starting point.Some remarksLet me make it clear again, this is a power supply that has very low noise, good DC stability, and good dynamic performance.He must meet my requirements, for my purpose.I do a lot of measurements on sensitive electronics, and sometimes it is necessary that my LAB power supply can deliver more than 2-Ampere with great stability and low noise.As an example, I develop voltage references that are in small ovens.And testing these ovens i need a high performance power supply.I do not want to say that everyone should build this power supply.I want to show that you can achieve very good specifications with a fairly simple set-up.If you want, use part of this design for your application.This design is not meant for production, but only for my LAB.And if I want to use more expensive parts, I do that because I think this is necessary for my application.Kind regards,Bram

author: blackdog
author_rank: Frequent Contributor
timestamp: March 21, 2018, 08:15:14 am
content: Hi,Many comments are made about the output network of this power supply.And yes, this is built differently than with most power supply's.And Why?...Perhaps it is good that those who make comments about this configuration read the following document.www.bramcam.nl/NA/NA-01-PSU/Calex-Power_Impedance-Decoupling.pdfPerhaps this document makes things more clear.Kind regards,Bram

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2018, 10:13:14 am
content: Another way to look at it: a diffusion network. This gives an approximately Z ~ 1/sqrt(F) characteristic, which has equal components of resistance and capacitance. In a sense, it's the most lossy capacitor you can have, over a wide range of frequencies, and therefore at the least risk of underdamped loads above the controller's cutoff frequency. (The purpose of the control is to set Zo very low, so within the passband, one should not expect damping to occur -- attaching an LC network to the output, such that the resonance falls in the passband, should indeed be poorly dampened, an indication of the low output impedance of the supply.)Tim

author: Wortel
author_rank: Newbie
timestamp: March 21, 2018, 02:30:16 pm
content: Hoi Bram,Ik zie dat je de LM317 current source als voorbelasting van een hele tijd geleden er weer ingezet hebt, en volgens mij, net als vroeger, met de uitgang aan de verkeerde pin. De constante stroom loopt bij zo'n 3-pins regelaar toch van de output pin, *door* de weerstand, dus rechtstreeks aangesloten op de sense (of ground)-pin?Ik heb je NA-01 blog op CO helemaal gelezen, groot respect! maar is dat nu doodgebloed?Grtz,Wortel

author: blackdog
author_rank: Frequent Contributor
timestamp: March 21, 2018, 08:02:09 pm
content: Hi Carrot,No, the project was not dead, but when I was busy with the extended testing of the 230V net filtering, it went very badly with one of my family members.So it is still a bit of a sensitive point...But I had already decided to make a smaller finnishd version first (2.5A), so that I can place it better between my other equipment in my LAB.And 2.5-Ampere is sufficient for most of my applications, i started building of the next test version, see the picture below.The 5V Reference on this board is a MAX6350, also a fine part.The basic schematic is ready, the reference section, Power Off Glitch Protection, and control loops and power section are extensively tested and ready for use.Depending on the used opamps, the compensation capacitors can be slightly adjusted for optimization. (C16 and C16)Because there is now a somewhat smaller transformer for me, the 230V grid filtering will also be adjusted a bit.And if I have tested with the new Analog Devices opamps: ADA4625-1 then together with the lower 2.5-Ampere output current, also the 220uF capacitor over the output can be smaler.This in turn will make the peak currents in the connected D.U.T smaler, in fault situations.But these are no big changes.Anyone who still wants to apply DAC control can connect the top of R32 to the U-DAC (0 to +5V).You will have to calculate the voltage divider for the I-DAC yourself(0 to +5V).The simplest method is to replace P1 with a 1K resistor and R31 on the top of this new 1K resistor.If you want two current ranges, connect R27 or R28 to the output of the I-DAC.You will have to do it with this information, I will not give any further advice about it, you are on your oneWortel, thanks for detecting the error in the schematic, it is now Ok (i hoop)Kind regarts,Bram

