Low Noise, Reasonably High Power, Isolated, Programmable Power Supply - Page 1
Forum section: electronics / projects

author: Taarak
author_rank: Newbie
timestamp: July 18, 2023, 05:27:40 am
content: Hello all, and sorry for the very long post lol.I'm an electronics technician (working on becoming an actual engineer now), and I've built power supplies before. Some low power linear ones, a 96W switching one I have by my side me right now, and others. But I wanted to build one that's actually good, for a couple of reasons. Such being the challenge itself, having more power supplies to use for stuff, and just the learning opportunity.I want to build power modules, which connect to a backplane that supplies power and controls each module with a microcontroller. I want each module to be able to output 0-32V at 0-5A, with (hopefully) millivolt and milliamp precision, and <300µV of RMS noise (just to beat most lab power supplies out there). This is probably way too much to ask for a project that's coming from me, but that doesn't mean I'm not going to try it.Anyway, for the modules themselves I thought of using an isolated switching pre-regulator, and a linear post-regulator. The switching pre-regulator is going to create a power rail for the post-regulator at the programmed output voltage plus around 5V (stopping at 36V). It needs to be reasonably low noise, and it doesn't need to have very high precision or resolution. But I don't care too much about it right now, as I'm designing the linear post-regulator first.I made a base design, simulated it in LTspice to make sure it works minimally and went improving some things where I could. I've also researched some other projects, and it seems like people are doing things in a similar way to me, so I'm not too out of track it seems, but I'm going to explain what I did to try to get some insight anyway.So firstly, since I can guarantee that the input voltage is going to be only about 5V above the output voltage, I can use a single pass element and dissipate around 25W at full load under any output voltage, which should be doable. Because of that I'm using a single TIP142 as the main pass element, because it has a high gain and can dissipate a lot of power. Its maximum VCEat saturation on 5A is 2V, which is a lot but should be perfectly fine. Its VBEis specced at a maximum of 3V, which is massive, but even then with the 770mV max of the BC850C driver and drop of the base transistor is fine for the error amplifier at maximum output voltage (which has only 4V of headroom).I used the BC850C as a general purpose transistor, because it's "low noise" and cheap anyway. For the opamps I used the OPAx182, because its ultra precision is going to help, and it's available.The voltage error amplifier is essentially a non-inverting amplifier with more steps. It has a gain of 6.4, so an input of 0-5V from a DAC can generate 0-32V at the output. I don't really know what I did with R206 and C205 there, but C204 and R205 helped it not ring like crazy when coming out of constant current mode in the simulation.The over-current protection circuit works by essentially comparing the programmed maximum current output and the current going out of the power module. If IMON, which comes from a current shunt amplifier with a gain of 100 (so 5A at 10mR shunt means 5V, pretty convenient), is smaller than IREF, that comes from a DAC, the difference is negative. That means the opamp saturates to ground and nothing happens. When the measured current goes over the programmed maximum, the opamp applies it's massive gain to try to saturate to the positive supply, but as it is doing so Q203 activates and pulls VREFdown, so the output goes down and the current follows. This will essentially put the output into constant current mode.I saw some people putting this circuit between the error amplifier and the main driver transistor, but when I did that in the simulation the error amplifier saturated and didn't have time to go back to normal when constant current mode wasn't needed anymore, so it started ringing a lot (voltage jumps very high, constant current activates again, voltage goes back, constant current deactivates, cycle repeats). Pulling the programmed voltage down worked much better, at least in the simulation.And finally, R202 is there to give some constant load to the pass element, and C201 for some improved load response (it is very small because I want the over current protection to actually work).I did all this, but now I'm stuck because I lack knowledge, so I come here to ask for you all. Firstly, I need some way to be able to calibrate the error amplifiers if I want any resemblance of millivolt and milliamp precision, but I have no idea how to do so. I looked around on google (actually ddg), but couldn't find anything.Secondly, this circuit is probably highly unstable. In my limited testing it's working fine, but I didn't experiment with rapidly changing loads or anything like that. I understand the concept of phase and gain margin for measuring stability, but I don't know how to actually measure that, or what to do when I have the graphs that say it's unstable.And lastly, my attempts of measuring PSRR have resulted inwildinstability, and I don't know if that's because I tried measuring it wrong, or my circuit is just that unstable. I tried putting a voltage source with a small noise signal between the main power rail's negative pin and GND in the simulation, but that resulted in the output oscillating between 0V and saturation very rapidly.So, what are your thoughts? Can you point me to any resources where I can learn more about this kind of topic and try to solve the problems I'm having? Or just general thoughts about what I described. Also note that my google-fu is kinda weak, so even though I tried finding the resources myself, I couldn't find anything that I understood enough to use in my project, but if you do please let me know.Thanks!

