TL431 linear power supply - Page 1
Forum section: electronics / beginners

author: mike_mike
author_rank: Super Contributor
timestamp: December 11, 2018, 03:16:45 pm
content: Hello, I built the power supply from the attached image but I found that it is oscillating. What should I do to prevent the oscillations that appeared on the scope at the output of the power supply as a sine wave with vpp amplitude of about a few tens of volts ?While the power supply was oscillating, the voltage drop on the 1N4148 diode was 4V instead of 0.7V and there was a high frequency noise.Also, the load was about 6-7A at 25Vdc output.What should I do to eliminate the oscillations ?*in the tested schematic, R6 was 2 0.1R in parallel resistors.

author: xavier60
author_rank: Super Contributor
timestamp: December 11, 2018, 11:25:37 pm
content: Try adding some frequency compensation to the TL431. Start with a series 1K and 1uF between K and R.

author: AngraMelo
author_rank: Regular Contributor
timestamp: December 11, 2018, 11:52:23 pm
content: What is the purpose of R4 and R12?

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2018, 01:47:52 am
content: Quote from: AngraMelo on December 11, 2018, 11:52:23 pmThey improve the transient response by discharging the base-emitter capacitance, when the base current decreases.

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2018, 02:54:28 am
content: What is the frequency of oscillation?Get rid of C2. It belongs either at the load or where the leads enter the chassis.AC bypass R18 with a low value capacitor.Swap the positions of R8 and R9. Then AC bypass the top fixed resistor with a capacitor or series capacitor and resistor. This adds phase lead and works in combination with xavier60's suggestion.Lower the value of R4.

author: iMo
author_rank: Super Contributor
timestamp: December 12, 2018, 09:48:18 am
content: Also the anode of the TL431 and the cold side of R8 should be wired close to the LSP4, and the hot side of R9 wired close to the LSP3 (better do it with a separate wires/tracks).At 7A current even <1milliOhm wire resistance could cause a voltage drop creating oscillation in the control loop. Therefore you must separate "power" signal paths from "sensing" paths (wires) - a "star" config.

author: mike_mike
author_rank: Super Contributor
timestamp: December 12, 2018, 04:21:07 pm
content: I found a power supply schematic and I would like to use it to power my equipment.Do you know if it will oscillate or not ?

author: spec
author_rank: Frequent Contributor
timestamp: December 12, 2018, 04:39:47 pm
content: Hi mike_mike,Not surprising that you have frequency instability problems with a TIL431. Take a look at the frequency stability charts on the TL431 data sheet.On first inspection, the second PSU looks very hairy too with a load of gain, especially in the constant current mode which has two zeros in series. My advice would be to look elsewhere.

author: mike_mike
author_rank: Super Contributor
timestamp: December 12, 2018, 05:12:51 pm
content: Thanks for the reply.Please recommend a good power supply schematic.I need the following performance1. min 5A output2. 3-25V output voltage3. to not oscillate4. to have a simple schematic

author: spec
author_rank: Frequent Contributor
timestamp: December 12, 2018, 05:21:36 pm
content: No probsI will have a look for a suitable PSU, but I am quite busy at the moment. A number of members are experienced in PSUs, perhaps they could recommend some suitable designs as well.But I can recommend a complete lab PSU that will do your job for around £47UK, including shipping (the transformer for a DIY PSU would cost more than that). The lab PSU provides 0V to 30V and 0A to 10A, with digital readouts for both voltage and current. There are controls for coarse and fine voltage output and the same for current. I have three of these and use them for electronics, battery charging, you name it.https://www.ebay.co.uk/p/Adjustable-DC-Power-Supply-30v-10a-Variable-Precision-Digital-Lab-PSU-Clip-Cable/23025566378You can probably get this generic lab PSU delivered to Romania.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2018, 05:52:26 pm
content: Simplest possible supply that meets those requirements:- Phase controller or VARIAC- 18 to 20VAC transformer, rated 8-10A RMS- Rectifier, capacitor filterYou did not specify anything further in depth like regulation or noise, or adjustable current limiting, or cost, so #4 is easiest to optimize. This uses about six parts (including fuse and power switch), hard to beat.Tim

author: mike_mike
author_rank: Super Contributor
timestamp: December 12, 2018, 05:58:49 pm
content: Thank you.But I need a power supply made by me. I am a kind of maniac in electronics.I need that the power supply to be variable via potentiometer and stabilized.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2018, 06:11:00 pm
content: Literally just the first schematic, with an R+C connected between TL431 ref and cathode. Say 10k and 1nF for starters. Adjust values to give good step response.Tim

author: xavier60
author_rank: Super Contributor
timestamp: December 12, 2018, 10:19:33 pm
content: Quote from: mike_mike on December 12, 2018, 05:58:49 pmYou have not mention if you have tried adding compensation to the TL341 or not.That first power supply circuit has the best chance of working properly than most others you are likely to find.There are a few more small improvements that can be made. For example the overload LED can be moved to a better spot.The BD140 isn't necessary.

author: spec
author_rank: Frequent Contributor
timestamp: December 13, 2018, 02:13:51 am
content: Quote from: mike_mike on December 12, 2018, 05:58:49 pm

author: spec
author_rank: Frequent Contributor
timestamp: December 13, 2018, 02:21:28 am
content: This is a genuine question asked out of interest.Has anybody ever built a variable power supply that has actually worked using a TL431, not simulated but in the flesh. My experience with the TL431, and family, is that unless they are in a nice benign position, like the reference for a switch mode PSU, they are not happy.

author: spec
author_rank: Frequent Contributor
timestamp: December 13, 2018, 02:30:51 am
content: mike-mikeI have done a sketch for a PSU that replaces the TL431 with an opamp, but otherwise uses the architecture and components of the TL431 circuit. Hopefully I will be able to post the schematic in about 17 hours from now.By the way, there seems to be an uneasy relationship between the voltage regulation and the current limiting function.If you like, just remove the current limiting LED indicator from the circuit and see if the PSU settles down (the current limiting function will be completely disconnected and not involved in the circuit).

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2018, 04:28:48 am
content: The only reason I have not used the TL431 that way is why would I bother when better performing and easier to use operational amplifiers are plentiful? It is routine in isolated switching power supplies however.The problem is a little like using the 723 or most switching regulator controllers where stabilizing feedback to the inverting input comes from the transconductance output instead of the low impedance buffered output. The TL431 has a transconductance output, despite what the datasheet specifications show, so gain varies considerably from device to device and with load. If you build a series regulator with it, then the variable biasing conditions are altering the transconductance unless steps are taken to prevent it so the frequency compensation becomes difficult to control.A better question might be *why* the TL431 operates so benignly when used as a simple shunt regulator. It will normally have a relatively large load capacitance which rolls off the gain of its transconductance output for dominant pole compensation which is exactly how a transconductance amplifier is compensated. But in a circuit where it controls a series regulator, that cannot be done if fast response is desired and instead feedback from the squirrely output to the input implements the frequency compensation because people are used to working with voltage feedback amplifiers. It can work as the 723 shows but can also be hit or miss.This brings up an alternate compensation scheme; place a series RC network from the TL431 output to ground.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 13, 2018, 07:39:09 am
content: Quote from: xavier60 on December 12, 2018, 10:19:33 pmThe BD140 BE resistor sets a coarsely constant current through the TL431.BTW 1kΩ is too much. 330-470Ω would be more appropriate and might even help mitigate the self-oscillation trouble as 1mA is the rated minimum current for TL431.The BD140 at the maximum voltage allowed for the TL431 would dissipate approximately 250mW. It might be replaced with a BC327 if the ambient temperature (inside the PSU case) is not too high.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 13, 2018, 07:58:37 am
content: Quote from: spec on December 12, 2018, 05:21:36 pmFrom the pictures (if you look through the case aeration slots it looks empty inside) I guess it is a switching PSU.The linear ones are better as they can be easily improved by replacing the electrolytic caps (they are low quality and too low voltage) and adding a proper TO3 heatsink (as minimal cure).

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 13, 2018, 08:21:03 am
content: Quote from: mike_mike on December 12, 2018, 05:12:51 pmIt would be better to use at least a center tapped transformer, switching the secondary winding appropriately to halve the power dissipation.Do you already have a transformer?It would be possible to modify the 431 psu, and run a few simulation to get a coarse idea of the compensation network, but IMHO a classical 2 opamp circuit, with fixed feedback network and variable references and both voltage and current regulation, is much better.If you feel audacious I can provide you with a proper 2 opamps circuit, but just tested it in LTspice as my lab is not available at the moment.Here is aproof of concept schematic, just to get a rough idea.If you like it I can provide you with a complete and real world usable circuit diagram.BTW what do you plan to use the PSU for?

author: mike_mike
author_rank: Super Contributor
timestamp: December 13, 2018, 09:27:46 am
content: Currently I am waiting for a response from @spec. The current limiting led is not necessary. If you will use an operational amplifier, then please use a common operational amplifier for example LM358 (I have many of them). The input of the supply will be maximum 45Vdc, so there needs to be made something in order to power the op amp.I need the power supply to power leds, Arduino, motors, relays etc.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 13, 2018, 12:38:37 pm
content: Quote from: mike_mike on December 13, 2018, 09:27:46 amWhat does it mean 45VDC?i.e. is that just transformer secondary rmsV * sqrt(2) or an AC/DC switching module?LM358 might still work up to 24-25 VDC out with a single transformer. But you should use a 24VAC (or better 12+12VAC) 8+8A transformer to get a regulated 24-25V 5A DC output.The best opamp IMHO is LT1013 due to a high supply voltage, limited cost and wide IO range (and much better features than LM358). A cheaper alternative is MC33171/2/4Better opamps are more expensive and harder to find.Another solution is a voltage boost stage (common base or common emitter) at the expense of higher chances of self-oscillations.The Harrison topology would be better (i.e. floating control circuit) requiring just a further small power (few VAs) transformer and is usually more stable.

author: mike_mike
author_rank: Super Contributor
timestamp: December 13, 2018, 01:25:36 pm
content: 45Vdc means the voltage after rectifier and filtering.I would still like to remain to LM358 since my electronic components vendor does not have this part number (the one with LT).I just need something like a error amplifier controlling one or more transistors (npns), a ref voltage, a current limit (not variable). Also I want to have a output current of about max 5A and a voltage of about 3-25Vdc. A overload protection led should be very useful, but only if it is simple to implement.I can use a 30vca 10A transformer or an 24vca, 6A transformer.And I want to be very stable and I want to NOT oscillate... this is the hardest thing for me.Could you please help me with such a schematic ?