author: David Hess
author_rank: Super Contributor
timestamp: March 21, 2018, 10:00:10 pm
content: Quote from: blackdog on March 20, 2018, 10:12:24 pmThe first test you showed indicates a problem in the small signal response.The test conditions for the others are not clear enough to rely on but they do seem to show a large signal response problem which might be caused by the error amplifier not being able to pull charge out of Q3 and Q6 quickly enough. I would usually expect a base-emitter shunt resistors on Q3 and Q6, capacitive bypassing to the base of Q3 and Q6, and much stiffer drive to Q3 and Q6 which goes along with unloading the outputs of the operational amplifiers to improve precision.The precision design shown at the end of my reply for instance would nominally use twice the drive current to the pass element while having 2 orders of magnitude less loading on the error amplifiers. This actually understates the difference because an integrated pass element has much higher current gain; idle current through the buffer is high to keep the buffer transistor characteristics consistent.QuoteIt is not a major factor that I have ever noticed other than more care being needed in controlling the local frequency compensation. I always found transient response to be dominated by the overall frequency response which is the point here; your first test indicates a problem in this area so a low output impedance from the pass element is more important than it otherwise should be.Some audio amplifiers use a couple of small signal transistors to add local feedback to the output transistors and reduce output impedance even further.QuoteHow good was the load regulation though? Using remote sense, I would expect the load regulation using the ADA4077 to be almost unmeasurable. An ADA4077 is not required to deliver a DC drift of less than 0.2mV.QuoteInternally they already have anti-parallel diodes (which is one reason I don't like AD's datasheet; it does not show them) but this is shorting the inputs during large signal conditions and may be upsetting the filter networks for the current and voltage setpoints. If this is happening, then it is going to show up as long settling time and poorer large signal response which does appear to be a problem. Low differential input voltage operational amplifiers are almost always a poor choice where open loop operation is possible but in this case even with the feedback clamp network, this may be causing a different problem.QuoteThe performance is good. I just think it should be much better given the complexity and parts selection.QuoteIf it is good enough, then it is good enough and best is the enemy of good.I have designed and built my fair share of special purpose power supplies for internal production and testing. My favorite involved the design shown below but modified for extreme precision and low noise using an LT1007 precision operational amplifier, precision reference, and lead-lag frequency compensation which involved 2 more resistors and 2 more capacitors not shown in both the feedback and input networks to optimize dynamic performance. Load and line regulation from 0 to 1 amp ended up being too good to accurately measure at better at 1 ppm or better.Note that I am not suggesting this design to you or anybody else. I just used it as a handy example.I have been considering doing a precision floating variable output tracking implementation but at lower maximum current but the problem is handling power dissipation at low output voltages without excessive power dissipation.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 21, 2018, 10:10:19 pm
content: Hi DAvid,Thank you for your remarks, it is late now here in Amsterdam and tomorrow i wil explane some more things.Like the photo you probably saw first, that is the photo with some ringing, that is from my "torture test" for power supply's :-)Talk to you later.Kind regards,Bram

author: exe
author_rank: Unknown
timestamp: March 22, 2018, 10:00:52 am
content: Thank you very much, guys, for all the information you provided.I entered EE about three years ago (as a hobby). My first device supposed to be a DIY power supply. Now, three years later, after hundreds of hours of reading forums, tech literature, experimenting and building multiple prototypes I'm nowhere near finishing my very first project. But, anyway, please keep discussing. Topics such as frequency compensation or choosing the pass element are quite rarely discussed (at least I didn't find much info, yet alone measurements). Who knows, may be after this thread I'll finish designing my "ideal bench power supply".