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 18, 2023, 07:15:20 am
content: My first thought was if you want to challenge yourself with a new design, why would you choose a 1980 era tech. The backplane idea and switching pre-regulators suggested you are to engage on the system design aspect and not to center your thinking around a classic discrete NPN pass transistor regulator.If I were in your situation I would extend the Ananlog.com bench supply design to 5A , 30V and make it digitally controllable:https://www.analog.com/en/technical-articles/high-performance-portable-dc-bench-power-supply.html

author: Taarak
author_rank: Newbie
timestamp: July 18, 2023, 04:38:10 pm
content: Using a regulator such as the LT3081 will completely solve instability issues, will have guaranteed output to 0V (which I'm not really sure my circuit has), and completely eliminate any kind of noise issue I would have. It also seems like it can be controlled digitally easily.But it doesn't have sense pins (thought a makeshift load sensing method is presented on the datasheet), and worst of all it's very expensive. I would spend $13.54 per board if I was buying ICs in the thousands, and $34.12 in small quantities! I guess I didn't use an integrated regulator from the start because of this, and because I thought making it myself would be easy anyway ("it's just a linear regulator, how hard can it be?"). I'll keep that in mind anyway.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 18, 2023, 08:24:27 pm
content: Quote from: Taarak on July 18, 2023, 04:38:10 pmHow many do you intend to make ?You can find LT308x on Aliexpress, but still pricey.Another common approach to linear supplies is to use LM317/LM338 as a 'smart power transistor'https://www.lcsc.com/product-detail/Linear-Voltage-Regulators-LDO_HGSEMI-LM338T_C512788.htmland you create a small negative rail, so the control pin can go to -1.24V driven by an opamp.You can still use a low noise reference and a low noise opamp to create a low power/precision 0~30V, offset by -1.24V.The noise in the LM338 is then a much smaller % of output.One example :https://electronics.stackexchange.com/questions/533606/lm337-based-current-limiting-power-supplyThat uses the regulator as reference, for a simpler, but lower performance design.You can get current sense amplifiers these days, that make current display easy. Pick one with a wide common range, that can read to below 0V for shorts.This is cute, claims 0-50V/8.1A, with metering includedhttps://www.aliexpress.com/item/1005005622688705.htmlSimilar, simpler LCD that looks to display to 1mA - I find 10mA LSD is a painhttps://www.aliexpress.com/item/1005005253832884.htmland display only, useful for any power supply design -https://www.aliexpress.com/item/1005004957761374.htmlhttps://www.aliexpress.com/item/1005005028995593.htmlThose seem to use small MCUs like N76E003 as the ADC/Display manager, & their 12b ADC makes 1mA LSB at 10A optimistic, but a 5A one would get close to a true 1mA

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 18, 2023, 11:39:38 pm
content: QuoteYes, BOM costing is an engineering task. But why automatically race to the bottom?Seems you pretend it a commercial endeavor, and a market for this modular low noise power supply exist. How would you cost the engineering efforts going into the stability design and testing, then the thermal design and testing? How would you estimate in terms of $$$ the time it takes and the cost of opportunity being on the market vs. tinkering with discrete parts?Asking as you becoming an actual engineer. Perhaps a product manager too.

