Replacing op amp with higher bandwidth for better dynamic response in a PS - Page 1
Forum section: electronics / beginners

author: nour
author_rank: Frequent Contributor
timestamp: May 30, 2017, 05:35:21 pm
content: I am working on a precision power supply that is based on gerry sweeney design which I have mentioned on other thread:http://gerrysweeney.com/fully-programmable-modular-bench-power-supply-part-14/?wppa-occur=1&wppa-cover=0&wppa-album=7&wppa-photo=108the design I want to do in the power supply will be working from 0 to 20v @1A output, I want to have 1mV\1mA resolution from 0 to maybe 8-10V (not sure yet) and maybe less than this from 10 to 20V.He is using opa2277 with GBW of 1MHZ and SR about 0.8 v\US, he faced some issues when he was testing the design with dynamic load changes and the dynamic response according to him wasn't as expected. So I thought if I change the opa2277 to a higher GBW like, for example, 5MHZ maybe it will have a better dynamic response.Am I correct about that?If so, how much higher should I have?After some search I found those:OPA2227 : 8MHZAD8671 :10MHZMCP6V91 :10MHZ <-- this one with Vcc 6.5V absolute rating! not sure how to use it here!I want to know what is the most important parameters that I should be considering to make this replacement?In my list:Offset voltage driftInput bias currentInput bias current offsetInput offset voltage

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 30, 2017, 05:55:18 pm
content: notQuote from: nour on May 30, 2017, 05:35:21 pmnot necessarily, sometime slower opamp may do the job better, imho. its a combination of the whole control circuit path, ie the opamp + mosfet driver + power bjts... ymmv...

author: Pitrsek
author_rank: Regular Contributor
timestamp: May 30, 2017, 08:20:56 pm
content: It might, and it might not. It dependsIf you have it on bench, measure it.http://www.ti.com/lit/an/snva364a/snva364a.pdfIt's better to use soundcard, instead of the oscilloscope. Way better dynamic response. With 192kHz sound-card bandwidth will not be a problem.For SW, I used Arta. FYI for SMPS soundcard is at times not good enough. For linear it should be ok.Try to measure plant transfer with various loads/operation conditions. When you have your plant measured, you can design proper compensation networ.For measurement to be valid, the regulator must be stable -> really low crossover, or open loop. Open loop might not be theoretically correct(DUNNO), but I haven't had any problemwith it.EDIT:For transformers - audio transformers works kinda OKish, better yet source the biggest, baddest core with as high Al as possible, and wind your own. EMI suppression cores works well.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2017, 10:56:58 am
content: Usually it is not the OPs bandwidth that sets the performance of an linear power supply. Even if so, the limited bandwidth might be chosen intentionally in some cases. So a faster OP might make the circuit unstable.It is more common that the speed of the output transistors limits the speed. So if you want to build a fast supply one might consider something fast than an 2N3055. Another point is that transistors get slower at low current - so a fast supply essentially needs some standing current to prevent the power stage to run at low current.Anyway the shown circuit does not look really good in some places: there is way to much capacitive load to some OPs. So poor dynamic performance is kind of expected from this circuit. So for a fast / good regulator, I would ignore that circuit.If at all a higher bandwidth OP might help for the amplifier at the current measuring shunt.

