Finalizing the design of my lab PSU - Page 1
Forum section: electronics / projects

author: erikj
author_rank: Contributor
timestamp: August 26, 2014, 10:59:58 pm
content: Hi everyone!First time posting on this forum.There has been several good threads here lately about DIY lab power supplies. Reading them gave me some inspiration to try to come up with my own design.So lets start with some goals and ideas I had for the design (in no particular order).Fairly high power: 0-40V adjustable voltage and 0-5A current limit.I will be using some kind of tracking switched mode pre-regulator to reduce the heat sink requirements.Analog controls with coarse and fine settings for voltage and current, but adding digital control at a later stage should be an easy task.I am planning on using a 48V 250W switched mode power supply instead of a mains transformer.I want the ability to use external voltage sense wires.If possible, very low or no output capacitor in order to have very fast current limiting.I want it to be rugged and well protected...and as stable and accurate as possible of course...Anyway, this is the design I have managed to come up with so far. It is based on an early version of void_error's Lab PSU design, but with quite a few changes.I have simulated and also bread-boarded the main part of the schematic and it seems to be working OK.This is the most complex design I have attempted as of yet so I am now looking for some feedback and suggestions.SchematicSome thoughts and comments for the schematic:The pre-regulator part is not included. I will start playing around with that part at a later stage.SD1, SD2 are intended to clamp the voltage on R20, R21 if the main wires are disconnected but the load is still connected by the voltage sense wires. Maybe there is a better way?I used a Vref of 4.096V is to be prepared for digital control later and also as a reference for the voltage and current meters(Meters will be using a MCP3551 AD-converter, PIC micro controller and a LED display)Compensation capacitor C20 for the current control loop was added after some breadboard experiments. More on this below.Zener diodes Z1, Z2 are there to avoid losing regulation if the (front panel-) LEDs should fail or be disconnected.Differential amp for current sense is probably not needed, I added that to make the layout of the ground paths simpler.When breadboarding the circuit I've had some problems with oscillations in the current control loop.I was using a few 24V light bulbs as dummy load, and in parallel I put a 1000uF low ESR capacitor.Adding the capacitor when in current limit mode caused the circuit to start oscillating at around 80kHz.To fix this I tried increasing C7 to 10nF and then to 100nF, but that did not seem to help at all for some reason.I finally managed to fix the problem by adding a capacitor (C20) from base to collector on T2, something I remember seeing in some audio amplifiers.I am not quite sure what is going on there...Now to the questions:Do you see any mistakes or weird design decisions in there?I am using 3 out of 4 op-amps from the TL054, can you think of a good use for the last one?Most designs I have seen seems to be using high-side current sensing, is that a better solution than my low-side version?Is there a better way to stabilize the current control loop than what is in the schematic?Do you know a good way to test for overshoot and/or ringing without access to a DSO?Can you suggest a really nasty load that I can connect to check how the regulation performs?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: August 27, 2014, 11:07:06 am
content: Quote from: erikj on August 26, 2014, 10:59:58 pmHi erikj .Nice job.You say you simulated it ?. you have an lt spice asc file to upload ? .It's alot easier to offer feedback comment and suggestions if the simulation is available .In respect to the above :1/ Yes there probably is a better way to stabilize the CC loop than C20 ,I think youl need at least a small cap on the output .but it's hard for anyone to say , you have to look at a bode plot of the loop ,and thats where your spice sim file helps .2/ Not without a oscilloscope you need one to see the ringing . You then load the output and (step the load) with a fast pulse ,then you can catch the recovery response on your O Scope .Wiht this you can judge how stable your loop is by the number/amplitude of overshoots in the recovery response.3/nasty loads .for testing your CC loop .Test with a few sizes of inductors 100uH, 1mH,100mH,10H running at the largest current they can handle ,for a large inductor hook up a primary of a mains transformer (leave secondary open) .For nasty loads for a C.V loop try multiple low esr caps across the output at low currents (so large Rload or open) .Try multiple ceramic caps in parallel ,and then a large low esr electolytic. But without a scope to view results this is all bit futileRegards

author: dannyf
author_rank: Super Contributor
timestamp: August 27, 2014, 11:20:36 am
content: Just some random questions for you to think about:1) the choice of opamps: particularly when they are operating towards the limits -> near short or fully open. Will the opamps behave as expected? If not, what to do?2) what value does T6/T7 add?3) what value does D2 add?4) do you really need T2/T8?5) C4 / C7 seem too large.6) R23 should be grounded? What other functions can R23 perform?7) C5/C8 next to the opamps? Make it easy to add pwm output from a mcu....I posted a tried-and-true design that's much cleaner than this and can be easily converted to be driven by a mcu. Look it up if you want.

