The State of Regulators for Linear Power Supplies - A Rant (!) - Page 1
Forum section: electronics / general

author: robert67
author_rank: Contributor
timestamp: May 29, 2017, 05:21:10 pm
content: Hi, since this will be kind of a rant, I put this into the general chat ...I've build my first linear power supply about 35 years ago with a LM317 (adjustable voltage only). And a few weeks ago I gutted an old, defective LM723 based power supply (a 1983 EA-3013). So I thought it was high time to build another one (having now an empty case with a big transformer in it). Nothing fancy though: 0-30V / 0-5A (the specs of the one I just gutted). Now I searched the internet high and low for a ...good enough (don't care if it regulates down to a few 100mV / a few dozen mA only, about a little ripple on the output, or if it goes down a few dozen mV when the load goes from 0A to 5A)simple (meaning low component/pin count)one chip solution (yes, you will need in addition one or two power transistors to handle the 5A)And what did I found:"LM723 still does its job" (yes, with an additional Op-Amp for the current limiting, and a driver transistor for the power transistors)"I throw in the L200" (nice, but I still need an additional Op-Amp for the current limiting)"LM614" (4 Op-Amps and a adjustable voltage reference - nice, but why aren't the Op-Amps rail-to-rail, why? And yes, still needs a driver for the power transistors. And I've to figure out all the caps for compensation!)"LM10, LM432, TSM101, TSM106, TSM107, TSM103, NCP4300, TL103 ..." (basically two Op-Amps with a voltage reference, and the Op-Amps and voltage references internally somehow wired - why bother!)So my conclusion so far is I could go with the LM317 I've used 35 years ago, throw in an Op-Amp for the current limiting, and be happy (there is even a reference design for that).Any thoughts?

author: Messtechniker
author_rank: Frequent Contributor
timestamp: May 29, 2017, 05:48:59 pm
content: Quote from: robert67 on May 29, 2017, 05:21:10 pmYep. Simply the way to go even today when wanting to keep things simple.

author: The Soulman
author_rank: Super Contributor
timestamp: May 29, 2017, 06:20:14 pm
content: Yes linear regulators haven't changed much in the last years, luckily there are plenty of options from the old days still in production.The reason why IC manufacturers don't develop new linear regulators is simple: there is no demand.The name of the game to day is low cost, for any traditional power supply the mayor costs are the transformerand the large heat-sink.The arrival of (cheap) fast mosfets and cheap controllers meant that today switch-mode power supply's can be cheaply made omitting the transformer and large heat-sink.How cheap? Look at:https://www.aliexpress.com/wholesale?ltype=wholesale&d=y&origin=y&isViewCP=y&catId=0&initiative_id=SB_20170529101729&SearchText=power+supply+module&blanktest=0&tc=af

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 29, 2017, 06:25:42 pm
content: I thought the 723 had built in current limiting?

author: David Hess
author_rank: Super Contributor
timestamp: May 29, 2017, 06:28:44 pm
content: Quote from: NiHaoMike on May 29, 2017, 06:25:42 pmThe 723 has built in non-adjustable peak current limiting. You can set the current limit by selecting the shunt resistance but it is intended for fault protection and not an adjustable current output.

author: robert67
author_rank: Contributor
timestamp: May 29, 2017, 06:37:49 pm
content: Quote from: Messtechniker on May 29, 2017, 05:48:59 pmThanks. I was feeling like I'm getting to oldBut maybe I'll go 21st century at least and use the L200 (introduced around 2000?). I like the L200 reference design - using the additional Op Amp as a variable gain amplifier for the voltage drop across the shunt resistor - better than the LM317 one - using the OP Amp as a comparator with the low side of both input voltage dividers floating around depending on the voltage settings. Though the LM317 reference design gives you a "current limit" LED for free.

author: robert67
author_rank: Contributor
timestamp: May 29, 2017, 06:46:37 pm
content: Quote from: The Soulman on May 29, 2017, 06:20:14 pmYeah, nobody like heavy bulky power supplies any more. But they have so many uses:You can use them to flatten some crumbled schematics (which you spilled coffee on)You can use them to hold a door open when there is a draftAnd there is nothing more soothing than the 50Hz (in some regions of this planet 60Hz) hum when you max out a transformer

author: robert67
author_rank: Contributor
timestamp: May 29, 2017, 06:55:56 pm
content: Quote from: David Hess on May 29, 2017, 06:28:44 pmYes, just using a shunt resistor and the internal current limiting is "limited". It relies on the voltage dropped across the shunt resistor exceeding the base-emitter voltage of a transistor, which drifts between 670mV at 25°C and 450mV at 150°C. And that's not "good enough" for current regulation.