author: nour
author_rank: Frequent Contributor
timestamp: May 31, 2017, 11:18:53 am
content: Quote from: Kleinstein on May 31, 2017, 10:56:58 amWhat would the parameter that I should look at if I want to choose another faster transistor?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 31, 2017, 12:53:02 pm
content: The speed of transistors is usually given as the transit frequency. Darlingtons are usually on the slow side - here the definition of the transit frequency makes it no a good parameter for speed any more. One also has to check for a good SOA, to have a transistor suitable for high power dissipation.The slow early power transistors (e.g. 2N3055) have a transit frequency in the 1-4 MHz range. This is good enough for a normal lab supply, but can be the limiting factor. There are transistors made mainly for audio amplifiers with a transit frequency in the 10-50 MHz range, that could allow a slightly faster loop. However this also needs care with parasitic effects, especially parasitic inductance in low impedance areas. So there is a limited return in using super fast transistors.Possible types would be D44H, 2SC5200, 2SC3281, 2SD1047. A problem is that these transistors tend to be a little more expensive and chances are to get fakes if nor from a reliable source.The first point would be to start with a suitable circuit. If done right it is possible to use a 2N3055 for a fast supply too. Really poor transient performance is usually a problem with the circuit, not due to slow transistors or OPs.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 31, 2017, 01:50:50 pm
content: Quote from: nour on May 31, 2017, 11:18:53 amfaster respond and high power dissipation capability? but alas you seldomly see both go along together in jellybeans part, you might end up using RF power amplifier which is not cheap. faster is sometime not a good thing, what we want is controllable output without much oscillation. better a slowly ramping PSU than highly oscillated overshooted PSU. this is something i learnt from more expert people in this forum, i'm no expert myself. the trick usually is, get whatever parts you can afford and work from there, or ask suggestion if that part is suitable or not in this forum. the hardest part is try to get compensation network (capacitor, resistors feedback) play along well together in the whole feedback loop, and different design requires different network, different component needs different value compensation network. in school we learn closed loop unit transfer or somesort that require Z domain, those are nasty math, more components give more complicated formula, and then bode and nyquist plot, highly academical. we usually try and erroror only hope on simulation, and more experienced people will get to it much quicker. better if you have loop stability test setup for your design as linked by earlier member above..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 31, 2017, 02:05:03 pm
content: Oh. God. Oh god no. Kill it with fire, please!It may kill itself with fire if you're lucky.Tim

author: nour
author_rank: Frequent Contributor
timestamp: May 31, 2017, 02:43:45 pm
content: Quote from: T3sl4co1l on May 31, 2017, 02:05:03 pmWell, I am lucky

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2017, 02:57:53 am
content: The only reason this design would benefit from faster operational amplifier is because it is terrible. Various part values would need to be adjusted to take advantage of them. It has a lot of complexity just to make up for design mistakes.Quote from: nour on May 31, 2017, 11:18:53 amFt or transition frequency is usually the parameter to look for. Suitable high power bipolar transistors will be in the 2 to 4 MHz range which is plenty fast for good performance. The step up in lower power designs or where more transistors in parallel are acceptable is something like the D44H11/D45H11 series (1) which are lower power but have an Ft of around 50 MHz.(1) There are a lot of similar but more powerful transistors now intended for big audio power amplifiers which could be used but they cost like 3 times more.

author: floobydust
author_rank: Super Contributor
timestamp: June 01, 2017, 04:18:52 am
content: You don't need speed- just increase the output capacitance from the tiny 47uF?Unless there is a huge delay switching from CC to CV that needs full slew of the op-amps.When I look at pass-transistors, I am surprised the lowest cost with good SOA is 2SC5200.TIP147 Darlingtons are so slow I thought.

author: Pitrsek
author_rank: Regular Contributor
timestamp: June 01, 2017, 09:22:00 am
content: Take look at LT3081. Might save you a lot of trouble. Feedback is taken care of for you, you just adjust the set point. Or if you wont better current limit, you can add current limiting feedback loop. Search around, I've seen quite a few diy lab supplies with this puppy.Starting point:http://www.linear.com/solutions/5086