author: Andreas
author_rank: Super Contributor
timestamp: August 27, 2014, 12:58:26 pm
content: Quote from: erikj on August 26, 2014, 10:59:58 pm240 W over 1 transistor that is capable to handle 200W @ 25 deg C case temperature-> you will need LN2 for cooling.alternately around 2 additional power stage transistors.The current limiting may be too slow for quickly changeing loads:The largest problems I had with a load that switched from nearly idle at maximum output voltage to maximum limiting current (nearly short).I simply used a large FET with a PWM-generator to test my PSU this way.Finally I used a additional current clamping transistor between emitter distribution resistors and base of the power stage to limit the peak current to safe values.Edit: probably you will need 2 reverse diodes at the output of your 7805/7905. Otherwise the regulators may stuck during power on.with best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: August 27, 2014, 03:22:43 pm
content: Hi erikjFirst, 1 powertransitor is not enough, for that kind of power 3 à 4 transitors, its als a plus that you can better distribute the heat.Look @ the datasheet of the MJ11016, Hfe @ 5A is around 5000, thats 1mA you dont niet de BD139 drivers...This wil only give you more phase lag (Thats why you schematic is generating 80Khz)The OP277 is a nice opamp, but... so slowwwwwwww, its whitin de loop of your currend controle, redesign this!The use op BIG compensation capacitors is a rookie mistake.If you have e relatif clean 48V supply remove T4 and T5, use a resistor, start with 5mA of current here.I can go on but than i rebuild your hole schematic *grin*At the output you need about 50uF/Ampere capacitor.You can take a look @ this design, its in Dutch, use google translate.And this is the hole artikel, its BIG and a lot of info.http://www.circuitsonline.net/forum/view/110029/1Kind regarts,Blackdog

author: erikj
author_rank: Contributor
timestamp: August 27, 2014, 05:42:44 pm
content: Quote from: Kevin.D on August 27, 2014, 11:07:06 am1> I am using TI-tina for the simulations, and I don't have very much experience with spice. I also wasn't able to find spice models for all components and used generic ones, so I am not sure the simulations are very accurate. But I'll look into that some more this weekend and see if I can produce some spice files.2> Actually I do have a scope! But it's an old analog one, so everything I can test will need to be periodic. Maybe I can use a mosfet to switch to load periodically as Andreas suggested. But as I don't have a signal generator I will need to build something to drive it... Or maybe I'll just get a cheap DSO3> Thanks! I'll have a look in my junk-bin and see what kind of inductors and caps I can find, I think I have a large transformer somewhere.

author: erikj
author_rank: Contributor
timestamp: August 27, 2014, 07:01:43 pm
content: Quote from: dannyf on August 27, 2014, 11:20:36 am1> I have been thinking that the negative voltage swing might not be enough to turn the transistors fully off when at full output voltage. If that is the case I'll just I add a diode in series with each led to push the emitter voltage a bit higher. I could also use -12V to supply to the op-amps, but then I'll risk pulling the base too low, and need to add some kind of protection for that. Or am I missing something else here?2> The idea for T6+T7 was to get a small current needed for the pre-regulator. I will probably be using a circuit similar to the one in EEVblog #329.3> I added that to protect T3 from reverse base emitter breakdown, but that was before I added SD3 so it is probably not needed now.4> I might not need them, the reason they are there is to handle the rather large output voltage swing. If I can find a good op-amp that can handle an output swing of 40V or more I could remove them.5> OK, I'll try something smaller on my breadboard6> I considered connecting it to +12V instead but wasn't sure how the DC-DC converter would handle that. Not sure what else I could use it for?7> I guess that could work, but wouldn't that have some unwanted effects for the compensation?Quote from: dannyf on August 27, 2014, 11:20:36 amThanks, I'll take a look.