author: Taarak
author_rank: Newbie
timestamp: July 19, 2023, 12:03:03 am
content: I want to put 4 modules into a single enclosure, and make maybe 2 enclosures. So I'm not mass-producing these, at least I don't intend to for now, but it's still going to be a considerable amount of money spent on components that I'd like to minimize a bit.As for PCB.Wiz's post, using a simpler linear regulator as a "smart" pass element seems interesting. It's cheaper than the LT3081 and will probably make achieving low noise and unconditional stability easier than doing everything with discrete components. The LM338 you mentioned has, at least what seems to me, pretty garbage load and line transient response. But I guess that driving the adjust pin with an opamp externally will make it respond better.And for the metering, I wanted to everything in the module itself. The backplane-thing with the microcontroller will communicate with the board to set the DAC outputs anyway, I might as well include the metering. For that I thought of two possible methods, the first being just finding an appropriate digital power monitor. I already have the 10mR shunt for the current limit circuit, and some precision digital power monitors are pretty cheap. The second option is using a two channel ADC to measure the current directly from the analog current sense amplifier that's already on the schematic, and the output voltage with a voltage divider. Whatever one of these I choose, with a 16-bit ADC or digital power monitor I can theoretically get up to 1mV and 100µA of resolution.Using an ADC directly is probably going to be more precise, but then I need a way to reliably calibrate the voltage divider to maintain accuracy. Just like with the other opamps if I still end up using them.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 19, 2023, 02:21:57 am
content: Quote from: Taarak on July 19, 2023, 12:03:03 amIf you have a MCU already, Parts like INA236 claim gain errors of 0.1% on their internal dividers.They will have much better LSD precision than the AliExpress modules, whose main appeal is 'ready to go'QuoteYes, almost any 1.2V regulator could be used, with sharing resistors if you use < 3A modelsOnce you add the small negative rail, a lot more options arrive.There are also parts like LD1084/LD1085 etc that have lower dropout than LM338.

author: Taarak
author_rank: Newbie
timestamp: July 19, 2023, 06:58:41 pm
content: Quote from: PCB.Wiz on July 19, 2023, 02:21:57 amThe INA236 has 0.1% precision internally, but the sense resistor itself is normally 1% or worse. And it also doesn't have 1mV resolution, it's 1.6mV because of the higher maximum voltage it supports. That's why I said the ADC method would probably be more precise, if I can calibrate it easily.It would be easy to put a digipot in the middle of a voltage divider with high value resistors for fine adjustment. But then I'm afraid that the resistors themselves will be too noisy to achieve the 300µV RMS noise goal I set to myself.

author: temperance
author_rank: Super Contributor
timestamp: July 20, 2023, 02:07:39 am
content: For a beginner learning about transistors, MOSFET's, amplifiers and feedback loops,... tinkering about different gain stages required to reach the stated goal can be a very valuable experience. (or a lesson or two hundred in things which don't work out as expected)QuoteLook up what's required for stability. Search for "Op Amps for Everyone Design Guide" published by TI.-From here you can continue to look into the gain stages you introduced between the op amp output and the power supply output where the feedback is taken from. How much phase shift the stages introduce before the op amp reaches unity gain? How much phase shift can be tolerated?-How does the phase shift introduced by the power stage change when applying different loads including reactive loads. It's a lab power supply so it must be able to handle all those situations. How can the phase shift be avoided or "improved" (think about output capacitors and the ESR and snubber networks across the output in parallel with the attached load). If I you can't improve the output stage anymore, how much do you have reduce the bandwidth of the error amplifier in order to not run into trouble? (look at page 7 of the LT3081 data sheet. Some hints are in there in the transient response measurements with and without output capacitor)Don't aim for an ultra high speed feedback loop. The OPA op amp without compensation is to fast for what you are planning to do.