author: ggchab
author_rank: Frequent Contributor
timestamp: June 01, 2017, 11:59:02 am
content: I was also interested by Gerry Sweeney's design and watched his videos about his power supply. I remember he made several versions and tests to determine the best values of various capacitors and resistors. I think he was pretty happy with the latest version. I would like to know what are the most fundamental errors in his design.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 01, 2017, 05:09:37 pm
content: There are several weak spots in Gerry Sweeney's design:The first is the output stage: it use darlington transistors controlled via the base current. This make the output stage relatively slow and also current dependent. The resistors at the collector have essentially no function at the emitter they would help with current sharing a little. The minimum current is set by just a resistor (330 Ohms) - this means low standing current at low voltage (e.g. < 1 V).The fast local loop with the 2N7002 fet and the transistors works essentially against the 47 µF output capacitor, with only the resistance/inductance of the fuse to isolate it from an external load. This makes the circuit sensitive to the parasitic properties of the 47 µF cap. To low ESR could make the circuit unstable, so could be something like a low ESR 100 nF cap at the output at certain DC currents. It might be stable, but this can depend on the exact layout.The tolerances of the capacitors around the OP13.1. can have a significant influence on the phase shifts of that circuit parts. So it may help to stabilize, but it could also promote oscillation.The regulator around OP12.2 makes it mainly integral type, with very little proportional contribution (some comes from C44 and C46). So chances are there will be quite some ringing if not instability.Combining CC and CV loop with D9 likely includes a drawing error. As shown C44 would make the two OP fight each others for the transition time: depending on which OP is more powerful (within the tolerances), this could cause trouble and a slow cross over from CV to CC mode.The CC mode compensation looks like very slow: 2.2 K and 15 nF gives a time constant of some 33 µs. Even in linear mode this gives a simulated capacitance in the 300 µF range. The bigger problem is likely recovery during CV-CC transition. Assuming a 12 V supply for the OPs, this would result in something like 0.3 ms before current limiting would set in. It may be just fast enough the 8 A fuse at the output would not trip on a dead short, but I am not sure about that. Anyway there would be quite a current spike (well above the set current limit, more like 10-30 A (limited by the base currents)) in case of a short. The limited bandwidth of OP13.2. might be a reason why such a slow compensation is needed.The resistors used to couple the sense inputs are not protected: in case the main load lines are not connected or for some reason interrupted, a high current can flow through those 10 Ohms resistors and very well burn them. Without those resistors non connected sense inputs result in full output voltage. At least this problem is solvable relatively easy.The OPs 13.1, 13.2 and 12.2 see too much capacitive load (something like two 4.7 nF caps in series). This can easily make them oscillate, or at least cause quite some ringing.There are a lot of capacitors / time constants involved in compensation. So transient response will be rather complicated (not a simple exponential or only two, more like 6 terms) - not sure how the function will look like, but it can be slow and ugly. Usually one should avoid such a large number of RC elements: they might help with stability (if well adjusted) but tend to cause a poor time domain response.For details and to see more possible problems one would today use a simulation.