author: Ian.M
author_rank: Super Contributor
timestamp: May 29, 2017, 07:11:54 pm
content: I posted a LTspice sim of the LM723 based Philips PE 1535-00 bench PSU earlier this month.https://www.eevblog.com/forum/projects/how-do-make-a-current-limiting-knob-using-lm723-for-linear-reg-power-supplies/msg1212923/#msg1212923It does have fully adjustable current limiting but its pretty crude. However it might be a good starting point for figuring out what's actually needed to add on adjustable precision current limiting by pulling down the COMP pin ,without running into major instability issues.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 29, 2017, 07:16:07 pm
content: The typical linear lab supply circuit is more or less based on 2 OPs, a reference and than a power stage. So there is not that much to gain making a special chip with the 2 OPs and the reference. Lab supplies also are made for different power ranges, thus a special chip for all sizes would be kind of difficult. This is especially true for modern version with a SMPS part before the linear regulator.The L200 is way older than from 2000, more like 1970, not much newer than the LM317.The LM317 is made as a voltage regulator for an internal supply to a given load. So it is not a good choice to build a bench top supply of lab supply - it was never made for this. A lab supply needs a different kind of frequency compensation, more tolerant to a difficult load. Especially adding a kind of after-though current limiting is a good way to build a poor lab supply.It is a good thing to have the control part and power part separated - so don't complain about an extra external driver needed in many LM723 designs.

author: jolshefsky
author_rank: Regular Contributor
timestamp: May 29, 2017, 07:16:34 pm
content: Quote from: robert67 on May 29, 2017, 05:21:10 pmThank your lucky stars that the you can use a reliable old design without being mocked that it doesn't have an internal lithium battery for round-the-clock WiFi "Internet of Things" connectivity.

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 29, 2017, 07:24:43 pm
content: In the past, I also built several LM723 power supplies, including a switchmode one (which appeared in an Unitrode app note).Thus, I'm also a bit nostalgic about it. Actually, quite nostalgic.The 723 is similar to the 555 in which its ultra-simple architecture allows to be understood easily by experienced and beginner users alike. And which allows the circuit to be applied in many different ways..That is the reason that both are still being used, and a Google search will turn up +200k hits.The major issue it has is that it has no thermal protection for the output transistors.Of course, being a linear regulator, it is also bulky and inefficient. And as other posters have correctly indicated, nowadays a complete SMPS will be cheaper, smaller and far more efficient.

author: robert67
author_rank: Contributor
timestamp: May 29, 2017, 08:01:23 pm
content: Quote from: Kleinstein on May 29, 2017, 07:16:07 pmThanks, I didn't know that. The ST datasheet is of January 2000 without mentioning previous revisions - so I just assumed it's from2000.Quote from: Kleinstein on May 29, 2017, 07:16:07 pmI read that statement several times on different boards. What I don't understand is why there is a "Constant Voltage/Constant Current Regulator" reference design for the LM317 (for the LM150 too) in the datasheets then.Quote from: Kleinstein on May 29, 2017, 07:16:07 pmOh, but I'll keep complainingYou see, my design goal is a "simple" (low pin/component count) and "good enough" (see start of the thread) power supply. So I take advantage of any product from any vendor that can lower my pin/component count, and I will accept a certain loss in quality for that. And I will continue to throw a fit until TI, ST or whoever announces a chip that only needs two pots attached to build my power supply

author: robert67
author_rank: Contributor
timestamp: May 29, 2017, 08:10:17 pm
content: Quote from: jolshefsky on May 29, 2017, 07:16:34 pmAnd I do, I definitively do, several times a day! I fear the time will come when I can no longer switch my soldering station on, when the internet is down. Or solder parts that are not in the (of course cloud based) database of the soldering station. Or I need different soldering stations for parts from different manufactures, because not every soldering station vendor has an agreement with every parts vendor. The world is evil

author: David Hess
author_rank: Super Contributor
timestamp: May 29, 2017, 10:13:29 pm
content: Quote from: robert67 on May 29, 2017, 08:01:23 pmI agree with Kleinstein and others that integrated regulators are just not designed for lab power supply applications. There is a market there but it is occupied by discrete designs and except for extra simplicity which is not required, there is no advantage to a more integrated design and some disadvantages. Lab power supplies are just not all that complicated.Where I disagree is that integrated regulators do have a part in simplifying a lab power supply design. An LM317 or similar is an easy way to drive the output stage and provide SOA protection, thermal protection, and short circuit current limiting. An LM395 would be even better for this but they cost a lot more.A design using an LM317 like that is not going to be quite as high performance (it will need more output capacitance) but I consider the trade off worth it.QuoteThere are some parts like that but they will require external power pass elements to meet your voltage and current requirements; the LT3081 comes close.One problem with a fully integrated implementation is that the performance of the reference and error amplifier are limited by temperature swings from the power element.