author: Shay
author_rank: Regular Contributor
timestamp: July 20, 2023, 07:34:52 am
content: Using IC regulators as basis for lab power supply usually isn't good idea, if current limiting is something that is desired. Adding a current limiting induces a lot of issues related to stability, and the results are slow current regulation. Usually it isn't worth it. It's best to either use IC with current limiting support, like the LM732.A good basic approach for reliable and high-performance PSU is utilizing 2 op amps, one for voltage and other for current, and ORing the output via diodes, so the op amp can only sink current from the pass element transistor.Such implementation is utilized by HP3610E power supplies and the schematics and support is available on google.Using a switching pre-reg is good as it can improve the efficiency. However, you must make sure to filter the output heavily, as the PSRR at such high switching freq and currents is poor. A transformer tap switching is another common approach.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 20, 2023, 08:18:37 am
content: Quote from: Taarak on July 18, 2023, 04:38:10 pmNo it would not solve stability issues. Voltage regulators are made for a well behaved load. A lab supply should be build to work with essentially any load.Another problem with integrated regulators that have the power stade and regulator all in 1 case is that they will get hot (often quite hot with there small case) and this limits the precision. The main advantage that many of such regulators offer is thermal protection of some kind.Adding current regulation to a regulator not made for this can be a big hassel.The linear regultor part is actually the easier part. The tricker part is getting the isolation without much EMI and common mode background.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 20, 2023, 04:18:45 pm
content: The Elektor supply is a good example of prereg, and discrete linear stage:https://ia800804.us.archive.org/20/items/ElektorMagazine/Elektornonlinear.ir2014-09.pdfYou can make the design juices flowing and make it to produce 5A.

author: Taarak
author_rank: Newbie
timestamp: July 20, 2023, 07:17:20 pm
content: Quote from: temperance on July 20, 2023, 02:07:39 amI skimmed through some sections of that document and I think I read a condensed version of that at school. It had nothing about stability though, which I think is my main issue right now, and it seems like that document talks about that as well.Quote from: Shay on July 20, 2023, 07:34:52 amThe LT3081 has integrated current limiting, if I was going to use that. Other than that, I didn't think it could have more stability issues, because they're slower than discrete transistors.QuoteSo essentially put a pull-up on the main driver transistor, and the opamp with the lowest voltage will sink current and set the output voltage? I tried doing something similar to that (voltage opamp drives the transistor directly, current opamp pulls the driver transistor's base down when constant current mode activates) and, because one opamp is always saturated, the output rings absurdly when going in or out of constant current mode. I'll try with the diodes, it just needs to have less than 470mV of voltage drop to make sure the driver transistor can go to cutoff without a negative supply on the opamps.QuoteI'll need an isolated regulator anyway (to not have one million windings on the transformer), so I might as well use it to increase efficiency. To improve noise I can maybe use a third regulator (mid-regulator?), one of those low noise switching ones from Analog or similar, if it doesn't get too expensive.Quote from: Kleinstein on July 20, 2023, 08:18:37 amI forgot that I would still need an external feedback loop when using an integrated voltage regulator, if I want any kind of precision. It would probably still have lower noise though.Now as for the calibration I was talking about before, I really wanted to do everything digitally so it could be integrated with other equipment and calibrated automatically. But that would be too expensive, so for now I'm going to use a normal 1k trimmer potentiometer with a low value resistor in parallel to reduce the "sensitivity" for the voltage feedback. I'm probably going to do the same again for the voltage readback, with a normal ADC.For the current I'm probably going to divide the output of the current sense amplifier by two with the adjustable divider (because the x50 version is harder to find than the x100, and 5mR shunts are harder to find than the 10mR ones), and use a DAC that has a 2.5V output range for the current, as well as 5V for the voltage setting. Then use another adjustable amplifier for the readback, if the ADC only supports 5V input range.

author: temperance
author_rank: Super Contributor
timestamp: July 21, 2023, 03:19:09 am
content: QuoteWhy would ready made IC be less noisy compared with a discrete circuit?I think you should also make up your mind. High power isolated with switching power supplies and quiet in terms of noise don't really fit into one sentence unless you want to invest an enormous amount of time in getting the common mode noise from the switching power supply down without running into an other set of problems like an inevitable increase in leakage inductance when trying to reduce the transformer stray capacitance unless you revert to resonant converters. But resonant converters and adjustable over a wide range also don't fit very well into one sentence unless someone invented something like that when I wasn't paying attention.I don't mean to discourage you but to give you an idea of the things to take into consideration.Just my two cents:If you are in need of a low noise supply, it's probably for powering circuits which don't require much power like battery powered precession devices. A dedicated low noise power supply with fine adjustments is a better option for such application.For high power applications, the opposite is true and even 100mv noise is not an issue.

