Simple 4-quadrant PSU with remote sense - please recommend topology/schematic - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: April 10, 2016, 08:03:12 pm
content: Hi!can you please recommend a topology or schematic or app. note how to build the simpliest possible 4-quadrant PSU?Requsted properties are output voltage of up to about +-1V and +-5A with remote voltage sense on load. Being capable of controlling output voltage (and possibly current limit) and measuring real output U and I. (ADCs and DACs with common ground)The setup should serve for PV cell characterisation on a local university. Non-comercial project, I have no profit from it. 've been just asked to help them.I have been thinking the whole weekend how to do it and haven't thought out any usable circuit. :-/ I am stuck with no freaking ideas. Have come accross some interesting schematics, like that one Operational Power Supply from LM12 datasheet... but that does not use remote voltage sensing.Can you help me please help the uni? :-)Thank you,YanPS: What the hell are doing those emitters of Q1 and Q2 in series with the caps?!

author: samofab
author_rank: Regular Contributor
timestamp: April 10, 2016, 09:51:11 pm
content: Google service manual for Keithley 228A, you'll find manual with schematics on xdevs. It's old, but quite simple.

author: Marco
author_rank: Super Contributor
timestamp: April 11, 2016, 12:22:40 am
content: Did you google yet?EDNhas a pretty basic one which is a slight variation on a datasheet circuit to use LT1970 with external MOSFETs. Expensive IC, but who cares for a one off.

author: timb
author_rank: Super Contributor
timestamp: April 11, 2016, 12:32:06 am
content: Check the LT1970Adatasheet out. There's a good example circuit on the last page for a +-5A version using two external FETs. Either use a cheap voltage reference and potentiometer to set the voltage/current limits or use a quad DAC + MCU.I'm currently working on a supply based on this chip and it's coming along very nicely. (I'm also in the simulation stage of a +-100V/100mA discrete four quadrant PSU/amp.)

author: amspire
author_rank: Super Contributor
timestamp: April 11, 2016, 12:53:21 am
content: The LT1970A would be fine if you are using a resistive or inductive load. It will not be stable with a capacitive load larger then a few nanoFarads. If your load is a resistor, then it is a great chip.

author: timb
author_rank: Super Contributor
timestamp: April 11, 2016, 09:06:20 am
content: Quote from: amspire on April 11, 2016, 12:53:21 amNot true, at least in my case. It's been stable with large capacitive loads. They even talk about this in the datasheet.

author: retrolefty
author_rank: Super Contributor
timestamp: April 11, 2016, 06:19:51 pm
content: Could someone give me/us a simple definition or example of a "four quadrant" PS unit? How does it differ from say a standard linear DC regulator.

author: samofab
author_rank: Regular Contributor
timestamp: April 11, 2016, 07:01:36 pm
content: "four quadrant" PS can source or sink current of either polarity. You can think of it as a battery that can be either charging or discharging (with a relay to reverse polarity). Quadrant 1 is for example positive voltage, positive current.

author: timb
author_rank: Super Contributor
timestamp: April 12, 2016, 01:05:39 am
content: Quote from: retrolefty on April 11, 2016, 06:19:51 pmAs others have mentioned, it can source *and* sink current. When sinking current, it acts like an electronic load; when sourcing current, it acts like a power supply. It's also bipolar in either mode (meaning it can handle negative and positive voltage).Here's a nice graph:Most power supplies only work in Quadrant I (and statically in Quadrant III by swapping the + and - leads).In short, they're basically big ass op-amps. Some times they're even sold as such, like the Kepco BOP series bipolar operational supply.

author: Yansi
author_rank: Super Contributor
timestamp: April 12, 2016, 08:31:25 pm
content: Thank you for explanation, but I know quite well what those quadrants mean.Without remotesense, the supply circuit can be quite simple, as per the schematic from the LM12 datasheet I have posted in the #1 post.With remote sensing, the "simply" is just gone. (When trying to avoid precision very very expensive ICs).By the word "simple" I have meant more like "jellybean part like" solution, not a fancy unobtanium superexpensive IC with zero availability, like most LT stuff I have been working before.So yes, I have tried to google something up, but found mostly nothing but bunch of that LT stuff which is not really needed here or many 4Q supply designs but without remote sensing.Thank you, will look for that Keithley 228A manual, should be interesting.EDIT: Found the Keithley schematic. Will be helpful, indeed! But am a little worried about precision of the diff amps (like the voltage sensing one when using jellybean parts like OP07...EDIT2: Can someone please explain the Q1 and Q2 in the #1 post schematic with LM12 power opamp?