author: erikj
author_rank: Contributor
timestamp: August 27, 2014, 08:25:21 pm
content: Quote from: Andreas on August 27, 2014, 12:58:26 pmIf I run the supply without the pre-regulator, then yes, it is definitely too much for the single transistor. But liquid nitrogen cooling would be an interesting experiment thoughMy thoughts were that the pre-regulator will be keeping the voltage at 2.5-5 volts (I will see what works best) above the output voltage. If the voltage gets too high the high side switch of the buck converter should turn off, and all the transistor would need to handle is the remaining energy in the output inductor and capacitor + the energy in the extra filter L1+C1. That should last maybe 1 millisecond if my calculations are correct.The safe operating area for pulses is not specified in the datasheet though, so maybe I am pushing my luck here.I'll definitely consider adding another one in parallel.Quote from: Andreas on August 27, 2014, 12:58:26 pmThanks, that is something I had not considered.Maybe L1 will keep the current from rising too quickly, I need to do some more calculations to see if this will be needed.Quote from: Andreas on August 27, 2014, 12:58:26 pmDo you mean diodes from output to ground, or from output to input?

author: erikj
author_rank: Contributor
timestamp: August 27, 2014, 09:22:13 pm
content: Quote from: blackdog on August 27, 2014, 03:22:43 pmI chose the opa277 for the low noise and offset voltage. What would you suggest instead?I guess I could adjust the offset with a trimpot so that shouldn't be a problem.Quote from: blackdog on August 27, 2014, 03:22:43 pmThat link you gave didn't work.Looking forward to reading that article!

author: blackdog
author_rank: Frequent Contributor
timestamp: August 28, 2014, 10:48:22 am
content: Sorry Erik,I got up twice to the wrong link posted ...I am old-)Refres the page, the original link works now...Take your time, its long, at the end you wil find information how to keep the dissipation low in you power transistor.About the IC for current sensing, high precision is good for the opamp, but with a slow IC there you make to much phase lag, and your current control loop wil generate!Your current sense opamp need to be as least as fast as the opamp voor the current loop, this because you current sense IC is inside this loop.The opamp that you use fore the current loop is not designed to have a second opamp in its feedback loop.Sorry for bad english...Kind regartsBlackdog

author: void_error
author_rank: Frequent Contributor
timestamp: August 28, 2014, 01:28:34 pm
content: Wow, so many bench PSU designs on this forum.You might want to take a look at mine, it's tested and stable on breadboard.https://www.eevblog.com/forum/projects/bench-power-supply-design/90/Note that not all components have values assigned yet.

author: Andreas
author_rank: Super Contributor
timestamp: August 28, 2014, 05:05:01 pm
content: Quote from: erikj on August 27, 2014, 08:25:21 pmHello,look at D13,D14 in blackarts design.he also has the peak current limiting transistor (Q6) that I mentioned.With best regardsAndreas

author: blackdog
author_rank: Frequent Contributor
timestamp: August 28, 2014, 07:10:18 pm
content: Hi Andreas,Thank you for my nice second name => blackartsOre, you find my design Black-Art *grin*Kind regarts,Blackdog

author: erikj
author_rank: Contributor
timestamp: August 28, 2014, 08:48:30 pm
content: Quote from: Andreas on August 28, 2014, 05:05:01 pmThanks, I'll add the diodesQuote from: blackdog on August 28, 2014, 10:48:22 amThank you! I have some reading to doGuess I'll have to order some faster op-amps.

author: David Hess
author_rank: Super Contributor
timestamp: August 31, 2014, 07:26:42 am
content: Quote from: erikj on August 26, 2014, 10:59:58 pmUse whatever is more convenient. Sometimes one is needed over the other because of common ground considerations.I prefer using single ended level shifting to instrumentation amplifiers because it is faster and does not require resistor matching for high common mode rejection. I see you implemented remote sense however which is one place I might use an instrumentation amplifier although I have done without them there as well.QuoteI had a bunch of comments here about T2 and T8 adding uncontrolled gain within the feedback loop but I see now that you configured them with shunt feedback which neatly solves the problem. I might try adding a little bit of emitter degeneration to tailor their frequency response and provide stability. Instead of the shunt feedback amplifiers, I would have tried bootstrapping the operational amplifiers to increase their output voltage range so T2 and T8 could be emitter followers which would simplify frequency compensation.You have not implemented any lead or pole-zero compensation in the feedback loops to make them more stable.At least a small output capacitor should be included to help with frequency compensation and stability.The integration capacitors around OP1.2 and OP1.3 are going to increase the delay when switching between voltage and current modes because of integrator wind-up. Proper frequency compensation will minimize their value but maybe the integrators should be clamped.QuoteI am not sure why a DSO would be needed for this. A low frequency square wave, 1 kHz is about right, can be used to apply load steps to the output, voltage steps to the voltage loop, and current steps to the current loop while monitoring the transient response.QuoteBob Widlar apparently like to run a file across the output terminals generating a shower of spark to see if the power supply would fail.40 volts at 5 amps is a pretty big supply. I would consider adding thermal protection to the pass element but maybe that is unnecessary because you are using a switching preregulator. I would include an SCR crowbar on the output to protect any load from failure of the pass element or voltage control loop.