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 06:43:48 am
content: Quote from: not1xor1 on December 13, 2018, 07:58:37 amUPDATE #1 2018_12_14 not1xor1 is correct, my PSUs are in fact switching- I just had the covers offIt is a linear PSU, with a very low level noise output of 3mV, why do any guessing.Although these PSUs work well as is, there are a few things that you can do to enhance them. Check the quality of solder joints and general workmanship (my three are fine, but the quality can vary according to some reviewers)(2) Fit an output rail on/off switch(3) Fit better quality potentiometers, say ten turn for the fine controls.Incidentally these power supplies, at the price, can just be bought for the parts.The transformer itself would cost you over £50,and then there is the chassis, voltage and current meters,2N3055s,heatsinks, fan, knobs, caps, resistors, diodes ...I am thinking of converting one into a precision current source.

author: xavier60
author_rank: Super Contributor
timestamp: December 14, 2018, 06:50:30 am
content: Quote from: mike_mike on December 13, 2018, 01:25:36 pmMost power supply designs will oscillate without proper compensation, especially those that use op-amps.The power supply circuit in your first post will work well enough with compensation.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:11:32 am
content: Hi mike-mike,Attached below, is a schematic for your PSU as promised. Apologies for the delay, but it took longer to sort than expected, and other things got in the way too.Bear in mind that this is a paper design and I do not have a full specification for your mains transformer so I have made some assumptions. Also, right now, I do not have access to a lab so I cannot build and test a prototype for you. But I have analyzed the circuit and used a conservative approach whenever possible.You may find the circuit odd, but it is not an outline circuit like you normally see, and shows the required wiring including star points (SPs), padding, and decoupling.The two opamp options shown on the schematic are unique, and not any old opamp will do, although you could use other opamps with reduced performance. The OPA192 is the better of the two, but costs a bit more. But, in terms of your PSU there would be little difference in the performance. Mainly the accuracy of the 25V setting would be affected.The frequency compensation is very heavy, but that is intentional. If necessary the frequency compensation can be optimized at a later date, by a few simple component changes. The important thing is to get the basic circuit working at this initial stage.You will need very good heat-sinking for the output transistors and fan cooling would be advisable, but we can talk you through all that.Finally, I cant emphasis how important it is to do a good layout with the connections as shown in the attached schematic. The thick lines on the schematic indicate where thick wires/PCB traces are required. It is also essential to keep wires and PCB traces short and compact. The 100nF capacitor across the supply pins of the opamp, must have short leads and be physically connected to the opamp pins, or as close as possible to the pins. Likewise the padding resistors and other decoupling capacitors must be directly on, or close to the opamp pins.There is a physical layout that I would recommend, which will simplify construction a lot and optimize performance. Once you have had a look at the schematic, we can discuss the layout if you like.Unless otherwise stated, all solid capacitors are ceramic X7R through hole types, the physically bigger the better (not surface mount).The specification for the PSU is 0V to 25V (exactly), 0A to 5A output current.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 08:08:36 am
content: Quote from: David Hess on December 13, 2018, 04:28:48 amI take it then that you yourself or anyone that you know has built a successful TL431 PSU.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2018, 08:11:59 am
content: Quote from: spec on December 14, 2018, 06:43:48 amThere are lots of chinese PSUs looking the same and with small or large differences inside (some with a huge heatsink on the back some with just a piece of aluminium inside).I bought a similar one (30V 5A) for about 45€ including shipping.It has a huge transformer.The one you linked is 30V 10A. So it should have a 500VA transformer, but since they are coarsely overrated a 300VA one would be more likely.But if you zoom the pictures and look at the last one, you can clearly see that the case is empty.So unless it uses a low profile toroid it can be just a switching PSU.OK ...I found a youtube video... it is definitely a switching PSU.https://youtu.be/1_ZK68_c3Ok?t=145

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2018, 08:22:22 am
content: Quote from: spec on December 14, 2018, 07:11:32 amyou must be kiddinga 20V/µs opamp with a huge 1µF compensation capacitor...and what would be the advantage versus a proven and foolproof design using the traditional LM723 ?

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 08:30:25 am
content: Quote from: xavier60 on December 14, 2018, 06:50:30 amCan you show a circuit for this compensation using a TL431, especially one that you have built and tested successfully. Or perhaps know of such a PSU, that someone else has built and tested.I am genuinely interested as stated before.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 08:57:24 am
content: Quote from: not1xor1 on December 14, 2018, 08:11:59 amHell, you are right. I specifically ordered the linear type. My apologies.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:01:14 am
content: Quote from: not1xor1 on December 14, 2018, 08:22:22 amI said it was slugged.I made no claim about any advantage over anything. Why didn't you post a suitable 723 circuit? You could have saved me a load of bother.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:11:26 am
content: Quote from: mike_mike on December 13, 2018, 01:25:36 pmWith a 30VAC 10A transformer or even with the smaller 24VAC 6A transformer you are building more an expensive electric heater than a PSUYou must first take into account that transformer are rated for resistive loads. If you put a rectifier bridge and a capacitor, you get narrow pulses of high value currents which increase power dissipation through the transformer winding, so according to the value of the capacitor you have to properly derate the transformer. For average values (around 2000-3000µF per ADC) 60% would work.That means that a 30VAC 10A transformer is suitable for a 30VDC 6A PSU (with multiple electrolic capacitors in parallel for a total value of 12000-20000µF).The 24VAC 6A transformer is suitable for a 30VDC 3.6A PSU (7000-10000µF).And even with derating, in the worst cases you would need to dissipate approximately 200-220W or 100-110W. That is a lot of power.It would be much better to just use a rectifier bridge and capacitors to get a DC voltage and then use one of those programmable switching modules to get a regulated output. That would save lot of wasted power and expensive heatsinks.If you still want to build yourself such electronic heaters, given that you do not want current regulation, then the easiest solution is to just use the old LM723, even one of those datasheet circuits.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:19:59 am
content: Quote from: spec on December 14, 2018, 09:01:14 amthere are loads of 723 circuit diagrams/projects on the net, and lots of them even in the datasheets (ST, NSC, etc). So what would be advantage of yet another (in this case untested) design ?

author: xavier60
author_rank: Super Contributor
timestamp: December 14, 2018, 09:29:48 am
content: Quote from: spec on December 14, 2018, 08:30:25 amThe only times that I have used a TL431 as an active part of the regulation loop rather than as the voltage reference, are as low voltage control rail supplies. I used the same topology as the circuit in the first post, but with a medium power MOSFET or BJT acting as follower for the K voltage. Very easy to compensate with a capacitor between K and R.Just like with the circuit you posted, even if the feedback capacitor is oversize, the op-amp or TL431 becomes a very slow Miller integrator.Transient response remains reasonable because all that the op-amp or TL431 needs do is compensate for small changes in B-E voltage as load current changes.Response can optimized by reducing the size of the capacitor and adding some series resistance.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:44:42 am
content: Quote from: xavier60 on December 14, 2018, 09:29:48 amThanks- very informative.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:47:57 am
content: Quote from: not1xor1 on December 14, 2018, 09:19:59 amAs you say, there are stacks of 723 circuits around, some better than others, so I would suggest that linking to a suitable circuit would help the OP a lot.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 10:02:24 am
content: Quote from: not1xor1 on December 14, 2018, 08:22:22 amI took this as a joke, but just in case, I chose those two opamps because of their low input ofset voltage, very low input bias current, high output current, RRO, RRI(100mV beyond) and the ability to drive a relatively large capacitor. The cost is reasonable too. The slew rate is not relevant in this application.But, in general, there are endless situations, in fact most, where components are used way under their performance: 150mHz transistors used as audio amps, 20ns logic gates used to turn a LED on and off. A 10MIPS microprocessor used to alter the speed of a motor ...

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 10:33:23 am
content: mike_mike,Attached is a 723 PSU schematic which I found drifting around my laptop. This, is the kind of thing that not1xor1 and I were discussing.

author: mike_mike
author_rank: Super Contributor
timestamp: December 14, 2018, 10:55:44 am
content: Thank you for your help.What about the attached schematic ?I already asked on the forum but I still not get a response for some of the questions.Will it (the power supply) oscillate ?