author: Wortel
author_rank: Newbie
timestamp: March 22, 2018, 10:53:34 am
content: Hi Bram,Sorry to hear about your family going bad, hopefully things are going better now.You mentioned a smaller version of the NA-01 low noise PSU on Circuits Online if I remember correctly. Is the schematic available somewhere? That would be nice!Wortel

author: Zero999
author_rank: Super Contributor
timestamp: March 22, 2018, 11:05:33 am
content: Quote from: blackdog on March 21, 2018, 08:02:09 pmOh no, don't build it like that. The creapage between the primary and secondary is very unlikely to be sufficient, for isolation from the mains. All that's needed is a voltage spike and the tiny gaps on the perf board will arc over, connecting the secondary side to the mains.Put the mains and secondary sides on different boards, that way you can be sure they're properly isolated.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 22, 2018, 12:54:25 pm
content: Hi Hero999,Thanks for the warningBut i already now that, i dril out the holes around the 230V powerline connections before i do the real testing, this is only the reference section build.This is not a final circuit board, i drill a lot in these boards, like in one of my high performance preamps to get the paracitics down.If you want, take a look at this design of one of my low noise preamp, design for measuring at power supply's and some extra holes in the circuits board!Date/time: vrijdag 17 maart 2017 16:35:45https://www.circuitsonline.net/forum/view/135863Back on topic,The most important point in the development of a linear power supply is the phase margin of the control loop.A standaard opamp has about 60 degrees phase margin, every component you place in the loop will bring this phase margin down.Placing a power transistor(s) in the loop brings it down, if don't place a capacitor over your feedback resistor, almost certainty there will be instability horror.Look at the many power circuits on the Internet, most of them are compensated to "dead"...This is because the person who developed the circuit has not understood how the circuit works.Two examples, Dave Jones fist power supply, I knew from the beginning that it would fail.He finally learned his lesson with version-2 :-)The second example is e.g. Gerry Sweeney power supply, look at his video's.Hi did a fine job, except for the loop stability...Do not think that I am a gentleman who knows everything, as someone declaimed here on the forum...In the beginning of the 1974 i started with opamps like the uA709,my college and i were complaining about it alway was oscillating, what a bad opamp!But of course we were bad technicians, we did not read the datasheet and aplication notes, we were fools.Take a look at the LM709 data sheet, and see how universel this opamp is, even gains of 80 dB are possible with reasonable bandwidth.Would I still use the uA709, no there are now much better opamps for sale, this was only just to make a point clear.And regularly I have to learn, make stupid mistakes, I am just a human being.Thus e.g. only using a fast opamp does not make a good power supply.The intention is that when you develop the power supply, and that all parts of the schematic are well matched to each other.The Harrison design i use (from the late 1950's) is really beautiful.And my part of it, is using modern parts and tuning these for optimum performance.All designs with extra transistor and opamps in the loop will never reach the performance of this simple design.And again, most LAB power supply do not need this High End performance.I have a Dutch famos brand switching power supply, 30V and 5-Ampere, the dynamic performance is "not so good", why?There is a lot of capacitance on the output en the loop is not fast.But i still use it regular wen i need higer currents then 1-Ampere and when its not critical.The time is up, have to work, laters more.Kind regards,Bram

author: Cerebus
author_rank: Super Contributor
timestamp: March 22, 2018, 03:00:44 pm
content: Quote from: exe on March 22, 2018, 10:00:52 amDon't forget that a power supply is just - in essence - a DC coupled power amplifier. So, texts covering power amplifiers go over much of the same ground. If you want a discussion of pass element choice that's a place to look for it. Just make sure you stick to reliable sources, otherwise you'll find yourself going down the audiophool rabbit hole.

author: David Hess
author_rank: Super Contributor
timestamp: March 22, 2018, 06:54:58 pm
content: Quote from: Cerebus on March 22, 2018, 03:00:44 pmThis is especially the case because common voltage feedback operational amplifiers duplicate the topology used in audio power amplifiers with a low voltage gain differential input stage, high voltage gain level shifter, and unity gain push-pull output stage. Remove the upper or lower half of the output stage and a DC coupled audio power amplifier becomes a regulated power supply although some power supplies keep the other half of the output stage for better control or 2 quadrant operation.The transistors blackdog used in his design are ring or distributed emitter transistors with low Ft droop and hfe droop intended for audio power amplifiers and work great for high performance regulators. Similar old but now inexpensive parts include the TO-220 D44/D45 transistors which were sometimes found in high performance point of load linear regulators for CPUs and low noise voltage regulator designs.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 22, 2018, 08:56:44 pm
content: Hi,It is true what David says about the transistors that I used.And I also tested with the D45H, and that went not wel, kaboem!I tested with 4x D45H, this transistor is useful with a 30V 2,5-Ampere power supply, but only if a pre-regulator is used or say transformer tabs to lower the dissipation.The SOA characteristic is not so good of these transistors, in the Motorola datasheet you can see what the SOA is at 70C.At 30V Vce max 0.6-Ampere..., so you need a good heatsink.My preference is for the transistors that are in the schematic.Kind regards,Bram