author: timb
author_rank: Super Contributor
timestamp: April 13, 2016, 12:11:24 am
content: @Yansi The explanation wasn't for you. It was for @remotelefty who asked what it meant.Now, remote sensing is actually fairly easy to do for a four-quadrant supply.Here's the simple way to do it with no extra parts:Here's a slightly more complex way to do it using either a difference amplifier (such as the LT1990) or a standard op-amp and a quad resistor array:In either method, you would want to place high value resistors between the force and sense terminals, to keep the supply from maxing out if the sense terminals weren't connected. You could also use a relay or analog switch (as shown in the second image) and have a MCU control it.I don't know why you think the LT parts are unobtainium or expensive; they're not. (If they were Maxim, maybe!) They're available on e14, Digi-Key, Mouser, Farnell, etc.The LT1970A combines an op-amp, power booster stage *and* an adjustable sink/source current limit all in a single package. For under $10. That's a really good price for what you get, as it reduces your BOM cost significantly and speeds design time. If you were building this into a product that you would be producing hundreds of thousands of, I'd say go discrete and save the money. But for a one-off? Use a monolithic solution.You should go check out the prices of other power amplifiers on Digi-Key, $10 is about the starting price, moving up to $100-$200 (per chip) for the high voltage, high current stuff from Apex.I'm not trying to get you to use the LT1970A; if another power amp fits the bill better for you, by all means use it. I'm just saying they're all fairly expensive. That said, once you've tried to boost a regular op-amp by bootstrapping FETs to it's power rails, or tried to build and compensate a discrete power stage, you'll wish you'd just spent the extra money on a power amp to start with. Been there, done that. It's not fun.

author: Yansi
author_rank: Super Contributor
timestamp: April 13, 2016, 12:22:47 am
content: Those two sensing examples are fine, but both depend heavily on the resistor precision and/or resistor trimming plus opamp offset. I did want to not use such. I don't like diff. amplifiers. Always the weakest point.However, few experiments made clear, that OP07 with four common type resistors (one trimmed with a trimmer in series) will work good enough fort his job.The actual schematic looks something like this and I'm in the process of breadboarding it. Due to several reasons, the -Force terminal will go straight to system GND and the sense shunt will be floating on the +Force terminal, sensed by a second diff. amp. A little change in my "no diffamp" rules, but seems it will work. At least good enough. They do not require extra precision, 1% should be enough for them.Currently, I am more thinking about the ouput current limiting. Can someone please explain the bizzare circuit in #1 post? Q1 and Q2 respectively, emitters to the caps.

author: quantumvolt
author_rank: Frequent Contributor
timestamp: April 13, 2016, 03:47:55 am
content: Quote from: timb on April 13, 2016, 12:11:24 amAgree. I have been working on a DC Power Analyzer since 2013. I have been prototyping old discrete HP / Tektronix / Fluke etc. stuff, general power amps and beefed-up op amp circuits. In the end I chose the LT1970.OFF TOPIC: I have the 5 amp LT1970+FETs demo boardhttp://www.linear.com/solutions/4288. Due to age, health situation and obligations I have to strictly prioritize my projects. If anyone wants the board, I'll sell it for half of LT's price + shipping.

author: Yansi
author_rank: Super Contributor
timestamp: April 13, 2016, 07:37:54 am
content: Understand. This not a first PSU I build. I know how much trouble can be adding discrete voltage amplifying stages into OPamp FB loop. But this should be no trouble, as the required output voltage range is well in between the opamp supply rails, so a simple follower stage is needed.I will use two power darlingtons and maybe two more trannies to reduce the opamp output current, and will try some linearization tricks to get rid of the hysteresis (base-to-base voltage of 2,4V or more) in the complementary pair to make it more stable.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: April 13, 2016, 07:57:11 am
content: Quote from: Yansi on April 13, 2016, 12:22:47 amNothing bizarre abouit it, Q1 and Q2 work as analogue switches, connecting a compensation RC network when either of the two current limit opamps is engaged.CheersAlex

author: Yansi
author_rank: Super Contributor
timestamp: April 13, 2016, 09:29:52 am
content: Ah!! That is an interesting idea!I already know the idea of the "speed up" diodes D1 and D2, that does not allow to go the opamp to openloop and saturate, but the FB switching seems neat to also speed up the current limiter engaging. Veery clever.