author: Taarak
author_rank: Newbie
timestamp: July 21, 2023, 05:36:33 am
content: Quote from: temperance on July 21, 2023, 03:19:09 amWhoever created that IC already put whatever was needed inside it to achieve low noise, something which I might not know how to do. So it won't necessarily have lower noise, but maybe it will be easier to. Or I'm talking out of my ass, I never controlled a voltage regulator with an external feedback loop before. Whatever the case may be, currently I'm still simulating with a discrete TIP142 as the main pass element.QuoteI didn't really know what to expect coming into this project, so I set my goals high and am trying to achieve them.I'm going to need a separate supply for the digital circuits, and another one for the current sense amplifier and other amplifiers I end up using. So maybe I put a fixed isolated converter and convert everything down with intermediary, lower noise switching regulators. Or just assume that whatever is powering the module is already isolated and don't bother at all, but I want to at least try making each module isolate itself.QuoteYeah, I know, I don't even need a power supply with the specs I listed. But I want to build one anyway, because I want to feel like I accomplished something interesting I guess. And that's why I'm here, asking for general instructions on the things I don't know, so I can learn them, and hoping that you don't think I wasted your time.And about Shay's previous post, I tried "the diode method" in simulation. It's much more stable than the methods I used before when the load changes suddenly, but it's also slow to respond because the opamps keep saturating each other.So ideally I would change the reference input voltage with the current opamp, and the voltage opamp would just follow. But that gets very unstable when the load changes too much. If I can't compensate this somehow I'll just use the diode-or to control voltage and current.

author: macaba
author_rank: Regular Contributor
timestamp: July 21, 2023, 08:55:37 am
content: For what it's worth - this is a wheel that is worth reinventing, the amount you will learn is a lot and well worth it. I will enjoy watching your progress.As for control loops, this page came to my mind:http://www.djerickson.com/diy_smu/particularly the block diagram underneath the title of "Here is the block diagram for the analog and power amp boards." which shows the voltage/current control crossover. The full schematics are also available. You might consider doing something similar/simplified, a 2 quadrant version?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 21, 2023, 09:48:09 am
content: Quote from: Taarak on July 21, 2023, 05:36:33 amAs examples of this type of control, see these parts - they DIODE-OR voltage and current control paths.https://www.diodes.com/products/power-management/ac-dc-converters/secondary-side-controllers-and-switchers/

author: temperance
author_rank: Super Contributor
timestamp: July 21, 2023, 11:52:26 am
content: QuoteI absolutely agree. The practical insight gained by developing something like this and failing many times before something useful has been achieved is gigantic.

author: armandine2
author_rank: Frequent Contributor
timestamp: July 21, 2023, 12:03:44 pm
content: Quote from: dobsonr741 on July 20, 2023, 04:18:45 pmanother link to abovehttps://archive.org/details/elektor_2014/07.%20Elektor%20USA%20-%20September%202014/

author: temperance
author_rank: Super Contributor
timestamp: July 21, 2023, 12:20:39 pm
content: As for the current limit:Error amplifier bandwidth requirement:-the bandwidth of the current limit error amplifier must be much higher than bandwidth of the voltage error amplifier in the first schematic you've posted because the phase shift introduced by the current error amplifier is being amplified by the voltage error amplifier. But here is the problem: the current error amplifier consists of two gain stages.In other words: you have an 180° phase shift occurring in the feedback loop while the gain is still >1 when current is being limited unless the voltage error amplifier reaches unity gain before the phase shift introduced by the current error amplifier comes into play.-What is the bandwidth of the transistor stage in the current error amplifier? Some pointer in the right direction:https://inst.eecs.berkeley.edu/~ee105/fa03/handouts/lectures/Lecture20.pdf-What's the total phase shift introduced by the current error amplifier stage before the voltage error amp reaches unity gain. (don't simulate that. Calculate and look at the op amp data sheet) Knowing that, what would be the bandwidth required for the voltage error amplifier when the circuit switches to current limiting?-How can you increase the bandwidth of the current error amp stage? Do you need the transistor if saturation of the current error amplifier is unavoidable?Op amp saturation:why do you think your current error amplifier doesn't saturate in your first schematic? I think you misunderstand what saturation means. Saturation can't be avoided. When selecting op amps some are very quick to come out of saturation while some can be very slow.The quest for calibration:there is an other way to achieve what you need without calibration. A P16 bit resolution PWM passed trough a low pass filter can be used to precisely set the output voltage. (any timer of a micro controller can be used for that). You can now "fine adjust" the output of your power supply and use some 12bit ADC to read back the output voltage / current.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 21, 2023, 08:15:45 pm
content: A good device for linear power pass is the 2SC6082, this hi HFE, non Darlington part allows direct opamp drive for >>3A , and with a tracking smps pre supply, it should regulate 3A to 200mV dropout ballpark.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: July 22, 2023, 05:17:17 am
content: And there is LM395, the “Super reliable transistor”. Good thermal protection is built in and easy to parallel many of them. It’s really an LM317 without the reference and error amplifier, keeping the useful parts when driving with an OpAmp.