author: David Hess
author_rank: Super Contributor
timestamp: March 22, 2018, 11:02:25 pm
content: All of the usual warning about SOA (safe operating area) apply to the D44H/D45H series just like any other transistor. In addition, the high Ft and hfe of ring emitter parts makes them more susceptible to destructive local oscillations if care is not used. Just like with an audio power amplifier, the equivalent of a series RC Zobel network is often included from the output to ground.The audio guys wanted larger parts like the 2SA1943 to get costs down but that is just good for us if we can repurpose them for high performance power supplies. I tend to prefer smaller parts though because even if I have to use more of them in parallel, cooling is easier because the case to heat sink thermal resistance is distributed over a larger area and a lower total thermal resistance is possible. On the other hand, this does make for more parts and layout complexity and an increased possibility of local oscillation if care is not used.I have a list of the various On Semiconductor ring emitter parts intended for audio amplifiers and a quick check indicates that the D44H/D45H has about a 50% better ratio of power per dollar. It would not surprise me however if they are also counterfeited more than the less common larger transistors.I agree with T3sl4co1l though that these fast transistors are of marginal benefit in a general purpose bench power supply simply because it defeats the purpose of having excess high frequency performance at the end of a pair of test leads. This does not apply for a point of load regulator and special purpose applications where the load is close.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 23, 2018, 02:10:23 am
content: Relevant example: in a recent design for a power supply, I wrote into the specification that the supply will have a maximum equivalent inductance characteristic. The purpose of this is twofold: 1. the customer had been testing with external power, on leads of so-and-so length, therefore I can't do any worse than what is already known to work, if I meet this; and 2. the supply will have effective inductance due to control loop roll-off and output filtering, and this spec gives me a suitable (finite, nonzero) target to design to.Tim

author: blackdog
author_rank: Frequent Contributor
timestamp: March 23, 2018, 08:31:36 am
content: Hi guys,I am aware that with a power supply without external sensing, I have little control over what happens after the wiring at the load.Earlier I have made a comment about that.And once again I want to point out that this power supply is for controlling precision circuits, as I indicated earlier.Something like in the development of reference ovens, I want to have everything powerd from +15V.So the oven heating and the reference, connected via short and twisted wires.This happens in this way, both parts of the circuit go with separate wires to the power supply.These parts are not electrically connected, except on the terminals of the power supply.The heater can be a linear one, slow PWM(Arduino PWM) or a normal PWM circuit.Normally i remove the fast edges on the pwm circuit, to get the noise down and it is easier on the power supply control loop.And of course there is a lot of attention on decoupling.A remark can be, that i can test with two power supplies, so i have no connection at all.But that would not be the reality of the finished project.Furthermore, it is not too difficult to bring out the sense wires with some extra security components to get better control at the load for short 4-wiring applications.About the smaller power transistors, i did test this, but for 30V 5-Amps 4 of them where not enough to make it reliable.I like the D45H D44H transistors, but nut at this power level, and again price is not a issueKind regards,Bram