author: erikj
author_rank: Contributor
timestamp: September 01, 2014, 08:49:52 pm
content: Quote from: David Hess on August 31, 2014, 07:26:42 amI might try bootstrapping if I can't get this thing stable, adds a lot of extra parts though.Quote from: David Hess on August 31, 2014, 07:26:42 amOK,I will definitely need to learn more about loop compensationI'll start by trying to measure the step responses and then go on from there.Quote from: David Hess on August 31, 2014, 07:26:42 amGood point, do you have any suggestion on how to do that?Quote from: David Hess on August 31, 2014, 07:26:42 amGreat, I have now set up the voltage control loop so that it is controlled by the test square wave signal from my scope.I will try to get some screen shots and post them here later...Quote from: David Hess on August 31, 2014, 07:26:42 amGood, I might add something like that.I am starting to have some seconds thoughts about the single transistor, it is probably better to make something that will work safely even without the pre-regulator but that is limited by an over temp. protection circuit.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 03:10:22 am
content: Quote from: erikj on September 01, 2014, 08:49:52 pmIt does but they are all good parts.I would also consider using operational amplifiers which have a slightly higher supply voltage range.QuoteJust improving the loop compensation may be all that is needed because it will lower the value of the feedback capacitors.If that is not enough, then limiting the amplifier output voltage when the loop is broken will help. I usually hear this called "clamping" or maybe "anti-windup". Some operational amplifiers bring out special connections so it can be applied to internal nodes.Usually this just involves tying the output back to the inverting input with diodes or a zener clamp or something more sophisticated.QuoteThis is one application where I find little or even negative advantage to using a DSO over an analog oscilloscope. It is possible to calculate the bode plot from the impulse or step response but very few DSOs can do it.Quote3 amps would be pushing it if the full supply voltage was across the pass element because of secondary breakdown limitations.One trick I like to use is bolting an integrated regulator to the output transistor heat sink and using it as part of the pass element or control circuit. It can be used to provide thermal protection and sometimes safe operating area protection which varies with temperature.

author: void_error
author_rank: Frequent Contributor
timestamp: September 02, 2014, 09:50:42 am
content: Quote from: David Hess on September 02, 2014, 03:10:22 amI wouldn't go above 20-30W of dissipated power for a single TO-220 package transistor when mounted on a proper heatsink. For larger packages a ballpark figure would be no more than 50W. Multiple paralleled pass elements with low value emitter resistors to equalize VBEis the best choice here. This will also lower the total junction-case Rthwhich is going to be the Rthof each device divided by the number of devices.

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 12:23:28 pm
content: Quote from: void_error on September 02, 2014, 09:50:42 amI agree. Power ratings are normally given assuming a Tc (temperature case) of 25C so they are just a starting point before considering thermal resistance and operating temperature. Spreading the heat out over multiple packages has the effect of paralleling and lowering the thermal interface resistance.I have occasionally gone higher with very careful thermal design and construction including proper mounting torque and active cooling but it is not something I would want to rely on in a production design because like heat treating, it is easy to screw up with no visible signs until something breaks or explodes.Or you forget to cool the emitter ballast resistors and they turn red hot.

author: erikj
author_rank: Contributor
timestamp: September 02, 2014, 07:41:53 pm
content: I did some scope measurements for the voltage regulation part of the PSU to see what happens with different integrator cap values.As the source signal I used the square wave from my Agilent multimeter at 4.8kHz, 50% duty.The signal is connected toVsetin the schematic but without the 470nF smoothing cap (C5), I have also changed the voltage dividers to get about 10V output signal.Vertical setting is 2V/div for all images.The lower trace is the input signal and the upper is the regulated voltage. I am using a 12ohms power resistor as the load and runningwith no output capacitor.This is a 150pF capacitor. Horizontal 50µs/div. Not bad!This is also 150pF cap. Horizontal 2µs/div, there is a very slight ringing visible.1nF cap. Horizontal 50µs/div. The input signal is clearly changed by the feedback capacitor, this might affect the results a bit.80 pF (47+33) cap. Horizontal 2µs/div. Overshoot and a ringing at about 500kHz, this value seems a bit too low.So I guess 150pF is the value I should be using