author: floobydust
author_rank: Super Contributor
timestamp: May 30, 2017, 02:52:11 am
content: I've used LM317,LT338Afor years in bench power supplies.But now I need adjustable current-limiting, and less than 1.25V min. output.Looking around, lazy me purchased the $6 eBay0-30V 2mA-3A PSU kitand found it somewhat sucks, as this guy's blog mentions.I am left to designing from scratch, using a few op-amps and overkill on the pass transistors to avoid them failing.But I need reliable, low-drift, low-noise PSU. Other linear bench supplies are copies of 1970's designs (hail LM741!) with a few bugs.Modern MCU controlled like Rigol 832 aren't good enough, they have so much overshoot and EMI I can't use them for low-level analog design. But fancy LCD.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2017, 02:16:53 pm
content: There is not much performance improvement to be had by just using an operational amplifier better than a 741 in a high current power supply so if you want precision and low noise, I would use a separate low and high current design if your application will allow it. Other factors like reference quality (1/f noise and drift), feedback network temperature coefficient (especially in current shunts), and the temperature coefficient and resistance of copper in the output leads need to be considered.Further within the bandwidth of the control loop, the low frequency output noise of an integrated regulator used as a pass element will be reduced by the control loop. High frequency noise is filtered by the output capacitance. So at least where it counts, a low noise output stage by itself does not deliver the improvement of noise implied by just the difference in output stage noise.The schematic below gives a general idea of what is required for high stability. Kelvin sensing, a separate low noise and low drift reference, and a precision operational amplifier are combined to control what would otherwise be a noisy and drifty integrated regulator. I have used this same circuit to build high current blow out proof fixed power supplies with microvolt stability under line and load changes.In a general purpose power supply, the reference and amplifier would be powered by a separate supply instead of bootstrapped from the output but the single point common is still required for precision. Add a current shunt to the output and a separate single operational amplifier control loop for current limiting and you have duplicated some of those old power supply designs from the 1970s but with more precision. They had better operational amplifiers than the 741 like the 308 (and lots of others) and could have included the design features I mentioned above but did not bother because the added precision is not generally needed in a high current power supply.

author: robert67
author_rank: Contributor
timestamp: May 30, 2017, 05:50:46 pm
content: Quote from: David Hess on May 29, 2017, 10:13:29 pmThanks for the LT3081 hint.I had a look at the datasheet, and it (as well as the LT3086) come really close to what I want (simple and good enough). Two pots as well as some resistors/capacitors and you're done. You don't even need an external shunt resistor for the current regulation. The only drawback of these is that you can't beef them up using a power transistor (at least I couldn't figure out how), instead they are designed to be used in parallel if you need more current.They even have demo board for a real good lab power supply using the LT3081 (http://cds.linear.com/docs/en/lt-journal/LTJournal-V24N2-02-df-BenchSupply-Szolusha.pdf). Switch mode pre-regulator and all. But of course they override all internal current sources with external precision ones to avoid temperature drift.

author: BravoV
author_rank: Super Contributor
timestamp: May 30, 2017, 06:00:10 pm
content: Quote from: robert67 on May 30, 2017, 05:50:46 pmWe have a review posted by fellow member DutchGert, check this thread ...->Linear Technology DC2132A CV&CC Adj. Bench Power Supply Board Review

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 30, 2017, 07:13:14 pm
content: Here is a thread about LT3081 PSU:http://www.diyaudio.com/forums/equipment-tools/263471-triple-outputs-160w-lab-psu-epsux3-version-2-a.htmlI'm having a hard time setting the current limit digitally, if you can have a go at LTspice and see if you can get the current limit to controlled digitally.

author: Zero999
author_rank: Super Contributor
timestamp: May 30, 2017, 10:03:43 pm
content: Quote from: ZeTeX on May 30, 2017, 07:13:14 pmI don't see the issue. Use an error amplifier with its reference set by a DAC, to look at the current on the monitoring pin and pull the set pin down, if it exceeds the reference.For me, one of the most annoying things with the LT3081 is the requirement for a minimum load current. If the adjustment range is small, then a resistor can be used but this is not feasible in an application which can work down to 0V, as the resistor would need to be close to zero and would draw an enormous current at higher voltage settings. The only solution is a current sink. To really get down to 0V you need a negative power supply but a simple current mirror can get close to 0V without one.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 30, 2017, 10:39:44 pm
content: Quote from: Hero999 on May 30, 2017, 10:03:43 pmAnd get oscillations and instability, you are adding another external op amp that controls the current, another op amp that you will have to compensate and basically the same as taking LM317 and adding current limiting. Not an option."Frex" from the other forum found a solution, but I don't understand how it works.EDIT - by looking at the schematic he seemed like he is having a current source into the Ilimit pin, so I tried that and it works:

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 10:29:56 am
content: Quote from: ZeTeX on May 30, 2017, 10:39:44 pmWell no, it's not the same as taking the LM317 and adding current limiting, as you no longer need a separate current sense resistor and high side differential amplifier. The LT3081 does all of that for you.As far as the instability is concerned, yes you'll need phase compensation, but you need that with any linear regulator design. I don't see how it's any more difficult than using the LM723.QuoteThat also looks like a bit of a PITA. The current source needs to have a low drop-out voltage and level shifting will be required, if the reference is on the negative rail.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 31, 2017, 10:43:45 am
content: Quote from: Hero999 on May 31, 2017, 10:29:56 amif you need phase compensation like you said its the same thing as building PSU from scratch. I'm also quite sure that getting a stable current limit this way will be very hard if possible, you can't just look inside the regulator, and in this case, there is not point on using the LT3081, unless you want crappy current limiting e.g. LM723.OP wants a simple IC that you connect 2 pots, and have it all stable fast and working. I don't know how stable over different loads the LT3081 is, it might be good, it might not. If it was that good and easy like it looks in simulation, wouldn't any manufacturer use it? it must be cheaper than having a discrete everything.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2017, 11:06:46 am
content: Just recently I have run across old Motorola and National integrated power supply controllers but they are long discontinued.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 12:45:26 pm
content: Quote from: ZeTeX on May 31, 2017, 10:43:45 amThe current setting in the LT3081 circuit resistor doesn't dissipate much power, so there's no reason why it can't be a pot. What's difficult is controlling the current limit digitally, but even if it is a bugger to stabilise, using an IC such as the LT3081 gives you other features such as safe operating area protection, temperature and current sensing which you don't get with the LM723.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2017, 01:13:00 pm
content: The LT3081 and similar regulator offer temperature and SOA protection, because they need it: cooling is difficult and the SOA range is often very limited. AnWith the old 723 type design and also many OP based circuits, the tendency is to use much more powerful transistors so that no more SOA protection is needed and temperature protection is also relatively simple.One you add a second regulator loop, you need compensation for this, and to do this one would need an accurate model. Especially with simple one like the LM317, it is not that clear how well current control works with a low impedance load. So the LT3081 with an extra input to set a current limit can be an advantage - though the datasheet is not giving enough information for a good design.Quote from: David Hess on May 31, 2017, 11:06:46 amThere was the old MC1466, that was kind of suitable for a lab supply with reasonable quality current regulation. It was getting close to only needing two pots, a darlington power transistor and a shunt.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 31, 2017, 02:01:04 pm
content: The MC1466 looks very nice, you can get it for 6$ from eBay, and there is a datasheet:https://www.mikrocontroller.net/attachment/160010/mc1466l_tiff.pdfSomebody copied the design:http://chemelec.com/Projects/MC1466/MC1466.htm

author: StillTrying
author_rank: Super Contributor
timestamp: May 31, 2017, 03:06:01 pm
content: Quote from: Hero999 on May 30, 2017, 10:03:43 pmIf you count down to ~100mV as 0V, you can get around the the minimum load current by connecting a resistor from the output to the (now lower valued)set resistor.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2017, 03:14:56 pm
content: The MC1466 is an old obsolete part. It is way beyond the point of not recommended for new designs. If at all, take a few ideas from it, but not an antique chip. Chances are the one you now get from ebay of similar sources are fakes for those who desperately need a replacement.It has an interesting points to the design, different from most other lab supply circuits used today: it uses a trans-conductance amplifiers before combining the voltage and current loop, followed by a common capacitor for compensation.The reference part is also kind of old style, this sets the minimum voltage for the chip rather high.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 05:07:40 pm
content: Quote from: StillTrying on May 31, 2017, 03:06:01 pmI don't see how that would work. You'd then have more current flowing through the set resistor, than intended.Perhaps I've misunderstood what you. Please post a schematic, showing exactly what you mean.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 31, 2017, 05:39:40 pm
content: Quote from: Hero999 on May 31, 2017, 05:07:40 pmMinimum set current is really not a problem, just a current mirror as a current sink would probably work:

author: schmitt trigger
author_rank: Super Contributor
timestamp: May 31, 2017, 05:53:52 pm
content: Among some of the very nice regulator ICs that National used to offer, was a dual polarity tracking regulator.The LM325, It is obsolete now.Could be used stand alone or with some external pass transistors for higher currents.One of the many discontinued products from National's heydays.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 06:01:40 pm
content: Quote from: ZeTeX on May 31, 2017, 05:39:40 pmI already posted that!https://www.eevblog.com/forum/chat/the-state-of-regulators-for-linear-power-supplies-a-rant-(!)/msg1221498/#msg1221498Yes, it works down to the saturation voltage of the transistor, which is quite low at 4mA. As I said before, to really get to 0V, you need a negative power supply.

author: StillTrying
author_rank: Super Contributor
timestamp: May 31, 2017, 06:15:53 pm
content: Quote from: Hero999 on May 31, 2017, 05:07:40 pmI should have said 2 resistors.I'm not saying it's very good, but for a one-off, I think you could get the minimum Vout down to about 50mV if you had to, but 100mV would be safer.I've not used some of these either because of their min output current, but about a month ago I noticed that some of the 5mA min. seem have changed to 0.5mA min. or perhaps I've just imagined it.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 06:20:46 pm
content: Quote from: StillTrying on May 31, 2017, 06:15:53 pmWhat IC is U1? I thought we were talking about the LT3081 here which has more than three pins.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: May 31, 2017, 06:24:36 pm
content: Quote from: Hero999 on May 31, 2017, 06:01:40 pmOh yes, sorry, my bad,you would probably need a negative supply for something else so this is not a problem.

author: StillTrying
author_rank: Super Contributor
timestamp: May 31, 2017, 06:34:31 pm
content: Quote from: Hero999 on May 31, 2017, 06:20:46 pmAll the LT3081 setting and monitor pins are constant current sources, so I can't see the problem, the monitors are only valid down to 0.4V output voltage though, I can't see a min output voltage for the current limit setting pin.

author: robert67
author_rank: Contributor
timestamp: May 31, 2017, 06:49:28 pm
content: Quote from: David Hess on May 31, 2017, 11:06:46 amWhat were the names / numbers of those regulators?