author: exe
author_rank: Unknown
timestamp: March 23, 2018, 10:28:05 am
content: Concerning discussion "how fast bench PSU should be", I'd say I want a faster one because I use them to supply, say, esp32 which has built-in WIFI. So, with a poor power supply it will reboot while transmitting. To some extend this can be fixed with more output capacitance. Although, I didn't do any measurements, so, may be, just putting a smaller cap closer to the load could help.Another topic interesting to me is how to parallel caps. Say, my power supply has output cap, and my load has an input cap (they may resonate). So, may be low-esr output caps is a bad idea? But I'm too far from understanding these matters.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 23, 2018, 02:57:53 pm
content: Hi,My opinion1eA LAB power supply must be nice to his load, realy taking care of it.2eThis implies, among other things, that he has the smalest possible capacitor at the output and has a rapidly responding current limitation.3eNo power On/Off and enable switch abberations.4eLow Ri and low noise.5eStable with a wide range of loads.But, the design i'am showing here is not for large inductive loads, a 2 of 4 quadrant powersupply should be used then.6eThere is no "one size fits all" powersupply.7eAs stable as posible with high capacitve loads, normally not a problem because the wiring resistance to the load.Just an example of which I think the designer has not understood how to do it right.My Rigol DP832 power supply, 2x30V at 3A and 1x 5V 3A.On the output banana jacks there is 470uF!That tells me that they have not had good control over the loop stability during development.In fault situation the energie in the capacitors can kill your load...I handle this rule of tumb, use about 50uF for every Ampere output current (for linear LAB power supply's)Something else with this power supply, the sense wires were bundled together with the live wires, so stupid ...A simple different twist fixt the problem, see this topic:https://www.circuitsonline.net/forum/view/130740#highlight=dp832The better you design the loop control(phase margin), the lower the output capacitor can be. (within a certain range)My design has two capacitors and resistors on the output so this wound not ring by itself (ofcouse i tested this :-) )The serie resistors will almost certainly kill the Q of these components.Kind regards,Bram

author: technogeeky
author_rank: Frequent Contributor
timestamp: March 23, 2018, 03:42:13 pm
content: Quote from: exe on March 23, 2018, 10:28:05 amThis is where "high speed" power supplies like theKeithley 2304come into play. There are HP models for these as well. These are also known as "mobile communications" power supplies. There are also similar models that are known as "battery simulators". Dave has done a teardown of one of these. The primary difference between a high speed power supply and a battery simulator is that the battery simulator has circuitry to control the output impedance to a selected value.These power supplies usually have very little output capacitance. I am not 100% sure, but I believe in general these power supplies have a dual push-pull topology. They are essentially the positive voltage half of a SMU (source measure unit).One way you can determine if the supply you are looking at is a high speed supply or not, even without reading the detailed specification, is looking fortransient responsein the specification. In theend (page 111)of the Keithley 2304a manual, you can see that they specify the transient response time to be less than 50 (resp. 40) microseconds. This may or may not seem fast to you, but keep in mind for these high speed power supplies this is a1000%change in load. Most supplies will only specify for a100%change in load. The numbers listed in the manual are the worst case. In practice, I've measured times as low as 5 to 10 microseconds for a 1000% change in load.In any case, the overall point of these power supplies is to be able to simulate/emulate the battery or power source for a mobile communications device (anything with a transmitter, really). It can source current (normal usage) and sink current (emulating a battery being charged). In such situations, you often use a small amount of power normally until the wireless radio transmits. At this point, the current usage can increase by a huge amount, but only for a small amount of time. So the overall picture to the power supply is a small quiescent current with a pulse train superimposed on it. All of thiscouldbe done using an oscilloscope, current probe, and a battery bank.Note that these arenotlinear power supplies. They have extremely good noise characteristics for switching power supplies (almost as good as linear, which is an impressive feat).: battery simulator.