author: ggchab
author_rank: Frequent Contributor
timestamp: June 01, 2017, 06:54:12 pm
content: Many thanks for the detailed explanations. I will spend some time to understand them properly.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2017, 07:17:31 pm
content: Quote from: Kleinstein on June 01, 2017, 05:09:37 pmI wonder if those resistors were needed to keep the local feedback loop from oscillating. I suspect they were placed on the wrong side and should have been in series with the emitters.The local feedback defeats the purpose of using a common emitter output stage for better current mode performance so might as well use a more stable common collector output or Sziklai pair.Or maybe the designer wanted the lowest dropout possible? A Sziklai pair would have improved that also.QuoteOr a lot of performance was discarded; my first power supply designs all used massive amounts of dominant-pole compensation to overwhelm other problems. If slow output transistors or slow operational amplifiers are used, then they can provide this for free.QuoteIf C44 was not there this would make more sense. This method can work but is more finicky than using two diodes.QuoteI would not mind knowing *why* the fuse is on the output instead of the input. I have seen a couple designs where fusible emitter ballast resistors were used. Maybe the fuse *is* because the current limit performance is too slow.QuoteThis is definitely a problem; both control modes have an extra stage which is not needed and contributes extra phase lag. Arguably this leads to extra precision but I am dubious that simpler control loops would not be just as good and they would be faster.The designer accepted the disadvantages of ground side current sensing and then used a second stage in the current control loop anyway. That might help match the frequency compensation between the control loops but it would not be my first choice.QuoteThis part is just weird. I reviewed the terrible TI datasheet for the OPA2277 and while it can handle this level of capacitive loading, it will not improve performance over doing the frequency compensation correctly. There are some rare instances where capacitively loading the output of an operation amplifier can work but this is not one of them.QuoteI get the feeling that the designer added capacitors until the circuit worked rather than developing a model for the frequency compensation. These days I use local frequency compensation to prevent local oscillations and global frequency compensation to prevent global oscillation.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 01, 2017, 08:21:30 pm
content: Most of the above is totally correct.Beyond that, the parameter in an OPA you are looking for might not be the small signal bandwith, but slew rate, large signal bandwidth, settling time and output current. With the exception of the output current, these are mostly connected with the small signal bandwith, but comparing several OPAs for those parameters can yield interesting differences at same SSBW ratings.If you expect your LPS to work correctly in CV AND CC modes, do NOT increase the output capacitance.A good reference is National Semiconductors LB28 (General Purpose Power Supply) from 1974.Further, to test properly, you need a dynamic load. For improvisation, look at Hickmans 'Designer's Power Supply' (Electronics World Jan.1995), or 'Netzteil-Tester' Elektor-Halbleiterheft 1992. There are some more that can be tailored just for your application.Then, there are some personal preferences/experiences about LPS:Also, I do not understand why someone trying to build a high quality laboratory PS builds his auxiliary voltages in such a dubious way. (if it is not high quality, it is not worth the complex interfaces)I would use a proper transformer for creating the symmetric voltages and, if possible/available, use a symmetric tracking regulator to supply your OPAs. It is one thing to try and experiment what can be done with workarounds, but there has to be a reason that none of the professional LPS designers do it.Maybe I am old-fashioned, but for a dependable LPS, fancy circuits are not a proper substitute for access to some electronic electromechanical components.

author: David Hess
author_rank: Super Contributor
timestamp: June 01, 2017, 09:33:40 pm
content: Quote from: Neomys Sapiens on June 01, 2017, 08:21:30 pmI think the reason for this is if you are doing a design for production, then extra floating windings on the custom power transformer are effectively free. For an amateur design, getting those auxiliary windings means adding a second albeit much smaller transformer which makes more complex circuitry with more specialized parts and a compromise in performance attractive.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 01, 2017, 10:56:48 pm
content: Ok, that is obvious. But (apart from my precious stash of RC4194 and SG1501s) those parts are not really hard to come by nor are they especially expensive. For the auxiliary XFMR, a PCB-mounted type will do most of the time and a lot of small transformers with usable winding combinations can be scavenged. Even with my Mil/Aero resources, I still do. But telling the new practicioners to collect stuff would probably amount toNot long ago, a coworker mused about breadboarding a circuit/device, for which he would have needed a switch, whose root makes contact to one NO contact and then to an additional NO contact. (Folgeschliesser)Next day I gave him one which even included a nice mounting bracket. It came from an old telephone.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 02, 2017, 09:56:56 am
content: With a digital controlled supply or just for a digital display one often needs an axillary supply anyway. In the rather common circuit (most of the cheap Chinese supplies use it) with floating regulator powered from the extra supply this can also be used for the display part. So the extra transformer or winding is not just for the regulator. While many of the older designs use something like a +-12 V here, one can get away with a lower more digital friendly voltage, like a +-5 V.The nice thing is that one can use essentially the same circuit (adjust a few resistors, transistor types) over a large voltage range. So it is kind of strange that there are so many poor quality lab supply projects found in the web.The alternative type of regulator with a low output impedance power stage, that is shown here, has some advantages (e.g. less output capacitor possible), but is more difficult if a good performance called for. So while you can get away without the extra supply one usually needs a negative one. It is only a low quality circuit with slow regulation (and thus high output capacitance) that is really more simple than the floating regulator.