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: December 14, 2018, 12:38:02 pm
content: The main issue with amplified IC regulator arrangements is high volts drop. The LM317 has a rather high burden voltage in itself, and adding a further transistor and series resistor considerably worsens that. Although, it is a workable arrangement if top efficiency is not important.The gotcha with your TL431 arrangement is that if the volts setting pot develops a bad spot on its track, the load is going to have its magic smoke forcibly removed, because the output will go straight to max. Putting the pot on the low side of the potential divider eliminates this risk, at the expense of the control being rather nonlinear. If it's a one-time preset that doesn't matter too much though.To achieve proportional control of voltage whilst being 'o/c wiper safe' generally requires a noninverting opamp arrangement fed from the pot's wiper, as in spec's reply.

author: David Hess
author_rank: Super Contributor
timestamp: December 14, 2018, 12:51:06 pm
content: Quote from: not1xor1 on December 14, 2018, 08:22:22 amThe compensation capacitor is way too large but a fast operational amplifier like that would be selected for lower noise. I have done the same thing many times using an LT1007 or OP27.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 14, 2018, 05:56:17 pm
content: Quote from: spec on December 14, 2018, 08:08:36 amGood evening spec,Have you?Tim

author: mike_mike
author_rank: Super Contributor
timestamp: December 14, 2018, 06:11:32 pm
content: Quote from: spec on December 14, 2018, 10:33:23 amI built this schematic and it works as expected.The problem is that LM723 will not be available more time from now and I need something that is always available from more than 2 manufacturers.So this is why I want to use LM317 and pass transistors or TL431.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:07:56 pm
content: UPDATE #1 2018_12_18 The attached schematic has the opamp input connections reversed- see reply #77 for revised schematic.Hi mike_mikeI got the message that you wanted to use parts from your spares box, so I did the attached schematic for you:(and from the above post I see you need components second sourced in Romania)

author: mike_mike
author_rank: Super Contributor
timestamp: December 14, 2018, 07:11:36 pm
content: Thank you.But I cant see were is the current limit or the short circuit protection...

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:17:55 pm
content: Quote from: mike_mike on December 14, 2018, 07:11:36 pmNo sweat.There isn't any current limit- didn't you say that you didn't need current limiting in one of your posts?

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:21:14 pm
content: Quote from: David Hess on December 14, 2018, 12:51:06 pmI don't mess about with compensation.

author: mike_mike
author_rank: Super Contributor
timestamp: December 14, 2018, 07:21:48 pm
content: I need current limiting. I dont need current limiting led.I need something like a transistor and a power resistor for current limiting. And I dont need that the current limit to be variable.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:24:20 pm
content: Quote from: IanMacdonald on December 14, 2018, 12:38:02 pmYeah, that and the temperature cut off.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:32:44 pm
content: Quote from: T3sl4co1l on December 14, 2018, 05:56:17 pmGood evening to you too Tim.Apart from the obvious vref applications in a benign environment, I have never been able to do anything with the TL431 and its family. So, I am biased and, for me it is one of those avoid chips. I wrote a critique of the 431 on ETO a couple of years ago, but can't seem to find it.

author: mike_mike
author_rank: Super Contributor
timestamp: December 14, 2018, 07:35:54 pm
content: Quote from: mike_mike on December 13, 2018, 01:25:36 pmHere I wrote that I need a current limit.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 07:43:38 pm
content: Quote from: mike_mike on December 14, 2018, 07:21:48 pmGot it. I just read all your posts on this thread and afraid I misread one of them. I am having one of those days.Will see if I can pop in some current limiting on the version three circuit (reply #45).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 14, 2018, 09:09:05 pm
content: Quote from: spec on December 14, 2018, 07:32:44 pmBasic scheme has already been mentioned:(I do in fact sometimes draw schematics; most of these are from my back catalog, hence the simple style.) Which the OP has already been set on the path too -- everything else is just an embellishment of this: adjustable voltage, current limiting, beefed up follower, better pull-up bias, etc.It's just a three-terminal op-amp, no mystery to it.Tim

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:19:02 pm
content: Quote from: T3sl4co1l on December 14, 2018, 09:09:05 pmThanks.

author: spec
author_rank: Frequent Contributor
timestamp: December 14, 2018, 09:20:55 pm
content: UPDATE #1 2018_12_18 The attached schematic has the opamp input connections reversed- see reply #77 for revised schematic.mike_mikeCurrent limiting added to PSU circuit version 3 of reply #45, as per attached schematic:

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2018, 12:29:53 am
content: Quote from: T3sl4co1l on December 14, 2018, 09:09:05 pmThe common emitter output makes it more like a three terminal operational transconductance amplifier which I think is where people get into trouble frequency compensating it as an error amplifier. It would be interesting to measure the transconductance versus current.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2018, 12:59:12 am
content: It's dominant pole (voltage mode) compensated, so it's a really bad gm amp, at least. It's true at DC, yes (but, DC isn't where we need compensation).Obvious difference: loading the output with C is fine for a gm amp, fatal for a '431!So it's more like an op-amp where they forgot the pull-up / follower part of the output.Tim

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2018, 02:15:10 am
content: I was not sure about how much effect the internal compensation had but of course the part has to be unity gain stable. The TI datasheet includes ashmoo plotof stability versus capacitve load which has the typical characteristics of a unity gain compensated operational amplifier with stability the worst at low gains and low output current.If anything, that makes things worse. It is too bad the world could not standardize on something like the LM10; it works well as a shunt reference also.

author: mike_mike
author_rank: Super Contributor
timestamp: December 15, 2018, 05:46:04 am
content: Quote from: spec on December 14, 2018, 09:20:55 pmC2, C17 and C15 are 10uF non polarised capacitors ?Does they really need to have such a big value and they really need to be non polarised ?R11, R16, R6, R13 could have a more usual value ?

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 15, 2018, 09:28:39 am
content: Quote from: spec on December 14, 2018, 09:20:55 pmsome notes:- The common emitter stage inverts the phase so you have to swap inverting and non-inverting inputs.- I've not made any calculations yet, but I think it would be safer to replace some transistors with BD139/40.- Probably it would be hard to compensate.I'll see if I can simulate it.

author: xavier60
author_rank: Super Contributor
timestamp: December 15, 2018, 09:51:06 am
content: Quote from: not1xor1 on December 15, 2018, 09:28:39 amWhen the op-amp's inputs are reversed, its minimum gain becomes unity.This can be a problem if there is too much gain elsewhere in the loop.I know from recent experience.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 15, 2018, 10:19:21 am
content: Quote from: xavier60 on December 15, 2018, 09:51:06 amWhat do you mean?In the classical floating PSU design, where the opamp inputs are swapped, the overall gain can be set to 0.

author: xavier60
author_rank: Super Contributor
timestamp: December 15, 2018, 11:04:45 am
content: Quote from: not1xor1 on December 15, 2018, 10:19:21 amWith the floating type regulator I built recently, the reference ended up on the CV op-amp's inverting input and the voltage feedback divider on the non-inverting input.Even if a large capacitor is connected between its output and inverting input, the AC gain can never go below unity.This isn't a problem in this situation because there is no extra gain between the CV op-amps output and the MOSFET's Gate.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 15, 2018, 11:37:15 am
content: For an output limited to a 2.5-25VDC range there is no need of a voltage boost stage. In the circuit below a LED and a diode allow the LM358 go a couple of volts beyond its worst case maximum output.BTW the zener should be at least 2W and the load resistor (1kΩ across the output stage and ground) should be at least 5W.The 2 leds display CV / current limit mode.I forgot to change Q8 (a BC327 would be OK) and to put a reverse diode on the output.

author: xavier60
author_rank: Super Contributor
timestamp: December 15, 2018, 11:52:00 am
content: That's a neat way of driving the overload LED. Is D11 still needed? It would be adding to the regulators dropout voltage.

author: mike_mike
author_rank: Super Contributor
timestamp: December 15, 2018, 12:29:05 pm
content: I am very very impatient to see a final version of this schematic, in order to build it.I can built it only starting from the next week.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 15, 2018, 01:21:07 pm
content: D11 is still very useful to allow a lower minimum output voltage. Due to the LED, the OP can not pull the voltage down so far.One could get away without if D2 and R14 are also left out. The LED D10 current would than be only limited by the OP internal current limit - should be OK with the LM358, but could be too much with another OP.If worried about the dropout, one could use a separate rectifier and filter stage for the current source around Q7 and Q8. This could give some 1 V extra, as there can be less drop due to ripple.

author: Cliff Matthews
author_rank: Unknown
timestamp: December 15, 2018, 10:04:26 pm
content: Dumb question since 30v 2-watt zener could be $$... Would jelly bean LM317 and small heat sink be just as stable?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 15, 2018, 10:14:41 pm
content: Just increase the resistor value, it doesn't need to be that beefy.C4, C7, D7 and D11 also aren't needed (at least not for a -- so far unstated -- ripple spec).Tim

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 16, 2018, 07:39:19 am
content: Quote from: Cliff Matthews on December 15, 2018, 10:04:26 pmIt depends on the transformer characteristics since the rectified voltage might drop a lot with a "heavy" load (I set the inner resistance of the voltage source to 0.3Ω to coarsely simulate that).I used just a zener in order to simplify the circuit.Nothing forbids to replace it with a zener/LT431 + BJT for a shunt regulator or a classical 2BJTs + zener series regulator which behaves as a capacitance multiplier providing low ripple even when the input voltage is too low (I've no idea of the behaviour of LM317 in that case).BTW I forgot to add a bleeding resistor to discharge the input capacitors.The capacitor in the constant current source should mitigate a bit the power-on overshot.Later I'll post an updated circuit.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 16, 2018, 08:02:34 am
content: Quote from: Kleinstein on December 15, 2018, 01:21:07 pmYou are right about D11.A resistor to limit the LED current is mandatory as LM358 can source up to 60mA according to the datasheet (ST) so it will blow a classical indicator led (and hurt your eyes).Besides that overloading the output of the LM358 would degrade further its poor characteristics due to the inner thermal gradient.A charge pump might be used to get an higher voltage rail. In that case the constant current source might even be replaced by a resistor since the current variations through it would be much reduced, that is approximately a range of 55V...32.5V / R vs 25V...2.5V / R (to provide 5...27.5V to the base of the darlington). That would also allow to use a low power zener to supply the LM358 thanks to reduced zener current variations.