author: David Hess
author_rank: Super Contributor
timestamp: March 23, 2018, 03:45:59 pm
content: Quote from: blackdog on March 23, 2018, 08:31:36 amWhether external sensing is used only matters at low frequencies. The contradiction is designing for fast loop response to provide low output impedance at high frequencies when the inductance in the external leads prevent the control loop from even seeing the load at high frequencies.QuoteI have done this before down to the microvolt level at load currents of amps and it works fine but capacitive decoupling at the load is what lowers the impedance at high frequencies. The high frequency performance of the power supply becomes less relevant at the leads become longer.When using remote sense, be sure to limit the difference between the force and sense lines at the power supply with diodes or resistors to prevent the output from running away damaging the load if a sense line becomes disconnected.Quote from: exe on March 23, 2018, 10:28:05 amUsually fast closed loop designs with high frequency transistors are used to provide a low AC output impedance while using only a minimum of output capacitance improving the constant current performance. A standard power supply might have 47uF per amp of output capacitance to provide a low AC impedance while a high frequency design can get by with less than 0.1uF and a carefully controlled ESR to preserve stability.QuoteThe power supply's output capacitance forms part of the frequency compensation and if it has an ESR which is too low, excessive phase shift can lower phase margin compromising transient response or even causing oscillation. So the output capacitor may have a very controlled ESR to prevent problems. The bypass capacitors at the load are much smaller and located remotely where the series resistance is greater so low ESR does not matter for them.This can be seen in the application circuits for regulators like the 78xx and 79xx series where a small low ESR ceramic bypass capacitor is shown at the input if the input bulk capacitor is located remotely but the output shows *only* a small high ESR tantalum or aluminum electrolytic capacitor. These regulators are well behaved and will not care about a 0.1uF ceramic capacitor at their output but this capacitor does nothing useful except perhaps keep RF out. For power supply bypassing purposes, it needs to be placed at the load instead.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 23, 2018, 04:25:05 pm
content: Hi David,I talkt about remote sensing on the Dutch forum, and explaned that if you want a fast reacting power supply, you need to build it like a HF amplifierTwisting high current cables, twisting your sense wires, keep the buffer capacitor close to the power stage, the output connectors close to the power transistors etc.And of course the development of a fast power supply means that the wiring to your load is becoming increasingly important to keep the Ri low.I did some testing with my HP 6632B with a 1-meter long connection cable with remote sensing.Of course, capacitors were needed at the terminals for some good dynamic behavior, but you cant go to high.There is a lot of usefull information about sense wires in the PDF of the KeySight 6632B.Long cables together with sense wiring can almost never result in a fast power supply due to the induction of the wiring.You cant winActually, we should have a separate section on this forum, which contains all the basics for designing a power supply for various applications where we can put our joint knowledge together.There is so much misunderstanding about the design of power supplies, look at the many compensated to dead designs on the Internet...Kind regards,Bram

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2018, 08:17:07 pm
content: It depends on the application on how fast the current limit should engage. Sometimes you want a very fast current limit, essentially limited by the output capacitor and sometimes one wants a current limit that is a bit slow and can deliver more current for a few ms. In the slow case one can usually tolerate more capacitance at the output.The simple single pole loop results in a constant output inductance for the regulator. However this inductance can be close to ideal and thus cause ringing together with a low loss load capacitor. For small caps a high loss output capacitor (e.g. the extra series resistors) helps to dampen the possible resonance, but this does not work anymore with large caps. It's only the lead resistance that can save the day. With external sense wires one may loose this resistance - so adding external sense wires can get tricky.For the sense wires one should also consider the case of the load wires interrupted and thus delivering power over the protective diodes / resistors.For very low output capacitance a push pull output stage with some standing current helps as this essentially the only way to provide a low output impedance even with low load current. At low current one way output stages often get slow and thus might get unstable there.