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 02, 2014, 09:34:32 pm
content: Quote from: erikj on September 02, 2014, 07:41:53 pmHi erik .Your supposed to test it under it's worst load conditions .Try that with 1uF's worth of CERAMIC caps and a very high value load resistor (or open ) .Regards

author: David Hess
author_rank: Super Contributor
timestamp: September 02, 2014, 10:59:55 pm
content: You have a couple of things going for you aiding stability.The kelvin sensing instrumentation amplifier divides the loop gain by 10 which largely offsets the transistor shunt amplifier which multiplies the loop gain by about 20 so that you need only a minimum amount of integration capacitance is not surprising. I think that explains why such a low value of feedback capacitance makes it stable.Similar math can be done for the current loop which needs to be checked separately anyway. I usually start off adjusting the frequency compensation empirically (It is difficult to know what the pass element is doing.) and then go back for an analytical solution.My usual strategy on the integration capacitors is to increase the resistance in series with the capacitor which will reduce the amount of capacitance needed.I think phase lead can be added in your design with small capacitors (or networks) added in parallel with R3 and R4. I usually do this earlier in the circuit path but you have those instrumentation amplifiers in the way. Eventually C20 should not be needed.When I have laid out boards in the past, I included parallel C and series RC phase lead and feedback networks which were then populated based on further testing.As Kevin.D says, testing for worse case load and operating point should be done. Usually at least a small low Q (relatively high ESR like an aluminum electrolytic or solid tantalum) capacitor is placed across the outputs to swamp other effects.If this is a production design, then the frequency compensation should be adjusted for the worst case components which implies some sample testing.What happens when power is first applied? Does the output glitch?

author: erikj
author_rank: Contributor
timestamp: September 03, 2014, 08:04:20 pm
content: Quote from: Kevin.D on September 02, 2014, 09:34:32 pmI wanted to start with something easySo I tried this... and the supply didn't like that at all. It went completely unstable and started oscillating wildly.Couldn't even get a good sync on the scope.Next I did some experiment with adding a cap or a cap + resistor across R14 to try to get a bit of lead compensation. This helped a bit, but did not solve the problem. I also simulated this setup and it turns out it is not stable in simulation either.I then tried swapping the BD139 for something faster, a BC550C. And that made a huge improvement! I guess the BD139 wasn't fast enough. I still had a large ringing, but it was much cleaner and eventually settled to the set voltage.Quote from: David Hess on September 02, 2014, 10:59:55 pmLots of good info, thanks.Quote from: David Hess on September 02, 2014, 10:59:55 pmI tried adding a 220p cap across R3 after changing the transistor, and that worked beautifully, thank you!Much better that putting it on R14.See below for the result.Quote from: David Hess on September 02, 2014, 10:59:55 pmAt the moment this is a one-off project for myself only.I tried cycling the power several times, and did notice a small glitch once, but only a volt or so on the output. I will have to test this again with the DC-DC converter in place. Currently I am running it from another lab supply.So here is the result with a 1µF ceramic cap as the only load. This is after changing transistors and adding a cap across R3.2V/div vertical and 50µs/div horizontal, 4.8kHz square wave input (same as before). Still not perfect, but a huge improvement.There is also a small ringing there that is visible when I zoom in.

author: void_error
author_rank: Frequent Contributor
timestamp: September 04, 2014, 07:37:42 am
content: I think the reason for the instability is the fact that the stage between the opamps and the series pass transistor has very little local negative feedback.In one of my early versions I tried something similar but I wasn't happy with it so I ended up with adding a discrete gain stage with a set gain slightly above the minimmum gain required for the maximum base drive voltage I needed (VOUT+VF+2xVBE). It basically amplifies the opamp's output by a factor of 10 (although 6 would probably be enough) while the opamp still controls the overall gain.Since you're doing the current sensing on the low side you could replace T6/T7 current sink with a 1k resistor to ground.Another thing you can do to make your circuit more stable is to add baker clamps to T2 & T8 to prevent them from saturating if that's the cause of instability/ringing.http://en.wikipedia.org/wiki/Baker_clamp

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2014, 10:52:21 am
content: Quote from: void_error on September 04, 2014, 07:37:42 amThe transistors are configured with shunt feedback stabilizing their voltage gain. I would be more worried that their operating current is too low to drive the output at full speed.QuoteThe transistors are so fast compared to the speed of the overall feedback loop that the storage delay from saturation should be insignificant.