author: timb
author_rank: Super Contributor
timestamp: April 13, 2016, 11:04:56 am
content: Quote from: Yansi on April 13, 2016, 12:22:47 amActually, it doesn't depend on resistor precision at all. It depends on resistor ratio. A quad resistor network with good matching isn't really that expensive, but if you're really cheap and it's for a one-off project you can simply hand select the four resistors and mount them in thermal contact.Besides, the second example doesn't depend on external resistors at all, because you'd use a difference amp with built-in precision resistors. AD, TI and LT all make them. They're not expensive.I'm not sure how difference amps are a weak point? To me, it would be a strong point, as they can often withstand very high voltages (the LT1990 has. +-250V common mode range, there's an AD part with similar specs) which is something you want in a 4-quadrant power supply. You never know what sort of voltage might accidentally get fed to it. Plus you don't have to mess around with trims for resistor ratio, etc. Offset can be compensated for easily by tying the output of a pot to the ref pin, though it's not really needed as most of these parts have under 1mV of offset.

author: Yansi
author_rank: Super Contributor
timestamp: April 13, 2016, 11:26:48 pm
content: I think precision is the right word, as you need 2 resistor dividers with as was said same ratios. Thats achievable only by using precision resistors with tight tolerances, or by trimming the damn thing for match. And even then the TCO of the resistor will play a role, when one needs some good CMRR of the whole diff amp.Yeah, sure, I can use precision opamp, IC, this, something... and buy an off the shelf 4Q PSU unit for $10 000. However this is not the case and the most "precision component" I will use here is OP07. As I've said, I will get no $ from this job and their required overall accuracy of the PSU is not too tight. 1% accuracy for PV cell measurement is good enough they've said. That has to be done using jellybean parts, hasn't it?Why I call diff amps the weakest point? Because very rarely I use precision diff amp ICs, always build them with standard OPamps and using common 1% resistor types. The need for the trimming, the temperature drifts of offset voltages or resistor TCOs always make it hard to make the circuit up to reasonable spec, using such jellybean parts.Lol, +-250V is not really what I need here. The requsted output voltage range was +-1 or +-2V at max. Sure, I understand the design should be as robust as possible, but I think withstand voltage of one tenth of those 250V will be more than enough for this experimental application.In some short time I will report some results, as I've already started breadboarding some circuitry, and it seems promissing, so far...With regards to the LM12 circuit and those two Q1 Q2 transistors: How will the circuitt behavior change when interchanging C and E in the circuit? Or why is the collector chosen to be connected to the opamp output?

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2016, 02:48:58 pm
content: Inamp/diff-amp doesn't feel like the natural topology for remote sensing a voltage, the exact matching of input impedance isn't all that relevant.Is there perhaps a variation on high side current sensing topologies possible?

author: Yansi
author_rank: Super Contributor
timestamp: April 14, 2016, 03:54:04 pm
content: Meh, i don't like it either. But I need it anyway, as I need to bring the exact voltage accross the load to an ADC.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2016, 04:09:30 pm
content: Quote from: Marco on April 14, 2016, 02:48:58 pmSo, what would you suggest?

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2016, 07:40:39 pm
content: Quote from: Siwastaja on April 14, 2016, 04:09:30 pmAs I said, something similar to current sensing, so differential voltage -> current -> voltage ... something like INA170 for instance. Considering it further though all you would really accomplish is make common mode rejection independent from the resistors drifting, offset and amplification would still be drifting along. The only way to get rid of that is with chopping techniques I think which seems a bit too complex here.PS. I'm not sure I actually understood the original problem BTW. Does the power supply actually have to regulate using the remote voltage? If it's just about sensing it use a differential ADC ...

author: mazurov
author_rank: Frequent Contributor
timestamp: April 14, 2016, 08:03:54 pm
content: Take a look at the LT3091 datasheet, there is an example of voltage-controlled 4Q PSU made out of it plus LT3081. Methods of increasing output current/improving dropout at low voltages are spread across LT308x-LT309x datasheets.

author: timb
author_rank: Super Contributor
timestamp: April 14, 2016, 08:14:22 pm
content: Quote from: Marco on April 14, 2016, 07:40:39 pmYes. Of course it has to regulate using the remote voltage! That's the entire point of remote sensing (aka Kelvin Sensing, Four Wire Sensing) is to regulate the voltage at point of load, instead of at the power supply output. This is to make up for I-R drops in the output leads at high currents, or for precision applications at low currents.A *difference* amplifier absolutely is the correct topology for this, as you obviously need to measure the difference between Sense + and -. I'm not sure what you'd gain by converting the voltage to current and back to voltage in this case?Anyway, this is how transistor power supplies have done it for the last 50 years. You *could* do it digitally, by sensing the remote voltage with a differential ADC, but that would only work if you're already controlling the output amplifier with a DAC. (So you'd have a local feedback loop with the output amp and a separate software remote sensing feedback loop.)There are other alternatives; LT and TI both make parts that do voltage drop compensation. Some of them are "dumb" and simply increase output voltage as the current increase, based on a fixed parameter for the wire length. Others are smart and actually measure the wire to calculate the drop through some sort of voodoo magic.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2016, 08:24:58 pm
content: Quote from: Marco on April 14, 2016, 07:40:39 pmDon't these current shunt monitor IC's rely on laser-trimmed matched resistors, too, or am I completely wrong here? It's only neatly integrated. So we are back to integrated difference amplifiers with integrated precision resistors, or using external precision resistors.I can't really see the problem OP is having. Precision resistors and precision opamps are the most basic building blocks, cheap and easily available, rather easy to design in. Maybe youcanbe very clever and MacGyverize a precision instrument from really jellybean parts, but please share the details if you are able to do it!