author: Taarak
author_rank: Newbie
timestamp: July 22, 2023, 06:18:33 pm
content: That's a lot of posts I didn't reply to, sorry. But anyway:Quote from: macaba on July 21, 2023, 08:55:37 amThanks! And about that link, I didn't understand fully what he did. He has the force and clamp settings into non-inverting adders, which work as a normal error amplifier but slowed I guess. The output of all of that goes to an integrator, that I don't understand the function of (maybe an AC-coupled low pass filter?), and the output of the integrator is divided with the output ground as reference. That then goes to a normal bipolar amplifier, which provides the positive voltage and current sourcing (with the positive branch) or negative voltage and current sinking (with the negative branch). I liked how he did the local current limiting though, looks like a nice safety measure when you have enough voltage headroom.Quote from: PCB.Wiz on July 21, 2023, 09:48:09 amIn other power supply schematics I find the diode-or control method being used, as well in ICs as you mentioned. For the moment I'll be using that too as well then.Quote from: temperance on July 21, 2023, 12:20:39 pmI'll still need to study more to understand why when the control loop is composed of two stages it gets so unstable. I know it's due to phase shift, but I still don't understand from where it's coming from or how to solve that.And in my first schematic the voltage opamp never saturates, as that would mean driving the power transistor to saturation as well. When setting the output to 0V it might saturate to the negative supply, but it doesn't need to.The current opamp is always saturated to the negative supply (the voltage from the current sense amplifier at the non-inverting terminal is smaller than the maximum current setting in the inverting terminal. (x - y) * Aol is a giant negative value when x is smaller than y, so it saturates to Vss). When the current at the output is too high and constant current needs to be activated, the current opamp needs to go up in voltage by a small amount, turning on that transistor, which is something it does very fast. The reference voltage on the voltage opamp goes lower and it follows by setting the output to a lower voltage. When constant current mode needs to be deactivated, the current opamp saturates to Vss again, which it can also do very fast because the voltage at the output is already very low.With the diode-or method the opamps saturate to the positive supply. When constant current mode needs to be activated, the current opamp has to go from positive saturation to a voltage lower than the voltage opamp, and that takes more time because it's slew rate limited. While that happens the current is higher than the set limit. This delay also happens when the constant current mode needs to be deactivated. It's only around 150µs to 250µs of delay though, so I might just ignore that.As for the calibration, I wanted to do it without using digital logic. That's because when I worked with component-level repair for a company I receivedso manyboards with a microcontroller that was either burned or had corrupted firmware for seemingly no reason. So I got somewhat traumatized, and since I don't want to accidentally put 160W at 32V on my little 3.3V prototype because my microcontroller corrupted itself I made everything analog. Not only that, but 16 bit ADCs that are also fast (16 bits to have at least 1mV/1mA resolution) are expensive. So for a first version I can spare some manual calibration and 4 $0.5 potentiometers from LCSC for reliability and precision.Edit: there's still the main microcontroller, but that's outside of the control loop. Since it needs to communicate with a specific protocol with the DACs and