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2018, 09:13:45 pm
content: Quote from: not1xor1 on December 15, 2018, 10:19:21 amI think what he is getting at is that the non-inverting configuration with a gain of 1 is less stable than the inverting configuration with a gain of -1 which for noise gain purposes is actually a non-inverting gain of 2. This is why some operational amplifiers are not stable as voltage followers and must be used at a minimum non-inverting gain of 2 or inverting gain of -1.A common solution to this is to raise the noise gain by placing a series RC network directly between the inverting and non-inverting inputs. By doing this for instance an OP37 or LT1037 can be used at lower gains even through they are specified for a minimum gain of 5.I have occasionally seen this crop up in power supplies where a specific operational amplifier could not be made stable no matter what was done unless the circuit was changed to raise the noise gain.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 17, 2018, 11:29:36 am
content: Here is a more complete circuit.Probably you do not need C12 in the real world as the instability of TL431 on startup would last fraction of seconds, but it was slowing the simulation (since Vdd rises slowly the TL431 doesn't get enough bias and oscillates, that agrees with the datasheet).BTW since the supply voltage of the LM358 is about 5V higher than the maximum output voltage one might even use the second opamp to implement current regulation (worst case max in voltage is Vdd - 2).

author: mike_mike
author_rank: Super Contributor
timestamp: December 17, 2018, 04:23:37 pm
content: There should be a NPN transistor where it writes "vdd" (with the base on vdd) ?

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 02:19:56 am
content: Quote from: not1xor1 on December 15, 2018, 09:28:39 amThanks- standard errorQuote from: not1xor1 on December 15, 2018, 09:28:39 amGood observation. I did look at the BC546/BC556 dissipation which will be 300mW worst case. I haven't done a thermal budget, but the BC546/BC556 limit on the datasheet is 500mW. I am not too keen on the BD139/140 for those circuit functions for various reasons. I was intending to recommend that mike_mike checks how hot the transistor cases are (by the finger test), and if necessary, add small heatsinks- no big deal.Quote from: not1xor1 on December 15, 2018, 09:28:39 amNot sure why you think this.Quote from: not1xor1 on December 15, 2018, 09:28:39 amBe great if you did do a bode plot of the circuit.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 02:50:41 am
content: mike-mikeI have been away from your PSU thread for a few days, but have now caught up with the latest posts.Attached is a revised schematic for the 25V, 5A Version 3 PSU with the opamp phase error corrected and other modifications as follows.Quote from: mike_mike on December 15, 2018, 05:46:04 amYes, they are X7R ceramic types for good high frequency performance.Quote from: mike_mike on December 15, 2018, 05:46:04 amYes, as above, but I have changed the schematic in view of your comments so that all solid capacitors are 100nF except one 1uF.Quote from: mike_mike on December 15, 2018, 05:46:04 amR11 was 330k, revised to 220k (330k is a normal value though),R16 was 24k and is now 240k and must be this value.R6 is 56R and must be this value (56R is a standard value)R14 is 14k and must be this value. But you will notice that it is annotated AOT (Adjust On Test). The actual value should be adjusted to give exactly 25V output voltage with the output voltage setting potentiometer set to maximum, but this is optional and up to you. I could have put another zener diode in the circuit to eliminate the need for AOT and then R14 could be a standard value. If you would like this option just shout and I will modify the schematic accordingly.About odd/unavailable values, you do know that you can make any value capacitor and resistor by connecting them in parallel/series.

author: mike_mike
author_rank: Super Contributor
timestamp: December 18, 2018, 04:34:23 am
content: @spec Thank you.D5 should be a simple zener diode or it is better to use a zener diode and a npn transistor ? Or there can be used an LM317 ?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 18, 2018, 06:49:31 am
content: Quote from: spec on December 18, 2018, 02:50:41 amThe regulator shown looks like it is rather slow with 1 µF at the OP and 100 µF after the transistor. The more normal values are about a factor of 1000 smaller. Is the stability (e.g. loop gain) check in a simulation ? With 2 low pass filtering stages and no phase boost there might be a stability issue, at least with a capacitive load.Even if the BC546/BC556 are specified at 500 mW, a TO92 case still gets rather hot at 300 mW, if there is no fan.So for the 10 mA current sink at the output the BD139 is a good idea. For the current source for the regulator one should get a way with a TO92 case transistor if the current is a little lower (e.g. 5 mA). This should be sufficient even for 5 A output.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 09:22:44 am
content: Quote from: mike_mike on December 18, 2018, 04:34:23 amNo probs- apologies for not correcting the obvious opamp phase error sooner.Quote from: mike_mike on December 18, 2018, 04:34:23 amYou could do any of those things: it is not necessary though. But, if for example, you would rather use an LM317, just say and I will post a modified circuit for you.D5 in the isue03 version is a simple zener diode and has two functions: To provide 15V Vcc for the opamp To provide the reference voltage for the PSU voltage stabilization loop.Because of the architecture of the issue 3 PSU, where there is a large voltage difference between the raw supply voltage and 15V rail, a single zener will be quite adequate for your stated requirements. Bear in mind that this PSU has been designed specifically with your stated requirements that in mind. Given a free choice the PSU architecture would be different, as would a few other areas, including the opamp and driver transistor which would probably be a depletion NMOSFET. The 2N3055 transistors would also change to a transistor with a much lower thermal resistance, junction to case.The other thing is that the LM358 current drawn from the 15V supply rail will be more or less constant at a maximum of 400uA, as the LM358 is not sourcing any current, only sinking current. This is one of the handy characteristics of the LM358: a constant current drain, as opposed to a varying current drain, is beneficial for the voltage stability of the 15V rail.But, I will post another PSU circuit to show how I would like to do the voltage reference using this architecture and if I have time, I will also post an LM317 reference version.By the way, this PSU gives an output voltage of 0V to 25V at 0A to 5A and will current limit around 6A. But the OV needs to be qualified: exactly 0V output is impossible to achieve without extra circuitry. This problem holds for any linear PSU, not just this one. On balance, and in the interests of simplicity and your requirements, I omitted the extra circuitry. But if you would like true 0V just say.The 0V ambiguity is due to leakage currents in the driver and output transistors, and there are effectively six transistors in parallel. Also the output transistors and driver will have a high junction temperature, especially with high currents and a low output voltage so the 0V potential will vary depending on usage. Temperature increases leakage exponentially.You will have, no doubt read the posts about the compensation being too heavy. Don't worry about this, it is intentional, as I have previously stated. There is little point in fine tuning the compensation until the PSU has been prototyped and tested. Once that is done we can optimize the stabilization, if necessary, by a few simple modifications, mainly capacitor changes.In common with many PSU designs, the approach is that the opamp voltage servo loop defines the absolute DC output voltage and also provides the very low frequency currents, the big electrolytic capacitor provides the medium frequency currents, and the solid capacitor (ceramic) provides the high frequency components. You can get a more in depth coverage of this area from the manufacturers application notes and from individual component data sheets.

author: spec
author_rank: Frequent Contributor
timestamp: December 18, 2018, 09:37:42 am
content: Quote from: Kleinstein on December 18, 2018, 06:49:31 amBoth of these areas have already been discussed.5mA Ik will not give sufficient current, worst case, to drive the output transistors.

author: mike_mike
author_rank: Super Contributor
timestamp: December 18, 2018, 06:56:14 pm
content: @specI need a final version of schematic, which uses LM317 instead of Zener diode and I will start drawing the layout.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 18, 2018, 07:56:14 pm
content: With the extra charge pump stage as shown the voltage might get too high for an LM317.So if at all one should use the charge pump to only give something like 2 times the normal voltage (diode to ground and not the normal positive supply).I don't see a real need for the charge pump. Already just extra diodes and Filter capacitor would give some extra headroom for the current source, as with an extra capacitor one can have less ripple than the main supply. For the low current needed by the OP, the zener might be sufficient - the LM317 would give some extra loss in voltage, but could still work.

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 04:13:59 am
content: Quote from: mike_mike on December 18, 2018, 06:56:14 pmOK will do

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 04:20:51 am
content: Quote from: Kleinstein on December 18, 2018, 07:56:14 pm

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 19, 2018, 08:13:00 am
content: Quote from: mike_mike on December 17, 2018, 04:23:37 pmthe current regulated by the zener is just few mAmperes so there is no need to transform it in a series regulator (if you meant that)

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 19, 2018, 08:16:02 am
content: Quote from: spec on December 18, 2018, 02:19:56 amI simulated just AC. The bodeplot doesn't look so good. It needs some more work on compensation.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 19, 2018, 08:28:56 am
content: Quote from: Kleinstein on December 18, 2018, 07:56:14 pmThe LM317 request concerned spec's schematic, not mine.Since the purpose of that circuit of mine was to get an output of 25V from a LM358 without any voltage boost stage, the charge pump ensures a proper and stable supply voltage for the LM358 even when the transformer voltage gets too low.Of course the circuit may work even without charge pump as showed in theother schematicI posted

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 08:57:08 am
content: UPDATE #2 of 2018_12_21 (R4 changed from 1k to 56R)UPDATE #1 of 2018_12_19 (add resistor to pot wiper)mike_mikeSchematic for 25V 5A PSU version 06, using an LM317 regulator, attached below. There are a few other minor changes here and there, including replacing all BC547/BC557 transistors with BC337/BC227 types. This will give more power dissipation margin for Q3 and Q8 and will make the constant current handover a touch sharper. But you can still use the original BC547/BC557 types if you like. The 1N400x diode in Q3 collector has been removed as it was unnecessary.I have done a thermal budget for BC547/BC557 in Q3 and Q8 positions and the maximum junction temperature, with 300mW dissipation and an assumed ambient temperature of 70 degC, is 125 degC, which gives a 25 deg C margin which is safe, especially as it is unlikely that the ambient temperature will be as high as 70 degC. All the same, with either transistor family, it would be wise to attach small heatsinks to the cases of Q3 and Q8.All resistors have been normalized and the need for an AOT resistor has been eliminated.A 1N400x diode has also been added at the output.All solid capacitors are X7R ceramic, through hole, and all polarized capacitors are aluminum electrolytic.

author: mike_mike
author_rank: Super Contributor
timestamp: December 19, 2018, 09:04:42 am
content: @spec Thank you.Can you please replace the transistors that will generate more heat (Q3, Q8) with some transistors that will support more power in order to not heat so much ?