author: erikj
author_rank: Contributor
timestamp: September 12, 2014, 07:14:45 pm
content: Thank you guys for the feedback so far!I have found some time to make a new version of the schematic, more feedback is welcome of course.Main changes are:Got rid of the diff-amp from the current loop to make it fasterAdded a simple peak current limiterParalleled the pass transistorsAdded output capUsed a resistor instead of a current sink for minimum load currentAdded some lead-compensation capsI also found a mistake in the first schematic, the op-amp was supposed to be OPA227 and not OPA277 which is much slower.The OPA227 was the one I was using for my tests, so the problems I had with the current loop probably weren't due to op-amp speed.I'm still looking for a nice fast quad op-amp for this project.I am not quite sure which parameters are the most important for this kind of application, so I made a short list of candidates I have found.AD8513 Nice specs, but CMRR seems a bit low. Not sure how important this is.AD8674 Is this fast enough?TLE2074 Offset drift seems a bit high.Which one do you think will work best? Or do you have other suggestions ?

author: void_error
author_rank: Frequent Contributor
timestamp: September 13, 2014, 02:10:21 pm
content: You don't really need R1, R3, R5 & R7, you can safely remove them.If you've done the math right the voltage drop on R2, R4, R6, R8 should be between 0.3 & 0.7V. If it's too low you're going to have the transistor with the lowest VBEconducting more than the ones with lower VBE. You could get away with a low voltage drop on those resistors if the transistors have nearly identical gain although I'm not sure that's the case for darlingtons... and their gain and VBEwill also change with temperature.I don't see any reason for the T9 & R35.

author: Andreas
author_rank: Super Contributor
timestamp: September 13, 2014, 02:22:29 pm
content: Quote from: void_error on September 13, 2014, 02:10:21 pmThink twice:For pulsing loads its the only path that limits 2nd break down fast enough.with best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: September 13, 2014, 05:19:31 pm
content: Quote from: erikj on September 12, 2014, 07:14:45 pmUsually the output capacitance and output transistor limits bandwidth at low loads so a faster error amplifier does not add much except for the need for more frequency compensation.QuoteCMRR will not mean much without precision dividers. Offset will not matter unless you need absolute accuracy which again, requires precision dividers. Neither affect load or line regulation.The AD8674 and OPA4227 are low noise but to take advantage of it, the impedances need to be below 10k. These amplifiers have input bias current compensation so there is no need to balance the inverting and non-inverting inputs and resistors like R16 and R25 do nothing useful. The FET input operational amplifiers do not need input balancing because they have such low input bias currents.Operational amplifiers with high slew rates will recover faster when switching between voltage and current mode since there is no clamping of OP1.1 and OP1.3 depending on the external compensation. That gives an edge to the JFET operational amplifiers. Add some clamping and this would not be a consideration.What are Z1 and Z2 for? Are they just to protect the LEDs?I would look into operating at a higher drive current, lowering the impedance of the feedback networks for T7 and T8, and buffering the outputs of OP1.1 and OP1.3 with emitter followers. This would include replacing the BC series transistors with BC327s and BC337s or something better. I wish there was a modern equivalent to the old 1+ watt TO-39 transistors.

author: erikj
author_rank: Contributor
timestamp: September 13, 2014, 07:18:26 pm
content: Quote from: void_error on September 13, 2014, 02:10:21 pmI the voltage drop in this case will be 5/4*0.1 = 0.125V max if the transistors are perfectly balanced (which they probably are not), so maybe that is too low then. It was hard to find any reliable info online so I used something low for the emitter resistors to avoid wasting power.I added R1,3,5,7 because I it seemed to be a common configuration in other designs, not really sure about their exact purpose there but some kind of balancing I guess. They are useful for measuring the individual base currents though, so maybe I'll keep them until I have the balancing part figured out.Quote from: Andreas on September 13, 2014, 02:22:29 pmYep, they are put there for the really short current spikes that are too fast for the current loop and as an extra fail-safe.

author: blackdog
author_rank: Frequent Contributor
timestamp: September 13, 2014, 08:59:03 pm
content: Hivoid_error,You are wrong, R1, R3, R5 & R7 are Needed!Have you ever heard of emitterfollower oscillations?erikjC1 is on the low side, you need a bigger energy reservoir for dynamic load changes.And, use about 50uF/ampere current on the output ( rule of thumb ).Look @ ADA4077-4 as opamp,Make R16 10K.Remove T7 and T8, use 1n4148 diodes en place a LED in serie, green for de voltage loop and red for the currend loop, less components and more stability.I like it more if the peak current limitor is build around the emittor resistors of the TIP142, but your system also works :-)But test it for stability!Do stability testing for you start with the circuit board.Take good care of the wiring around C1 and C20, These capacitors make you Ri @ higher frequenties and your loop stability depents on them.Kind regarts,Blackdog