ADCs to do anything the chance of it messing something up if it breaks is much smaller.Quote from: PCB.Wiz on July 21, 2023, 08:15:45 pmQuote from: dobsonr741 on July 22, 2023, 05:17:17 amI was looking at the 2SC6082 yesterday, and it seems to have much better specs than the TIP142 I'm currently using. I wouldn't say an opamp can direct drive it at 5A (minimum gain of 50 at 10A, which is 100mA at 5A, more than the opamp can provide), but it has only 400mV of maximum Vce at 7.5A. I could use only 1V of headroom and the power transistor would dissipate only 5W. Though I don't know if the PSRR would lower with the headroom, this seems to be the case in some integrated linear regulators that I remember.Anyway, it seems like the 2SC6082 is made entirely out of plastic externally. It lists a maximum dissipation of only 2W. I could parallel several of them, but LCSC has only 50 units from a shady manufacturer, and Mouser will have some directly from onsemi, in 2024. The TIP142 is more available at LCSC and directly from ST.The LM395 is cool, though it costs as much as a single TIP142 at LCSC and I would need 5 of them (or 6 if I wanted to be sure) per board because of the integrated current limit. A less expensive approach might be to botch a thermal sensor on the power transistor and use an inexpensive comparator to turn the output off when the temperature gets too high.And I have another problem still. I wanted to add a relay to the output, so the module could isolate itself when the output is turned off or when it detects an error. But power relays from Omron and Metaltex (a brazilian brand) have around 50mR of contact resistance, which create a 250mV drop at 5A. The voltage opamp needs to compensate for this, but at 32V output it doesn't have enough headroom to do that, since the maximum recommended power supply range is 36V.This made think that there will probably be more than 50mR of resistance in the PCB traces, internal wires and output connectors, so I probably need to add more headroom somewhere somehow. Right now I'm very busy with other things (probably doesn't look like that with the super long post, lol), but when I get the time I'll think of something and give an update.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 22, 2023, 09:01:56 pm
content: Quote from: Taarak on July 22, 2023, 06:18:33 pmYou have too much gain in that loop. An op-amp is unity gain stable, but if you add more gain it's the same as trying to use a de-compensated op amp with gain =1.On current feedback paths, you usually need to 'throw overboard' a lot of gain.Quote from: Taarak on July 22, 2023, 06:18:33 pmIt's always tricky when you have two loops handing over. some bench testing with nasty switched current loads will be needed.The other danger zone is when the current released, the voltage opamp is trying to drive 100%, so you may need to apply current-pulldowns in more than one place to avoid that slew-rate overshoot effect.Quotetrimpots are fine on low volume experimental designs, but keep in mind wiper noise. I've seen designs where wiper noise during adjustment, spikes voltages to MAX.QuoteThe 2W is 'free air' it has a 25W rating with the case TC=25'CI was targeting 3A max, as I was thinking you wanted 3A.You may want to parallel two anyway, on a decent heatsink, to spread the thermal loads and give you more choices on operation.eg for lower loads (couple of hundred mA or less) you could disable the switcher and run simply linear for lowest noise.Not many noise sensitive loads are also high currentMouser showsStock: 5,130 Can Dispatch Immediately On Order: 3,450 Expected 05-Feb-24DK says 323 In Stock Can ship immediatelyThe part has very good HFE curves in the 1-5A region, tho I agree 10A is pushing things.