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2017, 07:28:15 pm
content: Quote from: Kleinstein on May 31, 2017, 03:14:56 pmThis closely duplicates how the voltage and current error amplifiers work in some switching controllers like the ubiquitous UC2524. The switching section could be disabled and then ignored leaving a 5 volt reference and separate voltage and current error amplifiers with a combined output.I am dubious about any benefit over using separate operational amplifiers and if I wanted a cheap reference, I might include a 723 just for that. I wonder if there are any suitable transconductance amplifiers worth considering for this application.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2017, 08:25:42 pm
content: There are not many integrated trans-conductance amplifiers. So the obvious choice would be the LM13700. The properties should be OK for a power supply. However I have not seen a lab supply circuit based on this.For a normal lab supply the conventional way with 2 OPs - diodes and power stage is still OK. The main advantage of trans-conductance amplifiers could be a fast switch over from CC to CV mode and back. However a fast transition from CV to CC mode may not even be desired as it may mean voltage drops already close below the current limit.A slow CV-CC mode transition is a problem in some circuits, but there are solutions, if too much windup is avoided. It is usually more a problem with a power supply with a emitter follower type output stage. Especially the current limit is often designed slow, but this does not have to be this way.With a modern lab supply, the linear regulator part is usually only a small cheap part. The more expensive parts are transformer, caps, heat sink, and may be the digital control part with precision DACs. So a special analog part would not help that much - build from standard parts, the linear control part cost something like $1-2 and its still small compared to the rest.

author: Zero999
author_rank: Super Contributor
timestamp: May 31, 2017, 10:20:11 pm
content: Quote from: StillTrying on May 31, 2017, 06:34:31 pmI don't understand. The IC in the circuit you posted only has three pins, yet the LT3081 has five. If you replaced U1 with the LM3080 which doesn't have the other pins, just input, output and voltage setting, then I don't see how that 22R resistor would provide 4mA of load, over a wide voltage range. Suppose the 22R resistor did have 4mA through it, Ohm's law states the voltage across it must be 88mV, which is already 38mV above the 50mV shown on your diagram, before the voltage drop across R3 is even considered. If R2 is set to zero then the output will be 50mV but the current through R2 sill only be 2.27mA. Have you tried simulating it?EDIT:Oh, I see what you're trying to do now. Change the 22R for 10R to give 5mA of quiescent output current. Now Vout = R3*5*10-3+0.05VThe minimum voltage will now be 50mV, which will be good enough for most applications, but I'd be still tempted to change the 1k to 100R and 22R for 1R, which would make the minimum voltage 5mV and is small enough to ignore.

author: David Hess
author_rank: Super Contributor
timestamp: May 31, 2017, 10:29:51 pm
content: I was thinking of a "precision" integrated transconductance amplifier and maybe there was a part I was unaware of but the LM13700 is practically the only part available. There are a couple of ways to configure a single operational amplifier as a transconductance amplifier so maybe that has possibilities to avoid the need to include dedicated anti-windup.I would prefer separate voltage and current error amplifiers anyway so the current amplifier can directly do high side current sensing without voltage restrictions. That would not be possible in a combined part in all cases.

author: ZeTeX
author_rank: Frequent Contributor
timestamp: June 01, 2017, 08:04:22 am
content: How would transconductance amplifier compare to other PSUs with the common floating regulator?What makes it special? I have never used or learned about those amplifiers so I don't know much about them.