author: erikj
author_rank: Contributor
timestamp: September 13, 2014, 09:12:24 pm
content: Quote from: David Hess on September 13, 2014, 05:19:31 pmI am using 0.1% resistors for the differential amplifier. I wanted decent precision as I will be using 22-bit A/Ds and 5-digit displays for the current and voltage meters. I am hoping for <0.1% accuracy for the meters after calibration. If the drift is low I could compensate for offset and CMRR in software, but it would be nicer if I didn't have to do much calibration.Quote from: David Hess on September 13, 2014, 05:19:31 pmThanks, I'll consider removing the input balancing resistors.I am curious to know why impedance < 10k is needed? Is that because of the the current noise density?I have tried to figure out a way to clamp them but have not been able to find a good solution.So I think AD8513 might be the best choice then.Quote from: David Hess on September 13, 2014, 05:19:31 pmIt is more like protecting the supply from future user stupidity (from me probably). The LEDs will be indicators in the front panel, and usually I would not expect an indicator to be part of any important circuitry. So I was thinking that the supply should work without the LEDs and not lose regulation.Quote from: David Hess on September 13, 2014, 05:19:31 pmSome of the transistors might see a voltage of almost 50 volts, so I decided against BC327/337 for that reason and I wasn't able to find any higher voltage equivalents.How much current do you think might be needed?I could maybe increase it to 20mA or so. BC546/556 seems to be able to handle about 1W with a heat sink attached.Quote from: David Hess on September 13, 2014, 05:19:31 pmYep, I noticed how hard it was to find good fast transistors in the "a few watts" power range. And those TO-39 parts are easy to keep cool.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2014, 02:49:38 am
content: Quote from: void_error on September 13, 2014, 02:10:21 pmThey are often a good idea for snubbing emitter follower oscillations. The emitter degeneration resistors used to enforce current sharing are usually enough if they are at least Rb/hfe however if the transistors are mounted remotely, the wiring may have enough capacitance to require the base snubber at the transistor also.I would like to refer to this as snivet protection or moderation but fear enraging the tube enthusiasts. MOSFETs are more prone to this and maybe it would be more forgivable to use the term for them.Slow transistors are almost immune to this but it still happens and no Darlingtons are really slow. Lots of 2N3055 designs failed when they stopped using the 600 kHz single diffused processes.Quote from: erikj on September 13, 2014, 07:18:26 pmI usually figure on equaling Vbe with the emitter ballast resistor at peak current but the real value needed is less. If you ignore secondary breakdown effects, the minimum value can be calculated using either the Vbe or hfe change with temperature given a certain level of mismatch but it is seldom worthwhile.I have found that simple grading of the transistors for Vbe makes a huge difference if you want the lowest possible emitter ballast value.Darlingtons of course have approximately 2*Vbe so the change of base-emitter voltage over temperature will be doubled but I have never tested it empirically because I have not used them in parallel. The emitters are ratio matched and track with temperature so it would be fun and not difficult to calculate the actual emitter ballast resistor requirements. Hfe effects would be more interesting.QuoteI think it is a great idea. It is simple yet effective and prevents some rather catastrophic failure modes.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2014, 06:29:17 am
content: Quote from: erikj on September 13, 2014, 09:12:24 pmWorst case 0.1% resistor matching with a gain of 0.1 yields a CMRR of 49 dB so the operational amplifier CMRR will not be a limitation. Worst case 0.01% resistors would still only be 69 dB.http://www.linear.com/docs/41248CMRR requirements are relaxed since the change in common mode voltage itself is low.I have designed and built similar fixed power supplies with remote sense that had load and line regulation measured in microvolts or less. My favorite configuration is something like this:http://circuitdiagram-schematic.com/high-stability-regulator-circuit-using-lm108/QuoteYes. It is only approximate but dividing the voltage noise by the current noise yields the source resistance necessary to double the voltage noise which is a reasonable place to start. Resistors noise is usually insignificant.http://www.linear.com/docs/4345QuoteI do not have any good examples to link. Diode or zener clamps across the feedback capacitor are common but there are other ways using transistors or maybe current switched diodes. The idea is to prevent the operational amplifier from charging the integration capacitor excessively when operating open loop and the other error amplifier has control.Some operational amplifiers have special clamp pins or you can do it via an external compensation pin.Minimizing the integration capacitance shortens the recovery time but clamping is still necessary for the fastest response.QuoteLEDs tend to fail short but I agree with the concept. Tektronix just put an LED (and diode) in series with the output of the current loop amplifier to indicate current limiting.I was thinking it might be better to use a slightly higher voltage zener and a resistor in series with the LED. Then the bias current from the current source can be raised without affecting the LED.I am not fond of low voltage zener diodes though and by themselves, zener diodes have an appalling tenancy to fail open. A slightly more complicated design would use a resistor divider to fix the base voltage of a bipolar transistor so the transistor replaces the zener. Oddly enough the transistor Vbe would compensate for the LED forward voltage drop for better LED current regulation.QuoteI was thinking 20 milliamps but as you point out, the voltage is marginal on the BC327/BC337 and I think 1 watt is optimistic for any TO-92 transistor although I have heat sinked them in the past.QuoteThere are lots of surface mount options but I have been slowly compiling a list of suitable TO-126 transistors:2SA1507/2SC3902 160V 1.5A 1.5W 120MHz2SB1143/2SD1683 50V 4A 1.5W 150MHzBD139/BD140 100V 1.5A 1.25W 190MHz(?)