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2016, 08:28:39 pm
content: Quote from: timb on April 14, 2016, 08:14:22 pmWell that's treading into semantics, you need to output/amplify the difference but generally difference amplifier is understood as a specific way of doing it. The one opamp with 4 resistors difference amplifier, a chopping differential amplifier like the MAX4208 is clearly superior in most ways and is internally something fundamentally different (different from a 3 opamp inamp topology too, even though it's still called an inamp, semantics strike again).QuoteThe same thing as with current sensing, common mode rejection.Quote from: Siwastaja on April 14, 2016, 08:24:58 pmIt minimizes the number of high precision resistors needed ... and their drift doesn't impact common mode rejection by design. Doesn't seem a big win here though in retrospect.

author: timb
author_rank: Super Contributor
timestamp: April 14, 2016, 08:33:27 pm
content: Quote from: Marco on April 14, 2016, 08:28:39 pmRight. The OP wanted simple and cheap. Doesn't get much simpler than four resistors and an op-amp. (You don't even need the extra op-amp since you can feedback right to the power amp itself as I showed in another post.)For kicks, how exactly do you propose to convert the voltage to current?

author: Marco
author_rank: Super Contributor
timestamp: April 14, 2016, 08:43:11 pm
content: Quote from: timb on April 14, 2016, 08:33:27 pm"something like INA170 for instance"As I said before though, before you even commented ... "Considering it further though all you would really accomplish is make common mode rejection independent from the resistors drifting, offset and amplification would still be drifting along." So it doesn't really seem worth the effort.

author: timb
author_rank: Super Contributor
timestamp: April 14, 2016, 09:08:42 pm
content: Quote from: Marco on April 14, 2016, 08:43:11 pmAh, gotcha. I didn't realize that had a current output. I was thinking of some of the other INA current shunt monitors with voltage outputs.It would increase common mode rejection, but yeah not sure if it's worth the trouble in this particular case. An LT1990 difference amplifier has similar levels of CMMR and has a higher common mode range; there's also a couple of in-amps from TI and AD with better CMMR but at much lower voltages.Though, I can think of a couple of niche applications where doing bipolar differential voltage sensing to current could be extremely useful. So thanks for thinking outside of the box! I might be able to use this in a future circuit.

author: macboy
author_rank: Super Contributor
timestamp: April 15, 2016, 01:56:37 pm
content: I didn't see anyone mention using audio power amps as the power op-amp. On possible device is the LM3886 which can dissipate up to 125 W. Although marketed as an audio amplifier, it is fundamentally just a big power op-amp with 8 MHz of GBP and 115 dB open loop gain. The device is very well specified in the datasheet for things like SOA, pulse power limit, and many other parameters (no fewer than 45 graphs), and being an audio amp, it is designed to drive reactive and low resistance loads. Any comparable power op-amp specified (read: marketed) for industrial applications will cost 10x as much.