author: flowib
author_rank: Contributor
timestamp: July 23, 2023, 04:01:56 am
content: Allright.2SC6082 is indeed a good part to use as the linear pass element, it can be fed by a PNP+Red led transistor current source on the base set at approx 10mA, you can then use relatively inexpensive 5532 OR JRC4556AD opamps for control.I would Do a 24V shared bus for the modules. And use a cheap Buck controller to step down locally for an adittional +12V bus in order to run a UC3843 Flyback and a couple of DC/DC'sI'd use two 1W +-15V DC/DC modules, and use a I2C isolator to on each board.You can get increased performance by using a +5V or +10V reference with respects to +Out, you can find a dual DAC that has an internal reference chip, or use something inexpensive to get the drift and noise performance you want. You can use off the shelf resistor networks in DIP or SIP form to create the output divider, these parts are relatively inexpensive but provideApplication note 101 "minimizing switching regulator residue in linear regulator outputs" is a good read, its by the late Jim williams.I'd add wirepads for two ten turns, and perhaps a reed relay so you can switch between digital and local control with a pushbutton.

author: Taarak
author_rank: Newbie
timestamp: August 23, 2023, 06:50:20 pm
content: I'm back! And I hope there's no problem in reviving this "old" thread.Anyway, I was turbo-busy these last few weeks, so not that much progress was made on this project. Now I have more time to work on this again, though.As a general update, my school isn't cool anymore with me using their labs after I graduated, and I don't really have a personal lab. Do you guys have any recommendations on a cheap oscilloscope that can do very low level measurements, to properly test the power supply if it works? Or maybe a random cheap oscilloscope and a preamplifier I could build.Back to the actual project, some time ago I went to several electronics stores looking for some components, and the 2SC6082 was pretty much impossible to find. Some guy had one in a pair with another transistor, but it was clearly fake. Everyone had the TIP142 though, so I'll keep using that as the main pass element for now.I also had to improve the output voltage range on the pass element to allow more resistance on the path between the pass element and the load, which basically means increasing the maximum input voltage. Since the limiting factor is the opamp's maximum supply voltage, last month I tried doing something with current control by the opamp, using a current mirror connected to the control opamps that then control an I/V stage to keep the gain low enough. This means I could use a lower, fixed power supply for the opamp and the higher adjustable one for the pass element.However, at some point I received a promotional email from Analog Devices (lol) that included the ADA4510-2. This is a dual opamp that has garbage offset voltage and drift compared to the OPA2182 I was using before (still usable, probably), but is otherwise better in every aspect, including power supply range. It can accept up to 40V, which immediately puts the maximum output voltage at the pass element at 35 or 36V. This means that at the maximum output voltage and current, there can be over 600mR of resistance between the pass element and the load, which should be good enough.As is usual with Analog Devices, this opamp is much pricier, specially at low volumes, and the only distributor is Arrow with 98 parts in stock. This means that the alternative control methods aren't out of question, but the ADA4510-2 would simplify the design a lot it seems.For the current control, PCB.Wiz said a month ago that the two-staged loop is unstable because there's too much gain. Does that mean that the transistor after the current opamp (which I put there as an V/I stage, but also works as an amplifier) is too sensitive? It seems like maybe the current control opamp is too fast. Since the voltage opamp kinda works like an integrator, it is slower. That would make the current opamp pull down the reference voltage too much. When the voltage opamp finally responds, the current opamp releases the reference too much, and the cycle continues. Anyway, in the simulations I'm still using the diode-or control.As for the switching "mid-regulator", I found the LT8648S in Analog's website. It can accept up to 42V at the input, can do 15A, is extremely trivial to design around according to the datasheet, and can do under 10mV of peak-to-peak ripple in burst mode (the noisy one). If the isolation regulator can regulate precisely, without calibration, 41 to 42V, this part would be perfect. That is, if it didn't cost over $6, and almost $14 at low volumes. At least it's available, so it's not discarded.To finalize, the sensing circuit is more complicated to design than I anticipated. The ADA4510-2 has a maximum bias current of 10pA, which is perfect. But diodes for overvoltage protection such as the BAV199 (used in the DIY-SMU) can add up to 80nA, and it also needs protection against disconnected sense wires.This is normally done by sensing if the sense input is high impedance, using a pull-up. This pull-up will load the sense input, so the resistor needs to be very large, and large resistors create noise.The noise probably wouldn't be an issue if I could short the resistor when not using the sense wires, but the relay I intend to use is a small reed relay with single-throw contacts. This relay is cheap, but not cheap enough that I would want to use more. I also can't buffer the input before the pull-up because the opamp output is low impedance.With that the project continues I guess. When I get access to testing equipment again I'll actually build something to test everything. While that doesn't happen I'll continue trying to understand where and how to compensate each stage so that the output is as stable as possible, and design the "mid-regulator" and isolation pre-regulator.