author: Rerouter
author_rank: Super Contributor
timestamp: June 02, 2017, 10:24:48 am
content: I should point out the op amps bandwidth is being deliberately limited by the capacitors between the inverting input and its output, this is to reduce there rate of change, if you wanted to play around you could reduce the capacitance of the feedback path rate limiting caps and see how it goes, noting it may become unstable if you go too small in value (power supplies are built to be stable over almost any load, good ones anyway), faster op amps will still be rate limited by those capacitors.

author: David Hess
author_rank: Super Contributor
timestamp: June 02, 2017, 02:41:04 pm
content: Quote from: Rerouter on June 02, 2017, 10:24:48 amIt is more complicated than that because in 3 of the 4 cases, the inverting input resistor is also AC bypassed producing a capacitive voltage divider. So this design is also relying on the capacitive load driving ability of the operational amplifier.

author: floobydust
author_rank: Super Contributor
timestamp: June 03, 2017, 04:49:36 am
content: Quote from: David Hess on June 02, 2017, 02:41:04 pm+1OP's schematic got heavy lead/lag compensation in ways that are a bad idea.OPA2227not really for heavy capacitive loads that are present.OK Tim, kill it with fire, now I get it

author: Damianos
author_rank: Frequent Contributor
timestamp: June 03, 2017, 06:47:11 am
content: Very interesting design!The designer had a bag with a lot of 4n7 capacitors and decided to do something with them! So he threw them around the circuit!

author: ggchab
author_rank: Frequent Contributor
timestamp: June 03, 2017, 07:43:33 am
content: I watched the videos and most of the capacitors were chosen experimentally to obtain best stability and as fast as possible reaction to output variations. He used an electronic DC load for his tests. As far as I remember, results were not bad, despite the design errors clearly shown in this thread.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2017, 09:36:39 am
content: Quote from: Pitrsek on May 30, 2017, 08:20:56 pmWhat makes you believe a sound card is better for this than a decent oscilloscope?No sound card I know of has a bandwidth of 192kHz. I think you've confused sample rate with bandwidth, which generally isn't much more than 20kHz for most sound cards, although it wouldn't surprise me if there are some which go higher.As far as a sound card having more bits than an oscilloscope, that isn't much of an advantage, since with a decent oscilloscope it's possible to do oversampling, thus increasing the effective number of bits, beyond what the ADC has.Sound cards are generally AC coupled, which means they don't work at DC, which is important for using it to measure voltages on a PSU with a DC output. It's also much easier to destroy your sound card, than an oscilloscope.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 03, 2017, 12:19:35 pm
content: Quote from: Mechatrommer on May 31, 2017, 01:50:50 pmFaster can be better. Faster means it responds faster to changing loads. If you have a switching load that takes very short current pulses, your powersupply has to be able to deliver.Ofcourse, this is also what decoupling is for, but you don't want to have to rely on that. Ofcourse, phase margin is important, as you don't want over or undershoot.Linear has a few good appnotes on the topic of powersupplies. An example is their DC2132A document, which I think someone else already linked in. It'll easily outperform the design presented here on any spec with exception for cost.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2017, 03:23:58 pm
content: Faster also means better accuracy at medium frequencies (between the op-amp dominant pole and the loop cutoff frequency). Lower output impedance across the board.Integrator windup (the huge overshoot between CV and CC mode) can be avoided by linking the error amps more closely, or using a two-input error amp*.*These don't exist, but can be made with discretes, or assembled from two faster op-amps, a max(v1, v2) circuit (usually diodes, or something more accurate), and a buffer. The compensation networks are connected to the common output, not to each op-amp's output. Putting the max() circuit inside the loop solves it.The downside to this arrangement is the V(I) curve gets rounded off at the crossover point, instead of being rate-dependent-bumped-out as in the integrator-windup case. The rounding occurs due to ripple, variations in load, noise, etc. This is reasonable to expect, because the loop includes two diodes that are both biased nearly on, at the crossover point: any excess variation tips it over the edge, so it becomes doubly sensitive to noise, and rectifies faster peaks.Tim