author: Yansi
author_rank: Super Contributor
timestamp: April 17, 2016, 01:29:24 pm
content: Noone didn't mention, because this idea is wrong from a few perspectives of this PSU design:1) You cant easily heatsink the package like that MULTIWATT thingy of LM3886. The only one M3 screw for such package is just hilarious and any other means of mounting is just stupid. Too complicated. You cannot beat two TO3 mammas on a chunk of aluminium.2) Loop stability. In general, the audio power amps are mostly designed so they are stable only at certain level of gain. For example TDA2005 is the most know bastard, that just needs at least those 20dB to be really stable. I think the LM3886 won't be much different. And as long as I need only +-1V output range, 20+dB gain does not make much sense.3) minimum supply voltage. I doubt the LM3886 will work at about +-3V supply. Why would I use +-9V minimum supply for a +-1V output range? But this can be easily done with two TO3 mammas... no problem.Conclusion: integrated audio poweramps arereallynot suitable for this low voltage high current application. Difficulty of heatsinking, too high minimum gain for stability and too high minimum operating voltage are those culprits.NOTE: LM3886 is stable above 20dB gain, I was right. ...just have looked into the datasheet.Since last time, I have made some progress with breadboarding. The discrete power stage design is I think closed, schematic below. Sorry for not filling in the resistor values, these are still TBD. The breadbord version of the circuit is simplified and using little bit different scheme.I have also breadboarded the difference amp section for remote sensing. The whole is connected as on the image below below.Please don't be worried about those probably unknown to you semiconductor types used, these are some old beefy mammas made by our old national manufacturer, that has quit doing some decades ago.Now only CK1 is used and it seems to be stable. CK2 and RK2 not used (bcs of lacking knowledge how to calculate them. Can somebody help please?).However serious stability issues occur when capacitively loading the supply output. 100n or 3uF or whatever small cap on hand, wild oscillation starts. How to solve that issue, how should I tune the feedback loop so it will be stable with C loading on the output?Thx, Yansi

author: Yansi
author_rank: Super Contributor
timestamp: April 17, 2016, 01:42:54 pm
content: Huh? I've just might solved the stability issue.By looking in the Keithley 228A schematic, the control opamps seems to have only about 5kHz bandwidth (47K x 680p). I have calculated what I have there just to compare... and ahaa moment! 670kHz bandwidth... So instead of 250pF x 500ohm I have increased the feedback capacitor to 3n3 (meaning the bandwidth is about 96kHz) and bingo! It does now tolerate the 3uF MKP foil cap directly on the output.So I am quite happy now. Let's proceed to the current sense and feedback design.//EDIT: But still, after more slowing down the BW to only 14kHz it does not like capacitive loading of 1uF or less. :-/Yansi

author: Kalvin
author_rank: Super Contributor
timestamp: April 17, 2016, 02:25:03 pm
content: This application brief might give some help:"Unlimited Capacitive Load Drive Op Amp Takes Guesswork Out Of Design"http://www.ti.com/lit/an/snoa808/snoa808.pdf

author: Yansi
author_rank: Super Contributor
timestamp: April 17, 2016, 03:30:22 pm
content: Might, thanks, but did not :-/For me, a common undereducated hobbyist, such appnote looks like "buy this special opamp everything will suddenly work blah blah blah"But I know, it is not funny.

author: Kalvin
author_rank: Super Contributor
timestamp: April 17, 2016, 03:50:33 pm
content: Quote from: Yansi on April 17, 2016, 03:30:22 pmThe LM8272 discussed in the application brief has capacitors placed across the output transistors' base and collector increasing the Miller capacitance thus reducing the bandwidth when driving capacitive loads. I haven't been simulating the concept but it might help. Simulating your whole circuit and evaluating the phase margin with the Miller capacitors in place would give some practical insight, too.

author: Marco
author_rank: Super Contributor
timestamp: April 17, 2016, 05:44:05 pm
content: The output totem pole is not a pair of followers ... replicating this circuit semi-discretely seems a non starter.In loop compensationseems the most natural fit.

author: Yansi
author_rank: Super Contributor
timestamp: April 17, 2016, 06:16:57 pm
content: It not only seems, but it it really does fit! That is what I have exactly done, but based on stupid experimenting with the circuit. If you look in the circuit few posts above, I have added some small cap accross the opamp and that helped it stabilize.Now I have also verified by breadboarding the current limiter circuit from LM12 datasheet, with that transistor feedback thing. It really does work as advertised.Now I only need to add current limiter active indicator (some comparator craziness like in the Keithely 228A) and hooray, we can make us a PCB. (A then do the whole capacitor tweaking process again)

author: Yansi
author_rank: Super Contributor
timestamp: April 17, 2016, 09:21:40 pm
content: Oh goodness... Nobody spotted my superfail in the schematic? The buffering stage before the power darlingtons will burn like shit, when the +-3V power supply goes out first, or when the opamp saturates. FAIL right there!The buffer stage must be supplied from the 3V supply (I don't like that cause too much voltage drop accross 3 BE junctions) or I will throw em out, as those darlington mammas has enough h21e at 5A I can easily drive them with th opamp directly (as is also currently done on my breadboard).So here's the result, after few days of thinking and one whole day of breadboarding. Ugly, but works. Stable (or stable enough), current limiter works fine and so does current limit indicator, providing nice open collector output for the stupid arduino they want to drive this beauty withNow it's time for PCB layout. I'll try to have it on single sided board. Will throw in the full schematic for check or diy, if there will be interested people.Thx, Yansi