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 09:11:17 am
content: Quote from: mike_mike on December 19, 2018, 09:04:42 amNo sweat mike_mike.I would rather not fit larger transistors because their other characteristics are not so good. This whole dissipation business for Q3 and Q8 has been blown up out of all proportion (like a lot of things). Don't even give it a thought. Either transistor type will be fine.

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 09:23:51 am
content: I would add 4x4n7/100V (4n7-10n) capacitors in parallel with the bridge's diodes, and an 100k (100k-470k) resistor from the RV3's wiper to gnd (in case the wiper looses its contact).

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 09:32:29 am
content: Quote from: imo on December 19, 2018, 09:23:51 amThanks. I did consider commutation capacitors across the diodes, but on balance decided to pass, besides which they would mess up my drawing.Good suggestion about the resistor on the wiper. I will embody that.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 19, 2018, 09:48:44 am
content: Hi,Every time I have to laugh a lot about the standard errors that are made at Linear power supply circuits here on the forum...Why let's take the last schemes, C8 100uF, and R13 100 Ohm, you must be jokink...If your reaction is to make the circuit slower because it generates, then you're in a wrong way to solve the problem, YOU NEED SPEED!!!Every component within de opamp loop creats delay, this delay is your main problem, and also placing extra delay components (C8 and R13) wil make your problems worse.Extra components in de control loop of a opamp may not create more delay so that the fase margen of de whole circuit comes below 45 degrees.If you design a goed linear power supply schematic then there wil be 60 a 70 degrees phase margin.Small tip, place a capacitor over R16 a 240K say 0.1uF.If you ignore this, you will always start crying.Kind regards,Blackdog

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 10:14:01 am
content: Here we go again. Another smart arse pops out of the woodwork.As I have said before to others, lets see your complete circuit for this application.

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 10:19:54 am
content: mike-mikeI have updated the schematic for version 6 of the PSU attached to reply #89: nothing major just added a 100k resistor to the pot wiper.Keep an eye on reply #89 because I will update that as necessary.

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 10:23:48 am
content: Also add a 10uF/16V from LM317's ADJ pin to GND (noise reduction).

author: spec
author_rank: Frequent Contributor
timestamp: December 19, 2018, 10:28:54 am
content: Quote from: imo on December 19, 2018, 10:23:48 amI also considered that but decided there was no need. Take a look at the many posts on this thread to get a feel for the background to this power supply project. Still, at least you haven't started banging on about how slugged the opamp is at this stage.

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 10:43:33 am
content: You have done a nice LTSpice simulation above - you may try with say 15V output, switching from 10mA -> 5A -> 10mA output current (with, say, 200ms period and 10us edges) and adjust the loop when required..

author: taydin
author_rank: Frequent Contributor
timestamp: December 19, 2018, 10:58:49 am
content: Quote from: blackdog on December 19, 2018, 09:48:44 amQuote from: blackdog on December 19, 2018, 09:48:44 amQuote from: blackdog on December 19, 2018, 09:48:44 amAND after showing complete disregard to the efforts of others trying to come up with a good design, you say:Quote from: blackdog on December 19, 2018, 09:48:44 amNow THAT's what is funnyYou are making some valid points, but because of your terrible attitude, nobody really notices them and you lose credibility...

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 11:10:04 am
content: The fact is when the control loop is "slow" an abrupt load change at the output will propagate "slowish". Thus it will not track properly.If the user not1xor1 made the LTSpice source public, we could have played with it and finalize the efforts of the contributors..

author: blackdog
author_rank: Frequent Contributor
timestamp: December 19, 2018, 12:44:43 pm
content: Hi taydin,I have no problems with your opinion, because you are entitled to it.I hope that my remarks shock it this topic a little up.If you are aware of my technical remarks and you know that they are correct, why didn't you help the people in this topic?On this forum you can find a lot of information on how to make a good power supply and dummy load circuit.But I get the impression that people are getting more and more lazy while the info with a little search is easy to find.That's why I have to laugh and sometimes maybe cry a bit...I will also keep an eye on the topic en help if necessary.Here is a link about stability, there are more in this series, change the last digit in the link.https://training.ti.com/ti-precision-labs-op-amps-stability-4And some PDF's in a zip file with info of TI about opamp stabilitywww.bramcam.nl/Diversen/all.zipKind regards,Blackdog

author: taydin
author_rank: Frequent Contributor
timestamp: December 19, 2018, 01:25:10 pm
content: Quote from: blackdog on December 19, 2018, 12:44:43 pmMany times I can tell if some information is correct or not, mostly gut feeling and strong intuition. But my theoretical knowledge and practical circuit design experience is nowhere near the remarkable people in this forum (probably including YOUR level of knowledge). But even when I'm in forums where my knowledge is WAY ABOVE the demographics, I always try to present what knowledge I have without derogatory remarks. Try it, feels better

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 02:14:30 pm
content: Here is the loop model, you may play with it.The output is set to 15V, the Load is pulsing 10mA/5A, 100us edges.PS: I would use a higher voltage at the opamp input, ie. 0-10V for 0-30V output (see the 20k/10k divider).

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 05:15:15 pm
content: In the below schematics:1. with C2<1n loop oscillates2. with C1<220n loop oscillates3. C3 is not criticalWith the values as depicted it seems the response is "the best".Mind it is an simulation only.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2018, 06:00:27 pm
content: The values of 1 nF and 220 nF look much more reasonable, maybe a little on the fast side.For the regulator it could help to have a capacitor with noticeable ESR at the output. So something like 100 µF with 0.1-1 Ohms in series.

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 06:06:29 pm
content: Needs more tuning

author: Kleinstein
author_rank: Super Contributor
timestamp: December 19, 2018, 07:33:38 pm
content: For tuning it can help to look at the response in the frequency domain, e.g. with a load as AC current sink (at both a low and high DC current). For the final tuning also C3 would come in. I would expect a smaller value here to be better. Another point to adjust could be the 56 Ohms resistors - they don't have to be the same. A little more at the emitter could be better if C3 is there.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 19, 2018, 09:02:36 pm
content: Hi,Imo, try this if you want to test.Remove Q3, R5, C1 and R7.Use a diode between R1 (make him 100 Ohm) and the anode of the diode on the base of the BD243.Place a 4K7 resistor between the base and the emittor of the BD243.Make the currend source I1 10mA.Make C2 220pF (play with it for good stability)Make V3 30VConnect V2 opamp power supply to V3, so remove V2.Watch your opamp inputs, are they still connected the wright way in this test setup?The problem with the schematic and many schematic of power supplys.The average opamp has between 55 and 65 degrees phase margin.And for a good stable linear power supply, you need at least 45 degrees phase margin, 60 is better.If one or more transistors are behind the opamp, these transitors slow down the loop in which the opamp in the transistors is placed.These transistors all eat a bit of the phase margin.Below the 30 degree phase margin you come into the danger zone.Please , please, please, do not disregard this information, because it always goes wrong if you do this.It doesn't matter if you are developing a linear or a switching power supply or an electronic dummy load, this is always the case!See my previous post with links to info on this topic from the TI website, first class info from TI!Kind regards,Blackdog

author: iMo
author_rank: Super Contributor
timestamp: December 19, 2018, 09:21:53 pm
content: I posted the LTSpice source - see above - you may download it and play with it as you like..PS: you may want to install the "Bordodynov" libraryhttp://ltwiki.org/?title=Components_Library_and_Circuits

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 20, 2018, 08:35:51 am
content: Quote from: imo on December 19, 2018, 10:43:33 amI got inspired byanother circuit posted in a different thread by xavier60(see below), but I used the dual voltage boost (common emitter) stage to tame open loop gain by local feedback, that is a sort sziklai with gain greater than one. Such circuit makes it much easier to compensate and variations of the compensation capacitor are not critical (at least according to LTspice) so you can choose between faster or more damped...and you even save a BJT.I'll show the circuit later (I want to read the other messages in the thread before).Here is xaveir60 circuit.BTW the BJT in the current control feedback is another interesting trick, although I found it works much better, preventing overshots with no recovery delay, when used to avoid saturation by keeping the output of the voltage control opamp just a bit below (or above depending on the topology) the output of the current control one. I have to run more simulations to be sure it works in any condition though.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 20, 2018, 08:45:47 am
content: Quote from: imo on December 19, 2018, 11:10:04 amI apologize for my laziness, I've little time and to post the source I should have chased lib files in my folders and made a zip as various models (lm358, tip41c) are not included with ltspice.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 20, 2018, 09:17:04 am
content: Quote from: blackdog on December 19, 2018, 09:02:36 pmThat has been discussed before and I also posted a couple of schematic diagrams showing that there is no need for a common emitter BJT in the loop.I did not mind posting the simulation results, but both circuit have safe margin (70-90° or greater) as showed both in AC and transient simulations.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 20, 2018, 10:21:03 am
content: here is the circuit with the local feedback. Unfortunately it has a really ugly short-circuit recovery overshot.here is for comparison the previous circuit I posted, with just an opamp as voltage controller.