author: David Hess
author_rank: Super Contributor
timestamp: June 03, 2017, 05:34:12 pm
content: Quote from: T3sl4co1l on June 03, 2017, 03:23:58 pmYou can see this in fast response regulators like theLT1573which can use fast external series pass transistors like the D44H/D45H series.QuoteThey do exist and we discussed some of these in adifferent discussion thread! The old Motorola MC1466L works this way and there are a bunch of other out of production ICs.The UC3824 is still produced but given its cost and dubious availability, I would use an alternative design. The UC3524 pulse width modulator and the TL494 can be used this way by disabling and ignoring the switching regulator part and you get a free reference. One idea I had was to use two 723s in parallel through their compensation pins. These all rely on operational transconductance amplifiers linked at their output which solves the windup problem but they are hardly precision parts.Operational amplifiers which use external compensation with a pin that follows the output voltage can be paired up. This feature can also be used for clampling the output to prevent windup as was done in thisNational application note. Unfortunately, operational amplifiers like this are rare; the only ones I found that are still produced and commonly available are the LM301A (used in the National application note) and maybe the LT1008, LT1012, LT1097, and OP97. None of these are particularly fast although fast enough for general purpose power supply applications. Operational amplifier with clamp inputs may also be suitable but I have not checked them yet and they are even less common.Standard operational amplifiers can be configured with a transconductance output through their supply pins and I am thinking of trying this with my next power supply design. Would it be easier to externally clamp standard internally compensated operational amplifiers or configure them with transconductance outputs? I need to think about how this would work and draw up some schematics. The ironic thing about doing this however is that it results in much higher bandwidth so the slower precision parts above could be used but then their external compensation pin would not be needed as an output.

author: Pitrsek
author_rank: Regular Contributor
timestamp: June 05, 2017, 07:30:45 pm
content: Yep, I wrote bandwidth and was thinking sample rate. There is no problem in obtaining card with bandwidth bigger than 20KHz. Serious reviews usually covers frequency range measurement. For some(isolated) power supplies, even 20KHz bandwidth might be enough.Quote from: Hero999 on June 03, 2017, 09:36:39 amHaving more bits IS of A SERIOUS advantage. You are measuring very, very small signals. For measurement to be valid, your injection signal needs to be very small. If your injection signal is too big, you are moving your bias point and are not in a small signal region anymore - and the measurement is useless. Usually the signal is in a single mV range. And this signal is riding on PSU noise and/or switching ripple... You hit limits of 8bit scope ADC very soon, most scopes I've come across are not oversampling enough for this application.Another reason for recommending sound card is ease of use - you have generator integrated, just fire up Arta, or any other sound-measuring software that can do frequency analysis and you are set. For scope, you need generator and to write custom measurement scripts. Might be easy for some, might not for others.For stability measurements, you do not care about DC at all - you measure in AC. DC measurement is telling you nothing about stability.Why would you destroy it? Make a simple input board - 250V foil capacitor, protection diodes etc. Simple and effective.If anyone needs to measure/verify stability on a job, I would highly recommend skipping messing with scope/soundcard and getting proper FRA right away. You'll discover that you need it anyway.... and spend twice the price of the tool in the process. And if you are designing power supplies, FRA is awesome tool to have.For home/hobby, soundcard is good start, but has limits - not really good for high noise power supplies. If you are serious, you can roll your own FRA/synchronous detection, but it requires some dedication.