author: exe
author_rank: Unknown
timestamp: March 23, 2018, 08:34:18 pm
content: Quote from: technogeeky on March 23, 2018, 03:42:13 pmTo me, that's just a matter of pre-loading. Like, the "useful" load changes from 1mA to 1000mA. So, it's like 1000x change, right? But if I put an internal load sinking 200mA, the actual change for the PSU would be just 5x. So, this let's getting away with slower power supply. At least that's what I get in a simulator.Of course this wastes power, that's why I want this current to be settable.But this also opens another big question to me: how PSU behavior changes with the load. I think this should also affect stability.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 23, 2018, 09:12:03 pm
content: Hi exe,You can use my design and scale it down is you only need say 1,5 Ampere.The output capacitor can then be scaled down also, so the peak current wil also scale down, the power supply is fast enough.But maybe a nother design is better for you, one moment...Look at this, it is a first setup...The original schematic you wil find in app note 16 from LT, is uses en LT118 and a LT1010.My version is absolute not finished!It need extensive testing and probably the zobel network can be smaller.The first tests look promising, verry fast acting current limiting.The easy way for more current in the orriginal design is to use say 5x LT1010 TO220.It is easy to parallel them, the have a internal 7 Ohm output resistor and that maks them perfect for capacitiv loads.Read the datasheet and app notes on the LT1010.Do not use a high voltage power supply if your requirements for only 12V output voltage.This wil keeps the dissipation down and easyer to build.Kind regards,Bram

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2018, 09:35:10 pm
content: With a simple output stage the load current can effect stability. Usually the transistors get slow at low current. There is an additional change with the Sizlaki stage. With MOSFETs the effect could also be quite significant.So coss-over to instability usually either starts at low current because the output stage gets to slow or at high current because the output stage is fastest there. An internal constant current load is only a partial solution, as there can be times when this current is used to discharge an output capacitor when recovering from overshoot.There is also a difference between power supplies with a more current controlled output stage and those with an emitter-follower type. The emitter-follower type can be more sensitive to changing current. To get away from the low current problem, a push pull output stage can help.

author: Zero999
author_rank: Super Contributor
timestamp: March 25, 2018, 12:01:03 am
content: Quote from: blackdog on March 22, 2018, 12:54:25 pmThen please make that clear in future, especially as this is the beginners section.

author: blackdog
author_rank: Frequent Contributor
timestamp: March 27, 2018, 12:15:05 pm
content: Hi Hero999,You are correct, i dit not think of that...And I would like to add that if you test such a printed circuit board as I showed, you will isolate all mains connections!Also the live connection on the bottom of the board.When I test, I place a piece of plastic on the bottom so that I can not touch the live parts.And on the top side i use electrical tape to isolate the live parts.If you place a line transformer on the China boards, dril at least one row of holes around all the live connections.Make sure that all burrs from drilling are removed on both sides and clean the board with alcohol.So be carefull!Kind regards,Bram

author: exe
author_rank: Unknown
timestamp: May 27, 2018, 07:53:11 pm
content: Dear blackdog, what's the purpose of D13 and R39 onhttp://www.bramcam.nl/NA/NA-01-PSU/NA-PSU-50.GIF? As I understand, D13 protects BJT's base from being pulled too low. Is this correct?R39 remove leakage currents from the base? So, I expect some current flowing through it. Can I connect it before the shunt resistor so this current is accounted? (although, I guess, this won't make a big difference).

author: blackdog
author_rank: Frequent Contributor
timestamp: May 27, 2018, 10:52:33 pm
content: Hi exe,You are correct, for the diode and the resistor!It is a good tip.I will place R39 in front of the 0.1 Ohm current sense resistance to lower the minimum current.Kind regards,Blackdog

author: AG7CK
author_rank: Regular Contributor
timestamp: May 30, 2018, 09:46:12 pm
content: Quote: "The Harrison design i use (from the late 1950's) is really beautiful."Agree. Harrison manuals should be obligatory reading for anyone who is "building my own power supply".I find it amazing that none of the posters here seem to recognize that the topology you use is the hallmark of HP power supplies from the Harrison period until "modern" Agilent power supplies (and probably also Keysight-branded smaller bench supplies up until today [no schematics]).Anyone wanting to understand your "version" should start by reading Harrison Laboratories / HP Harrison / HP / Agilent manuals with theory of operation and schematics. There are dozens of them on the web. It hardly matters where you start because they are almost all based on the same theme: floating reference, dedicated bias supply and a current controlled linear power section.Those that are historically inclined can start herehttp://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1962-07.pdf.

author: exe
author_rank: Unknown
timestamp: May 31, 2018, 07:24:23 am
content: Thanks a lot, haven't read it yet, but looks like exactly what I need.