author: spec
author_rank: Frequent Contributor
timestamp: December 21, 2018, 04:40:51 am
content: Quote from: blackdog on December 19, 2018, 12:44:43 pmWhat a presumptuous, patronizing fellow you are.Rather than being so opinionated perhaps it would be better is you read the previous posts on this thread.Quote from: spec on December 18, 2018, 09:22:44 amAs I said already, lets see your full circuit to meet the OP's request- or are you the one who is lazy?

author: spec
author_rank: Frequent Contributor
timestamp: December 21, 2018, 05:24:24 am
content: Quote from: not1xor1 on December 19, 2018, 08:16:02 amnot1xor1I had missed this simulation that you did for the pre reply #89 circuit. Thanks a lot- very interesting.But you have omitted one fundamentally important component: R19 (10K)What is the battery on the non inverting input of the opamp doing there (the reference voltage is the variable).Also in #89 revised schematic:R13, R20, and R21 are 56R.All small signal transistors are BC337/327D1 (D5) has been removed and replaced by a traceR4 has changed from 1k to 56R

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 21, 2018, 08:19:02 am
content: Quote from: spec on December 21, 2018, 05:24:24 amYou're right. I had overlooked the fact that the reference voltage was on the inverting input. Of course the local feedback is affected by the resistor in series with that input, but there is little difference between 10k and 11k (i.e. the pot at its minimum) of source resistance.I also changed the transistors and the resistors, anyway the bodeplot is still ugly.I tried to compensate the circuit in a different way and it improved a little (phase margin in the 50-70° range according to the load). But I've no idea if that might work in the real world. Later I'll run a transient load simulation.

author: spec
author_rank: Frequent Contributor
timestamp: December 21, 2018, 09:01:15 am
content: Quote from: not1xor1 on December 21, 2018, 08:19:02 amNice work- thanks. I am just digesting the sim results along with breakfast.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 21, 2018, 10:10:06 am
content: Hi Spec,Thank you for the insight of my psyche, I didn't know that yet, keep up the good work!Hi not1xor1Your last scheme looks better with R7 1-Meg and C2 4N7.But because of the amplification in Q6 it becomes difficult to get the whole stable again.Because of this you need C7 1N and R14 1K.I think it is useful to know, that for every ampare output current you need about 50uF over the output, as a rule of thumb.The more phase margin the control loops have, the smaller this value can become. (and some other things)With this kind of "standard" circuits as in this topic, use around 50uF/Ampere for good dynamic behavior.Even if you have sufficient phase margin, the dynamic behaviour may not be sufficient. (not fast enough)It becomes even more difficult when you hang the i loop in the circuit,Fault peak current protectionI always use the BC337 as in your circuit Q9 to limit the peak current.and then with an extra opamp make second loop for an adjustable current control.R2 and Q9 then limit the current a little above the maximum current the power supply should supply.Q9 is much faster in responding than an opamp loop.So in case of a short circuit Q9 protects your power trasistoren and your D.U.T. better than the current loop.But only if you make C7-1nF and especially the previous schematic C8 of 100uF that your surge protection with Q9 will not work!The high value of R16, 240K can make a nice noise contribution.C1 of 10pF is too small, I take depending on the total circuit setup here 10 to 100nF.A larger value helps to reduce the noise contribution and parasitic capacities.But an extra resistor is needed directly at the opamp input, this to protect the input during short circuits.Then the energy in C1 is partly dumped in the opamp, one or two diodes also help to keep the opamp intact in these error conditions.If the opamp itself already has diodes, take into account that you usually should not exceed the 10mA peak current in or out a opamp input!Furthermore it is good to think about bias currents, the LM358 is in this respect an old opamp so that higher bias values are unfavourable for the noise contribution and offset drift specifications.Most power supply circuits will exhibit undesirable behaviour in the transition area between CV and CC.And then it also depends on the amplification of the circuit (R16 and R17) and the current on which the power supply is set.That's the hardest part of designing power supplies.Spyce helps, but you will also have to build a test circuit to run into things Spice can't do for you.I hope this helps.Kind regards,Blackdog

author: spec
author_rank: Frequent Contributor
timestamp: December 21, 2018, 10:29:24 am
content: Quote from: blackdog on December 21, 2018, 10:10:06 amNo sweat and thanks for your informative and helpful post.

author: spec
author_rank: Frequent Contributor
timestamp: December 21, 2018, 11:10:49 am
content: Quote from: not1xor1 on December 21, 2018, 08:19:02 amnot1xor1Breakfast was nice, but I choked on the sim results. So much for my initial slug approach to get the circuit going.Your compensation looks good for the initial configuration so, if it is OK with you, I will embody it in the schematic of reply #89.Taking BlackDog's comment about the high source resistance for the LM358 inputs. I only increased the value from the original 1k to 10k so that mike-mike would be able to use a 1uF capacitor rather than the original 10uF. Is there any way that we can use your compensation scheme and reduce the source resistance back to 1k. So would:R16=24KR17=1kR6= 1kR7= 100kC2= 47nwork?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2018, 12:39:20 pm
content: For low noise the feedback divider should not divide much more than really needed to stay inside the OPs working range and the practical reference range. So maybe up to 5 V after the divider. For a 30 V maximum this would be something like 51 K and 10 K. It helps to have the divide by 6 or similar stage, as in this case the simple capacitor in parallel to the upper resistor can be well used in the frequency compensation.Even the 240 K and 10 K divider is not bad because of high impedance, but more due to the high divider ratio. So 24 K and 1 K would be nearly as bad. With a TL431 or similar reference one would still have quite some noise from the reference itself. So only at small voltages (e.g. < 2 V) the noise from the LM358 would be really relevant.The extra transistor stage with gain makes the compensation quite complicated. So unless really needed because the voltage goes higher than the OPs output range, I would avoid the extra gain stage and directly control the Darlington output stage from the OP.To get a good regulation over a wide range of loads, including low ESR capacitive loads, it really helps if there is some output capacitance with ESR. There is a reason that nearly all lab supplies have some electrolytic capacitor at the output - though one may not need as much as often found. This is not just for the capacitance but also for the ESR of the electrolytic cap. The output ESR is kind of part of the compensation scheme. As a loss to the capacitor it still has an effect if there is an external load that includes a capacitor with less ESR.For the phase margin one should have a really good phase margin with a well behaved resistive load. With a tricky load like low ESR capacitor plus current sink the phase margin will go down. This is acceptable and essentially unavoidable. Still a good phase margin with a resistive load does guarantee stability with a difficult load. So it might help to adjust the compensation for the difficult case first and only than check the easy case.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 21, 2018, 02:58:10 pm
content: Hi,Some extra remarks....The LM358 is a slow opamp and with a well-designed circuit it should provide a reasonable power supply with the 2N3055 transistors.The TIP41 and the 2N3055 are both "slow" transistors and they do not help to make the circuit stable.Hint, make the base emittor resistor a little lower say 56 or 68 Ohm, which helps with the paracit capacitors of the 2N3055.And also this, there are so may different 2N3055 transitoren, Ft 0f 0,8Mhz, Ft of 3Mhz etc,So if someone present here a schematic with 2N3055 transistors, then you will actually have to take into account the worst version when it comes to loop compensation.A solution for better phase marginBetter is to use faster power transistors like the cheap 2CS5200 (the real one)Almost all 2SC5200 transistors have a Hfe of > 90 and if you use a BD139 as a driver and some coling,then you have power stage that does not affect the phase margin of the LM358 much.That doesn't mean that Q6 is a good idea, Q6 means troubleCan it be done with Q6, look at several HP power supply schematics, do not forget the ferriet bead.An exampleSuppose we take 4x a 2SC5200 and a max 5a power suply.If we then assume an Hfe of 90 (usually it is more, especially if the transistor gets warm)The base current in then 1.25A/90 = ~14mA per 2SC5200 which is then about 60mA of total base current for four transistors,there is then a little extra for the emitter base resistor, say total 70mA.Drive this 70ma with a BD139-16, Hfe > 100x, that makes the base current of the BD139 below 1mA.Make a 5mA current source and a diode from the opamp to de base of the BD139 to sink the current.This makes a fast power stage that does not affect the phase behaviour of the opamp very much.Testing output capacitorsThis is a setup how i tested, to make a good RC section over a wide frequency for the design i was working on.And a nother test.Prefered schematic/designI prefer the Harison setup with a floating power supply for the opamps, the extra transistor is olmost never necessary.www.bramcam.nl/NA/NA-01-PSU/Harris-Tech-Letter-02.pdfwww.bramcam.nl/NA/NA-01-PSU/Harris-Tech-Letter-03.pdfwww.bramcam.nl/NA/NA-01-PSU/Harris-Tech-Letter-04.pdfwww.bramcam.nl/NA/NA-01-PSU/Harris-Tech-Letter-07.pdfwww.bramcam.nl/NA/NA-01-PSU/HP-AN90A.pdfAnd then there's this one, very importantwww.bramcam.nl/NA/NA-01-PSU/Calex-Power_Impedance-Decoupling.pdfHow to test phase and gain marginAN-1889 How to Measure the Loop Transfer Function ofPower Supplies From TIwww.bramcam.nl/NA/NA-01-PSU/snva364a.pdfMore than enough information for the holidays.Kind regards,Bram