author: floobydust
author_rank: Super Contributor
timestamp: June 05, 2017, 08:44:55 pm
content: Whatever happened to the Heaviside, step function for testing voltage regulation? Tests at real current, shows over/undershoot. I always test this way.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 05, 2017, 09:01:39 pm
content: Yep, that's what I was referring to when citing the Hickman article. Before the availability of suitable mosfets, often done with a mercury relay.Especially when working in such moderate I/U ranges where this is possible, I would not substitute any other method for testing a PS circuit in real-mode.

author: David Hess
author_rank: Super Contributor
timestamp: June 05, 2017, 09:15:09 pm
content: Quote from: Pitrsek on June 05, 2017, 07:30:45 pmI will just have to disagree about the signal to noise ratio problem. I have done this whole procedure manually (1) using analog oscilloscopes and 8 bit DSOs without problems. Averaging works fine on DSOs (use an external trigger) and many analog oscilloscopes have selectable bandwidth filters if needed. It helps a lot though to have a good understanding of what you are looking for so you can ignore the knowns and find the unknowns.Sound cards are too slow except for low performance designs and have unknown characteristics. I would trust them for distortion measurements but not for phase or transient response measurements.Quote from: floobydust on June 05, 2017, 08:44:55 pmI still want a DSO which can differentiate and display the FFT magnitude and phase so I can use this to display full bode plots. If the frequency compensation is not too complex, then the step response is the way to go for fast results but it takes some experience to do it quickly.(1) It it well worth having automatic amplitude, frequency, and phase measurements. Of these automatic phase measurements are the most useful.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 06, 2017, 02:16:56 pm
content: To test a power supply one needs the step test: the small signal AC test can not get the large signal response. At best one could get the small signal response. Some lab supply circuits with capacitive load can show a kind of large signal instability due to wind up. So they are stable at small disturbance level (though likely not a large phase margin), but can show permanent oscillation after a sufficiently large step.The AC test to get the frequency response can be helpful to understand the loop. So it is more like a tool for improving on a circuit, not so much to do a final test or to check a given supply. An AC test on the output (e.g. getting the output impedance) could be helpful to see how the supply might behave with a different / more difficult load. For just the transient response it is difficult to say how the supply will behave with additional output capacitance or another load.A sound-card is usually too slow, except for very slow loops. I don't see a need for high resolution - one might want a high resolution if the whole frequency range is measured with a fixed gain. Measuring in smaller parts with adjusted gain can compensate for a lower resolution. However a high resolution ADC can not compensate for a lack of bandwidth.

author: Pitrsek
author_rank: Regular Contributor
timestamp: June 08, 2017, 06:50:12 pm
content: Quote from: David Hess on June 05, 2017, 09:15:09 pmThe design I tried to measure with DSO had very high ripple (really small output capacitors), I might try it again with something more sensible/normal to see how far I can get with averaging and filters.Quote from: Kleinstein on June 06, 2017, 02:16:56 pmAbsolutely agree, there is no substitute for step load testingQuote from: Kleinstein on June 06, 2017, 02:16:56 pmIts one of the checks. If you have small stability margin, your step load test will definitely sux. With open loop transfer function, you can tell where is the stability problem coming from. You can measure just plat transfer function/power stage transfer function, and then design your frequency compensation accordingly. With plant transfer function in front of you, you can easily see what compensation will you need. Load your measured data into spice, design compensation network, and voila. No more fiddling with values. clear cut.Quote from: Kleinstein on June 06, 2017, 02:16:56 pmWhen you need bandwidth, you need bandwidth, no argument. For many design, you don't need. Even 250KHz small buck has datasheet compensation below 10Khz.With gain, it can work up to some point. If your injection signal is too strong, you are no longer in a small signal area.There are many ways to skin a cat. Soundcard worked for me and it occurred to me that it's quite accessible(easy) way how to measure it. Yes, it has it's limitations(bandwidth).For those who think about making DIY frequency analyzer -http://golac.fr/network-analyzer-150mhz/