author: technix
author_rank: Super Contributor
timestamp: June 01, 2017, 10:14:03 am
content: You can build a hybrid power supply if you like to: a switch-mode tracking pre-regulator that can be bypassed, and a linear main regulator. The pre-regulator cuts the input voltage down to somewhere near the output, and the main regulator smooth out the ripple. This way you can get the full benefit of a linear regulator yet still have the (somewhat) high efficiency of the switcher. The switch-mode stage can be bypassed if the loss on the main regulator is lower than a threshold value to further lower the ripple under high voltage or low current applications.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2017, 03:58:50 pm
content: Quote from: ZeTeX on June 01, 2017, 08:04:22 amThese are two different things; a transconductance amplifier, or really an operational transconductance amplifier, replaces the operational amplifier which is used for the error amplifier. In a very simplified design, a single transistor is often configured as a transconductance amplifier and used as the error amplifier.QuoteA transconductance amplifier is voltage-in-current-out instead of voltage-in-voltage-out. If you removed the output buffer stage from a common operational amplifier, then it would become an operational transconductance amplifier. On some parts this is possible to do by accessing the transconductance output from a compensation pin.The advantage is that the transconductance outputs from the voltage and current error amplifiers can be combined so they see the same compensation point and there is no separate windup of the error amplifiers so switching between voltage and current mode is basically instantaneous which explains why integrated regulators with separate voltage and current error amplifiers work this way.Disadvantages? There are not many or any precision operational transconductance amplifiers and operational amplifiers which provide access to the compensation pin are rare limiting your options. Open loop voltage gain depends directly on the load at the output which is a blessing and a curse; you can tailor the frequency response to be exactly what you need but you have to do all of the frequency compensation yourself.The old but ubiquitous 723 regulator works this way; its transconductance output is exposed at its compensation pin and in theory two 723s could be combined at this point to yield precision voltage and current limiting although I have never seen this done. I wonder what available operational amplifiers could be used this way; there are not very many parts which provide access to the internal compensation point. The 301A is the only inexpensive one I know of. I think the LT1008, LT1012, and LT1097 could be used this way. It might be possible with the OP97 but Analog Devices does not provide even a simplified schematic so I cannot tell. Of these the 301A and LT1008 are the fastest but hardly fast.Incidentally, this is the same reason that operational amplifiers which provide access to the compensation point can also use this access to clamp the output preventing windup and solving the same problem. Below is an example where this is done to a 301A.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 02, 2017, 12:10:01 am
content: Here also, I recommend a look at NatSemi's LB-28. Interesting LPS based on LM101/LM108 and some LM195s (which is a very interesting part indeed) BTW, M.S.Kennedy published an Appnote showing that it is a LM117 bonded differently!Further, I could offer you a ITT design which is totally devoid of integrated circuits! 30V/3A with both CV/CC and a overcurrent switchoff mode. Mine uses double transistors for the differential pairs, but should they fail while in south-noobtainistan, you could substitute BC107s with a reduced performance.Then, you could try the UC1834 or the SG1532 to build a regulator around.Also, there was a circuit using a floating regulator from SGS, but I lost it.But there is nothing wrong with wanting to build another LMx17-based PS. As you said:Quote from: robert67 on May 29, 2017, 05:21:10 pmIf you refer to the one with the external PNP pass darlington, you could use a PNP and multiple LMx95s instead. Then get both in Rad-Hard, hermetic variants from Sescosem (or whodunnit now) and build the EPS for the last stand.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2017, 02:14:47 am
content: Quote from: Neomys Sapiens on June 02, 2017, 12:10:01 amUsing just the error amplifier of the 723 would be like this; it is only made up of a 6 transistors and a better one could be made with 5 transistors.QuoteI was not aware of these. The SG1532 is an improved 723 but the UC1834 is more like that old Motorola part and what I described with two 723s or two operational transconductance amplifiers with a parallel output. Given how much the UC3834 costs, I would only consider alternatives.

author: floobydust
author_rank: Super Contributor
timestamp: June 02, 2017, 02:48:23 am
content: None of the 1970's IC linear regulators seemed to have survived the 3-terminal wars, LM723 aside.Example MC1425, MC1466, MC1468/1568, LM300, LM325/326/327, RC4195 etc.The building blocks are the same: reference, error-amp, pass-transistor, current-limiting.TheLT3081is around $10 at Digikey and quite an improvement over $0.80 LM317.And in my experience, after the IC (LM317) shorts and burns up your $21.00 10-turn potentiometer, you decide to roll your own bench PSU that has more current say 3A, and goes down to zero volts. I got no problems with a big Wakefield heatsink, and having no EMI from a SMPS is so nice.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2017, 01:42:25 pm
content: The only time I have had a 3 terminal integrated regulator fail was when the output or adjustment pin capacitor discharge through the device do to loss of power at the input. When this is an issue, two external diodes can be added to prevent it. Some more recent 3 terminal regulators are immune or resistant to this problem.

author: Zero999
author_rank: Super Contributor
timestamp: June 02, 2017, 10:44:10 pm
content: I had a problem with an LM317 keep failing in a solar SLA battery charging application. Adding a diode from the output to the input didn't help. There was no capacitor at the adjustment pin and the solar cells had a series diode, which prevented the batteries from discharging back into them. I believed it was transients due to nearby lightning strikes killing the LM317 so I added a zener diode between the input and output and I haven't had any problems since.

author: floobydust
author_rank: Super Contributor
timestamp: June 03, 2017, 01:14:05 am
content: A company I worked for was designing 12V furnace ignitors, basically car ignition coils w/oscillators.Every lab power supply eventually got blown up. I took them apart and replaced the pass transistors, IC's etc. and most were missing the two critical protection diodes.Jr. engineers would not repair them, "let's buy a new one" lolI know several ways linear PSU's including the venerable 3-terminal regulators fail:1. Reverse voltage on output - from connecting batteries backwards or inductive load transients. Adding a diode fixes this and smokes very good.2. Higher input/output differential than max. - from line and I/P transients3. Higher output voltage than max. possible - Adding input/output diode Vout to Vin fixes this.4. Higher output voltage than setpoint - connecting batteries when output is set to a low V. Backfeed through pass-transistor if over VEB breakdown ~7V.5. Old age- the regulator's SOA protection screws up and things short.6. The ADJ pin capacitor dumping into the LM317 during a O/P short. Add the diode.