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2018, 08:36:18 pm
content: The slow 2N3055 were the more robust old style transistors. One hardly finds them any more. However the once popular 2N3773 are usually still slow. For a high performance power supply it really helps if the transistors are fast, but for a simple low cost supply one can still use the normal 2N3055 or TIP35. It still helps if the second transistor in the Darlington configuration is fast. A low power the BD139 is a good choice, at higher power something like D44H11. A slower supply would need a larger output capacitor though.@blackdog: the links provided are interesting, but have a trailing / too much.

author: blackdog
author_rank: Frequent Contributor
timestamp: December 21, 2018, 08:57:55 pm
content: Hi KleinsteinThank you for finding the errors in de links en telling me!Almost all links were broken, sometimes even a double address in one line and even one line whas started with http:www.eevblog.com in front of it...And when I clicked on a link, the pdf was downloaded... spookyI hope it is now OKKind regards,Bram

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2018, 03:09:18 am
content: Quote from: blackdog on December 21, 2018, 02:58:10 pmI think there is some benefit up to a 3MHz operational amplifier (except for likely needing extra frequency compensation) but the old JFET input parts were 3MHz and had much higher slew rates which is even more important for designs where the voltage or current error control loop sits in saturation. The old RC4136 was a weird 741 class operational amplifier which was 3MHz so this was not universal.Old slow LM358/LM324s, 741s, and 310As could almost always be used without any extra frequency compensation and perform just fine.QuoteI do not think anybody has produced the slowest 2N3055s for a long time now. If you want one of those, use the 2N3771G or 2N3772G. See below.Quote from: Kleinstein on December 21, 2018, 08:36:18 pmBe real careful about the 2N3773; I thought they all used the 0.8MHz process but apparently all new production at least from On Semiconductor is 4MHz. On the other hand, the 2N3773G has the highest secondary breakdown voltage of parts listed below at 100 volts. The MJ15015 and MJ15016 are not far behind at 90 volts.2N3771G NPN 40V 30A 150W 0.2MHz2N3772G NPN 60V 20A 150W 0.2MHz2N3055AG NPN 60V 15A 115W 0.8MHzMJ15015 NPN 120V 15A 180W 0.8MHzMJ15016 NPN 120V 15A 180W 2.2MHz2N3773G NPN 140V 16A 150W 4MHzMJ15022 NPN 200V 16A 250W 4MHzMJ15023 PNP 200V 16A 250W 4MHzMJ15024 NPN 250V 16A 250W 4MHzMJ15025 PNP 250V 16A 250W 4MHzQuoteThe D44H11 is a RET (ring emitter transistor) style so it may have other advantages as well. I commonly see them used in low noise and fast response regulators and of course audio power amplifiers. Oddly enough, the final design of the problematical single ended high voltage inverter Tektronix used in their 465 series oscilloscopes changed from a selected 2N3055 to a much faster RET D44H/D45H series.

author: spec
author_rank: Frequent Contributor
timestamp: December 22, 2018, 06:27:35 am
content: Thank you all for the interesting information in the last posts #122 to #127 above. But, I am afraid that much of the good stuff cannot be embodied in this PSU because the components are a given by the OP, who probably has limited access to components in his local. As stated a few times now, given a free hand, I would not have have used the components or architecture in the PSU of post #89, especially the opamp, or the extra transistor stage, or at least not in the common emitter configuration.I agree that the additional transistor stage is a PITA from a frequency compensation point of view, but it is the only way I could provide adequate output voltage swing and current sinking, while providing sufficient base drive for the output driver transistor. My scheme on the original design was, in the interests of frequency stability, to get the voltage gain of the transistor stage down to 1 at as low a frequency as practicable.It may be possible to drive the output stage directly from the LM358 if another emitter follower were added to drive the output stage. There is another aspect though, that concerns the relationship between the constant voltage and constant current modes. This is always a difficult area and IMO the safest way to do it is by the 'decreasing gm' method which precludes the use of a voltage drive of the output transistors directly by the opamp.The other aspect is that the OP wants to use an LM317 regulator and with its dropout voltage and 40V input limit that makes things difficult.About the 24:1 voltage divider. While I agree that it may increase the noise and voltage drift, I am not sure that it is that significant. But the attenuation does reduce the feedback and should help with the frequency stability, at least that was my thinking. It also allows some flexibility in tailoring the open loop response, as has already been done.If anyone has an improved complete, practical circuit, that meets all of the OPs requirements and is easy to read and that the OP can use to build a PSU, please post it (this is a genuine request).Also if anyone has an approach, in addition to not1xor1's work, for compensating the reply #89 circuit please post a complete practical circuit (of the compensation that is). That would not only be interesting, to me anyway, but also very useful.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2018, 11:01:01 am
content: The initial circuit right from the start is not that bad - it is mainly missing the frequency compensation. So with the first response one could call it problem solved. There are still some points to optimize and the selection of parts could be better, but nothing serious.For just a simple power supply the "normal" modern (e.g. 3-4 MHt Ft) 2N3055, TIP35 or similar should be sufficient in speed. It helps if in the Darlington circuit the smaller transistor is a little faster. But this is easy, as the BD139 is nothing special. Even the D44H11 is not that exotic.The voltage regulator with the Darlington emitter follower output and thus a low output impedance output stage normally does not need a fast OP for the regulation, unless one has high demands. So for normal use the LM358 is sufficient, at least if one does not divide down the feedback signal very much (e.g .down to 1.2 V), as this would need more gain.If there is a second loop for current regulation (and not just the transistor for a crude current limit), than one might want a faster OP in the simple circuit, as the slew rate of the OP would limit how fast the current limit can engage. The 0.3 V/µs of the LM358 would require some 100 µs to come down from 30 to near 0 in case of a short. Even with a faster OP, the current regulation is a little harder in this type of circuit.With only up to 25 V the simple circuit type is perfectly fine. Going up to the usual 30 V starts to get tricky as the supply for the OPs is limited.The LM317 was an idea for the OPs supply. Here the 40 V limit may indeed be a problem - though it only applies to the in to out difference, not the absolute voltage that is never seen by the LM317. Still I would prefer the simple Zener solution for the rather low current (e.g. 2 mA) needed for the OP.

author: iMo
author_rank: Super Contributor
timestamp: December 22, 2018, 02:26:49 pm
content: This works nice, imho..It oscillates without the C4 capacitor.Responses I_Load=10mA/5A for a list of C_Load capacitors.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2018, 03:02:38 pm
content: There still seems to be quite some ringing - so maybe C4 should be even larger. A suitable series resistor to C4 could also help.

author: iMo
author_rank: Super Contributor
timestamp: December 22, 2018, 03:46:41 pm
content: And with the current limiter, switching various R_Loads in/out.

author: iMo
author_rank: Super Contributor
timestamp: December 22, 2018, 04:14:08 pm
content: And with constant R_Load=10ohm (switched on/off) with various values of R2 (output voltage setting).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2018, 04:57:27 pm
content: There are two small problems with the simple TL431 based regulator:The first is relative obvious: one can not set a voltage below 2.5 V. This might be OK for a fixed supply to a circuit, but would be a slight limitation for a stand alone supply.The second possible problem is that the loop gain and thus the suitable compensation tends to depend on the set voltage, if the voltage is set by adjusting the feedback divider. The way it is shown in the last simulation, by adjusting the lower resistor this is to a large part avoided, but at the cost of a rather nonlinear scale and even higher lower voltage limit.So the circuit is nice and simple for a fixed supply, but not that suitable for a variable. For an adjustable supply it should be better to have a kind of OP to compare the feedback from a fixed divider with a variable set voltage.For finding the right compensation from a simulation I found it rather useful to look at the output impedance: So similar to the load test with a constant current load a current sink (source) is used as a load, but this time in AC mode and first without much of the output capacitance. Looking than at the AC output voltage than directly gives the output impedance. Ideally one want's a low output impedance of cause. The more important part is that the phase shift does not reach values above 90 degrees from a resistive load. Getting to close to an ideal inductance (90 degree phase) is also bad as this would cause much ringing if there is a corresponding capacitive load.Some of the higher frequency problems can be fixed with those RC combinations at the output, but the lower frequency part has to come right from the regulator. Only at the very low end (e.g. below some 100 Hz) one can tolerate more phase shift, as it would take extreme, non realistic capacitors (e.g. C > 100 mF with ESR in the µOhms) to make the regulator unstable.The nice point about looking at the output impedance instead of the more common loop gain is that a single simulation covers all possible load impedance cases. One still has to check different DC currents as the output stage can behave different at high and low currents. For the loop gain one would need to additionally test with different capacitors at the output to get the "whole" picture. Another point to check would be the hand over from current to voltage regulation and back.

author: iMo
author_rank: Super Contributor
timestamp: December 22, 2018, 11:39:20 pm
content: Bode

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 23, 2018, 09:09:31 am
content: Quote from: Kleinstein on December 22, 2018, 11:01:01 amThe initial circuit is not that bad ...if one just looks for trouble.So far have I been unable to make it work under all the possible conditions of load and feedback resistor.TL431 is handy for a fixed voltage regulator, but unless you severely compensate it (achieving awful transient recovery times) it is unsuitable for a bench psu.A PSU with fixed feedback opamp and variable voltage reference is not more difficult to build, but is much more reliable and easier to set up.And with a range of 2.5-25V, as required by the original poster, there is no need for a further voltage boost stage.I already showed two simple circuits which are simple yet offer much better performances than the other proposal.Of course one is free to do what he/she likes...