author: blackdog
author_rank: Frequent Contributor
timestamp: September 14, 2014, 08:27:18 am
content: Hi erikj,What is de reason that you are using T7 and T8?With 4x TIP142 you only need about 4ma total base drive, if the transistors getting warm it wil be far below that.Typical i measured about 0,5mA @ 2Ampere for the TIP142.A "Pull Up" current source of 7 a 8ma wil be more than enough for this design and the curent its about right for driving your CC or CV LED's.The best way to be sure that you dont have "switch On" transients is to delay the current source that drive the TIP142.Also watch out for "Switch Of" transients, a powersupply that kils a connected microcontroler with a 13V puls is a BAD power supplyKind regarts,Blackdog

author: erikj
author_rank: Contributor
timestamp: September 14, 2014, 05:05:00 pm
content: Quote from: David Hess on September 14, 2014, 02:49:38 amI don't know how to perform those calculations, so I think it would be easier for me to simply get some different values for emitter resistors (maybe 0.1 - 0.4 ohms) and do some experiments later. And also get a few extra transistors for matching.Quote from: David Hess on September 14, 2014, 06:29:17 amVery useful document, thanks.I did some measurements of the resistors and found that they are matched to about 0.02%, so op-amp CMRR shouldn't be an issue then.Quote from: David Hess on September 14, 2014, 06:29:17 amThe self-biasing transistor topology I am using makes this a bit tricky I think. But there should be some way to do it, I'll keep thinking.Quote from: David Hess on September 14, 2014, 06:29:17 amQuote from: David Hess on September 14, 2014, 06:29:17 amA shunt resistor is another option I am thinking about to reduce LED-current. Maybe shunting the LED with a 150 ohm resistor or something like that. Also I wouldn't need the zener.Why is it important to increase the drive current? Is it to make it faster?I was using BD139 in the first version of the schematic, but it turned out to be too slow. I think that base-collector capacitance might be the problem, as it should be fast enough in terms of bandwidth. The 2SA1507/2SC3902 looks better in terms of capacitance, so that might be an option. Also lowering the impedance as you mentioned might help.

author: erikj
author_rank: Contributor
timestamp: September 14, 2014, 06:33:35 pm
content: Quote from: blackdog on September 13, 2014, 08:59:03 pmADA4077-4 is a nice op-amp, I like the low drift and offset. The slew-rate is a bit low, and that might be a problem if I can't get the integrator-windup problem sorted out.You are right, C1 should probably be higher. How much depends on the voltage margin I will use for the pre-regulator.Quote from: blackdog on September 14, 2014, 08:27:18 amThe reason for T7 and T8 is all about the voltage range of 0-40V. I could maybe use a +-22V op amp and drive directly but I would be running it very close to the maximum voltage rating. I would need an op-amp supply of maybe -1 and 42 volts and extra regulators for that.There are other options I have considered like bootstrapping a regular op-amp or getting a few of those high-current high-voltage ones that goes up to 60V or more.Or I could do what you did in your design and reference the control circuitry to the positive output.

author: David Hess
author_rank: Super Contributor
timestamp: September 14, 2014, 10:59:07 pm
content: Quote from: erikj on September 14, 2014, 06:33:35 pmThere is probably no good reason to borrow trouble here. Similar designs using diode switching between constant voltage and constant current get by with slow 741 type amplifiers no clamping. I would clamp the integrators just for the design challenge although it is pretty straightforward. I am surprised at the lack of examples online.