author: David Hess
author_rank: Super Contributor
timestamp: June 03, 2017, 01:41:21 am
content: Quote from: Hero999 on June 02, 2017, 10:44:10 pmThe adjustment pin in an LM317 is also sensitive to ESD. Sometimes designers run across this when they build an adjustable regulator with a remote potentiometer for controlling the output.

author: Red Squirrel
author_rank: Super Contributor
timestamp: June 03, 2017, 05:11:41 am
content: Quote from: The Soulman on May 29, 2017, 06:20:14 pmIt's crazy how cheap they can sell this stuff for. Like $1 for a SMPS module. There's just no way to even compete with that. A single one of those parts like the capacitor is probably close to that price on it's own. Heck, the PCB itself.

author: floobydust
author_rank: Super Contributor
timestamp: June 05, 2017, 03:41:32 am
content: The problem is those PSU little PCB's from china are grossly over-rated, no safety approvals, have crap capacitors, unstable output and generally not reliable.I believe china has devalued their currency so much, to wage war and snuff out any competition. I can't even buy the raw materials for the prices they sell fully assembled products for.Your bench power supply has to be an old friend, something that you can count and not doubt, so I find a payoff spending a few $$ and making my own.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 05, 2017, 01:57:02 pm
content: I fully agree with you;there are two instruments in your bench that you should be able to fully trust blindfolded: the DMM and the Power Supply.

author: 2N3055
author_rank: Super Contributor
timestamp: June 05, 2017, 09:22:52 pm
content: Quote from: Neomys Sapiens on June 02, 2017, 12:10:01 amWould you be so kind to share that ITT schematic??Thanks!!Regards,Sinisa

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 05, 2017, 11:43:17 pm
content: Yes, will do. Although, the description is German. If you have a problem at some point, let me know.It was also published in the 'Funkschau' back then.I will first post the appnote, which is from a ITT booklet calledZ-Dioden, integrierte Stabilisierungsschaltungen und Spannungsregler - Grundlagen und Anwendungen, ITT Intermetall, 1977Maybe I can dig out my LTspice model of it.Mine does have some differences. I looked for a discrete negative auxiliary regulator and use it. Overtemperature protection of transformer and power transistors is by ptc sensors and a (discrete) circuit and a relay. when overtemp condition occurs, it cuts out the holding coil of my power switch.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 06, 2017, 08:50:06 pm
content: Here is the excerpt from that ITT booklet. For 3A add another pass transistor and check if the driver is sufficient. I will try to find my copy of the 'Funkschau'-Article too, as it was more detailed.The LT-spice model is not exactly the same, because of missing components. Also some connections are different, as I was experimenting or trying to explore a special state - I can't remember, but you can complete it easily. I think that I once wanted to improve the Tempco or something like that.Also interesting: The already mentioned article by I. Hickman (Experimenters Power Supply, Electronics world Jan97) includes use of the so-called McPherson-circuit (load sharing between pass transistors with 2 supply rails) I never saw another reference to it.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 06, 2017, 09:22:58 pm
content: Some more publications on LPS circuits:Stromversorgungs-Praxis , Nührmann, D. , Franzis 1987, 232p, ISBN3-7723-8581-8Transistorisierte Netzgeraete, Strobel, Franzis 6thEd 1980, 63p, ISBN3-7723-1366-3 RPB-seriesNetzgeraete mit ICs , G. Sehrig, Franzis, 3rdEd 1981, 64p, ISBN3-7723-0903-8 RPB-series

author: floobydust
author_rank: Super Contributor
timestamp: June 07, 2017, 09:09:05 pm
content: Thanks for posting the Mk3 ITT sch and LTspice sim.The triple emitter follower pass-transistor has no voltage gain, so I am surprised a discrete diff amp with a few transistors is enough. Everyone uses op-amps.

author: David Hess
author_rank: Super Contributor
timestamp: June 07, 2017, 11:56:46 pm
content: Quote from: floobydust on June 07, 2017, 09:09:05 pmA 723 or MC1466L design is no different with a single stage transconductance amplifier driving a triple Darlington. A huge voltage gain is not required for good performance.

author: floobydust
author_rank: Super Contributor
timestamp: June 08, 2017, 05:34:46 am
content: This is just like audio amplifiers - discrete verses op-amps. I guess for good voltage (load) regulation, less gain needed than what I thought.Look at HP 6632A power supply 0-20V, output stage is a class AB power amplifier! It can source/sink current. From this thread:forum/beginners/new-power-supply.HP 6632A, 6633A, 6634A Service Manual pdf(schematic pg 105/117)