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 23, 2018, 09:11:12 am
content: Quote from: imo on December 22, 2018, 11:39:20 pmthat's fine for a power oscillator

author: iMo
author_rank: Super Contributor
timestamp: December 23, 2018, 10:33:55 am
content: Stability test simulation made according to theAD app noteVout 15V/1.5A with:1. C4=100pF2. C4=1nF3. C4=1nF ser 56ohm4. none comp C4 - here at 300kHz the phase margin is 0deg - it oscillates

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 24, 2018, 08:17:43 am
content: Quote from: imo on December 23, 2018, 10:33:55 amyou must not forget that TL431 is specified for a minimum current of 1mA (although in some cases might be fine with as little as 500µA).R4 might work for the whole voltage range just for 1A of output current or so...once you replace it with a constant current source the open loop gain soars and the nightmare beginsBTW it would be better to start from 1Hz or even below...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2018, 10:41:37 am
content: FWIW, TL431 works just fine at <1mA, it's just not going to be spot on 2.500V. In particular, it drops to about 2.2V at light currents, around which it's pretty much off (~uA).Knowing this behavior is helpful when working with a clamping or startup threshold sort of circuit: any time the threshold needs to be sharper than a comparable zener (a 2.5V zener is a damn good resistor in comparison), but also accurate at higher currents. (For example, the fact that current rises from ~uA at ~2.2V, to 1mA+ at 2.50V, only means a clamping threshold is rounded off by a comparable amount.)Tim

author: iMo
author_rank: Super Contributor
timestamp: December 24, 2018, 12:33:42 pm
content: With above R4=4k7 the TL431's current will be from 10mA to 4mA for 3V/5A to 30V/5A output voltage/current..Mind it is an simulation only, with values and parts taken as an example..With 60V input and 3V/5A output the Q1 will dissipate 20W and Q2 260W of heat

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 25, 2018, 08:07:23 am
content: Quote from: imo on December 24, 2018, 12:33:42 pmWe must take into account the case of output voltage set to 25V with maximum load.In such case, when using a real transformer, the rectified voltage would drop several volts due to the transformer losses and the ripple, so the minimum rectified voltage has to be greater than:> maximum Isense voltage drop (about 0.7V) +maximum ballast voltage drop (in case of paralleled power devices i.e. about 0.7V) +maximum compound Vbe drop (1.5-2V) or ce saturation voltage (whichever is greater) +peak-to-peak ripple (1-6V depending on the value of the capacitor(s) and the maximum output current)And then the resistor feeding the base of the power devices should provide at least 1mA through the TL431 plus the base current for the power devices... let's make it a couple of mA in case of just 1A max output current, that makes : 2e-3 * 4.7e3 = 9.4V...That's a lot... at least 9.4W wasted by the power devices for no real purpose.Besides that the task of reducing the input ripple would be totally on the shoulders of that poor TL431...So now we have 3 options:-1) use a low minimum voltage constant current source (making the TL431 more prone to oscillations)-2) use a transformer with voltage much higher than required (wasting further 10W per A of output current)-3) use a charge pump (2 diodes + 2 capacitors voltage doubler) to get a high voltage railThe 3rd options might be the only viable in case of a TL431 PSU.So in this case the minimum rectified voltage can be as low as 28V which would allow to get more than 50V (more likely 53-54V) on the high voltage (low current) rail.If we just need 2mA (like in the above example) we must use a (50-28)/2e-3 = 11k base-feed resistor.But we could better use 2 resistors, let's make it 3.3k and 8.2k (or 4.7k and 6.8k) and place a 100µF electrolitic cap in the resulting node to ensure both soft start and ripple reduction.The maximum current through the resistor would be achieved with minimum output voltage and no load. In that case the high voltage rail might get as high as 70V-71V that would make less than 7mA...still safe for the TL431 with just around 17mW of dissipation while it would be better to use 2 1W resistors (although they would dissipate about 1/4 W).Of course, in case of higher maximum output current, the resistor values (and power) should be recalculated.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 25, 2018, 08:13:20 am
content: Quote from: T3sl4co1l on December 24, 2018, 10:41:37 amI know that TL431 would work with few hundreds of µA, but I can't imagine who would like a PSU whose output voltage may drop 10% or more depending on the load

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2018, 08:20:11 am
content: Quote from: not1xor1 on December 25, 2018, 08:13:20 amI know, it seems silly doesn't it, when we're designing a PSU whose output may drop 100% (hopefully not more!) depending on load!...Hyperbole aside, actually, note that the TL431 is at maximum current when the PSU output is minimum load -- it's regulating best there! So it really doesn't hurt anything, as long as, at the point where current limiting cuts in, on that very knee, there's still that extra 1mA available for the TL431.And again, the consequence is merely a softening of the knee, which is hardly immaculate in this case because we've been talking about Vbe sensing action in this thread.So, assuming the bias supply current is that precise, it really would work out just fine here.The bigger problem, then, would be ensuring current is actually that close -- in practice, it will be far less accurate (we're talking, say, a 1mA margin out of a 50mA or whatever total requirement!), and setting the guard band for that variance will take priority.Good to know, if that guard band runs out, it'll behave itself though.Tim

author: iMo
author_rank: Super Contributor
timestamp: December 25, 2018, 09:39:31 am
content: With a 6.3mA current source and 2n2 ser 100ohm comp.3V/5A25V/5A

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 27, 2018, 08:14:20 am
content: Quote from: imo on December 25, 2018, 09:39:31 amI confirm the circuit works as showed in your screenshot.Il looks like the problems arises from the placement of the AC source that yields awful results when placed like I did.I've to check more carefully the subject to understand what is wrong.ThanksP.S.since I suspected something is wrong in your measurement method I made a quick transient test, switching between 5mA and 1A on 25V output and the output goes awry... huge oscillations... see below

author: iMo
author_rank: Super Contributor
timestamp: December 27, 2018, 02:25:04 pm
content: Measurement method?? - That is the question!I took the LT/ADI method - see the link above.You previously did with a simple AC sweep.Your oscillation - are you using zero ESR output capacitors?Below with "real capacitors", no oscillation, a small ringing w/ 160mVpp amplitude..

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 27, 2018, 07:40:45 pm
content: Quote from: imo on December 27, 2018, 02:25:04 pmThat is a different method but should yield similar results.I used models of real capacitors too...I also tried to add a resistor of 0.5 ohm in series to each capacitor with no result.BTW this isLTspice XVII x64 Jun 15 2018running in linux.I tried both alternate and normal solver, 4 threads and single thread.No matter what I try I always get oscillations.Might that depend on the TL431 model I'm using here ...?in the comment I read:Quote

author: iMo
author_rank: Super Contributor
timestamp: December 27, 2018, 08:39:54 pm
content: Here is the latest LTspiceXVII, and with the latest Bordodynov library (LTspiceXVII_2018Dec21.zip).Below the TL431s offerings..PS: tried with several TL431 models and got similar results as above..And the library ishere, for the latest download go to "Additional links :"

author: xavier60
author_rank: Super Contributor
timestamp: December 27, 2018, 11:14:55 pm
content: Quote from: not1xor1 on December 27, 2018, 08:14:20 amI think that C1 and R17 should be made larger, I'd start with 1uF and 1K

author: Kleinstein
author_rank: Super Contributor
timestamp: December 28, 2018, 09:16:10 am
content: A larger value for R17 is likely a good idea. About half the values of R8 (feedback from the input) could be a good start.1 µF for C1 would be really slow and only a start. The shown 2.2 nF seem to be in a reasonable range, when looking at simulations from IMO.One should also keep in mind that the loop gain would change with the feedback divider.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 28, 2018, 10:05:21 am
content: Quote from: imo on December 27, 2018, 08:39:54 pmOK, but what really matters is to know which kind of TL431 library you got.The one I'm using clearly states:Model Type: Transient and AC.The one you're using might be a coarser model... might it not?(OK... I'm downloading the huge zip and will check that myself later)

author: iMo
author_rank: Super Contributor
timestamp: December 28, 2018, 10:14:11 am
content: Here with the 2n2 (C4 in my simulation) and R4 from 100-16k.For 25V/5A pulses and 3V/5A pulses.For 3V the R4s>=1k in order the simulation works "reasonably fast" (PS:With Behavioral model TL431A it simulates fine from 100ohm up).With 25V the R4s>400ohm make more ringing.

author: iMo
author_rank: Super Contributor
timestamp: December 28, 2018, 10:40:34 am
content: Quote from: not1xor1 on December 28, 2018, 10:05:21 amA good moveBe careful with the installation of the Library.You have to replace the 2 folders (examples, lib) in:C:\Users\..user..\Documents\LTspiceXVII\PS: make backup of your own models/parts.Do not touch folders in:C:\Program Files\LTC\LTspiceXVII\After the start LTspice will reindex the new Library.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 28, 2018, 04:49:13 pm
content: Quote from: imo on December 28, 2018, 10:40:34 amI already did that in past, but I prefer to add just the models I need.BTW I use my own paths and just add them to the search path in LTspice settings.In any case I see that there are 4 different TL431 models which added to the one I've been using so far are 5.No wonder the same circuit may produce so different results...I guess the only way to know which one works better is to build a few differentrealcircuits and compare the results.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 31, 2018, 08:01:13 am
content: I unsuccessfully tried to use the other TL431 models (LTspice reported some error messages). Probably the problem was due to the symbols (.asy files) I used.I did not investigate further for lack of time, but I tend to trust the model I used previously much more than the others as it was provided by TI.This last simple schematic seems to work under a range of loads and feedback resistors. If I had to build a TL431 variable voltage PSU (I wouldn't call that a bench PSU), I would start from this circuit.