author: Kleinstein
author_rank: Super Contributor
timestamp: June 08, 2017, 07:00:13 pm
content: You can get reasonable good regulation even with relatively low voltage gain - if the output stage already has a low output impedance. It actually helps for stability if the DC output resistance is not extremely low (which is useful with extra sense terminal only anyway). Many old designs with discrete transistors use this to get good stability. Change it to OPs and stability with a large load capacitance can become a problem.However many lab supply circuit use a much higher output impedance power stage, and these definitely need the high gain. Already the simple Darlington is not that low in output impedance if the current is low - so it kind of needs a minimum current.The class AB type output stage is one way to make a power stage with low output impedance at all currents.A discrete difference stage usually has relatively high DC drift. And today OPs are cheaper then a transistor array chip. So the OP is the obvious choice.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 08, 2017, 08:08:12 pm
content: Quote from: Kleinstein on June 08, 2017, 07:00:13 pmI would never doubt that. And not only the regulator components, but the reference will also not stand a comparison to modern components. I merely wanted to offer an interesting alternative. What I personally find intrigueing is the two current action modes. And the drift was the reasoning behind the more complex negative auxiliary regulator. Next thing to try would be to substitute some circuit for the 1.5k 1W resistor.After that, one would have to perform thermal testing.

author: Red Squirrel
author_rank: Super Contributor
timestamp: June 08, 2017, 11:55:26 pm
content: Quote from: floobydust on June 05, 2017, 03:41:32 amYeah would actually be interesting to do load tests on these. I would expect a PSU to at very least be able to sustain it's rated load continuously - I guess these dollar jobs probably can't. Still makes it hard to compete with though as the average joe is just going to go with whatever is cheaper. But I guess someone buying a bare PSU like that probably half knows what they're doing and may very well favour a more expensive one. Ex: a TDK, Meanwell etc. For bench PSU then it definitly pays to go with good quality as you want to be able to trust the readings too. Actually that's something I need to get myself... still using an old ATX PSU, good for various voltage rails but not good if I want as specific voltage or want to current limit etc. I want to build my own eventually but I should probably buy a good one so I know I can trust it, and I can test my own build against it.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 09, 2017, 09:16:27 am
content: Quote from: Neomys Sapiens on June 08, 2017, 08:08:12 pmThe 1.5 K 1 W resistor in the old ITT regulator circuit is mainly for limiting the LEDs current. So with more modern LEDs one could use less current and thus less power.The amplifiers only use something like 3 mA.A second mode that turns of the supply in case of over current is sometimes useful, but it might need some delay, no to engage too fast. There are a few other circuits that offer this.

author: IconicPCB
author_rank: Super Contributor
timestamp: April 16, 2019, 12:48:27 am
content: I am presently looking at a fixed voltage current limited power supply 28V, 50A continuous, based on LT3086.Idea being to use the regulator with a series pass transistor such that the the regulator provides current limiting and SOA protection - shutdown under adverse conditions.Raw power will come from a transformer rectifier with at least 10000uF per Amp.I am using a TIP36C to provide added current per module ( module spec 12.5Amp @28V input).Raw power will vary from 40V down to 29.5V drop out voltage. Transformer is yet to be trialed.Overall four modules will be paralleled up to provide the required current.So far initial trials of a single module look promising.I am having some difficulty in interpreting the graph of Vin-Vout versus Iload as presented in the data sheet.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 16, 2019, 07:38:52 am
content: At the high power level, using very large filter caps may no be such a good idea. Especially with a large and thus low impedance transformer the power factor will get rather bad because the current will flow in short pulses near the peak. With just huge caps, one would have to look at a 100-150 A rated transformer to get 50 A DC out. The rule of thumb factor 1.6-1.7 for the current is valid for smaller transformers and not much capacitance (e.g. 10% ripple). At the very high power level it may be more effective to also use an inductor for smoothing. Inductors, like transformers get better the larger they get.One problem with the boosted current version is that some of the regulator internal protections don't work anymore. The SOA limit of the transistors usually do not match the SOA limits of the regulators. So it still needs a check, and it tends to need a lot of current through the regulator and thus not so good regulation. I may be easier to build SOA limit / fold-back of the current limit from scratch than to try to use the regulators internal SOA limit, that may not be that well documented or accurate.At that power level I would definitely think about using at least 2 transformer taps. My favorite here is with electronic switch over like in class H audio amps. This is fast and can thus include the residual ripple, using the higher voltage only when really needed. 2 transistors in series also helps with the SOA, especially at higher voltage.As always with such projects - start small (e.g. 1 A) first: it's much less of the magic smoke to be released.

author: IconicPCB
author_rank: Super Contributor
timestamp: April 16, 2019, 11:14:15 am
content: I am relying on the fact that TIP36 has a reasonable Beta as such the regulator needs to supply a couple hundred mA ... and a base emitter resistopr can be used to match regulator and outptu transistor SOA along with regulator current limit facility.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 16, 2019, 02:52:43 pm
content: Quote from: robert67 on May 29, 2017, 06:46:37 pmIndeed! its like a cat's purr.And those TO3 transistors mounted in a badass heatsink definitively look nice.

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2019, 10:20:08 pm
content: Quote from: schmitt trigger on April 16, 2019, 02:52:43 pmTransformer hum is actually the second harmonic so 100 or 120 Hz because the force from the magnetic flux maximizes in one direction on every peak.Does that mean a cat's purr is produced by a scalar process?

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 17, 2019, 06:24:09 pm
content: Purring is actually more an absolute value function.