Nice power supply design - Page 1
Forum section: electronics / projects

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 19, 2023, 02:40:08 pm
content: I came across this in the Boonton 1121A service manual and thought it was a nice way of doing a supply that was both accurate, quiet and low cost. I've seen variations on something like this - usually with zeners - but this just seemed to bring it all together in an efficient way. it could easily modified for better performance if desired.Hal

author: Terry Bites
author_rank: Super Contributor
timestamp: June 19, 2023, 02:53:16 pm
content: I'd feel a bit sad feeding my 10ppm REF-01 into a TL072.It looks very sensible but probably needs a bit of modernising.

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 19, 2023, 03:06:04 pm
content: Well, yes... that's what I meant by modifying - if nothing else, there are 10V references with better specs that cost less. But, then again, the resistor network is 0.1% and it would take quite a bit of temperature change to make a dent in that. The main point was that it had very good performance as is and was also fairly inexpensive. Personally, i like the TL072 - I use them as jellybeans in a lot of designs - and a 1mv max offset gets lost in the weeds on this design. To me, one of the attractions to the supply design is that it's very much a set-and-forget supply - for the most part it will just keep working within specs for the life of the product. It's simple and just works... that counts for a lot to me.

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2023, 03:20:43 pm
content: Quote from: Terry Bites on June 19, 2023, 02:53:16 pmThetypicaloffset drift spec for a typical TL072 is 10 uV/K (no max spec).That is 1 ppm/K of 10 V, which seems reasonable, if not overkill.(Other versions range between 2 and 18 uV/K for typical offset drift.)

author: wraper
author_rank: Unknown
timestamp: June 19, 2023, 03:32:59 pm
content: Does this audio analyzer need supply voltages extra stable over temperature and slow load current change (if R111x are really good)? Otherwise I don't see how this any better than plain 78/79xx otherwise.EDIT: 78/7905 have somewhat better noise than 78/7915. But then why not use LM317/337?

author: MasterT
author_rank: Frequent Contributor
timestamp: June 19, 2023, 04:32:38 pm
content: I think this design has about 160dB open loop gain, so PSRR / ripple rejection 'd be very high (120 Hz). Nothing even close to 78xx or 317 can reach. Same time TL072 is not the best choice, considering low frequency < 1 kHz noise, where 1/f would introduce more troubles than main 120 Hz ripple. NE5532/NE5534 are better options, since input current not limited

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2023, 04:39:28 pm
content: The 5532/5534 were not intended for precision DC applications, and have relatively high input bias current.They do feature low noise and good slew rate, since they were originally designed for audio applications.(NE5534 bias current typ. 500 nA, max 1500 nA at 25oC. Offset voltage drift typically 5 uV/oC.)

author: MasterT
author_rank: Frequent Contributor
timestamp: June 19, 2023, 04:50:52 pm
content: Quote from: TimFox on June 19, 2023, 04:39:28 pmDesign was not intended as DC precise, I suspect that frequency range above DC - < 20 Hz were not considered at all. BTW offset of the TL072 is 5 mV, this is why I pointed to ne5532/34.If this PS gonna to be in use not in an audio application, and needs precision at DC, than something like OPA227, OPA2202 or OP07

author: David Hess
author_rank: Super Contributor
timestamp: June 19, 2023, 04:59:42 pm
content: The idea of enclosing an integrated regulator within the feedback loop of an operational amplifier and using a precision reference is an old one dating back to at least the 1970s as shown below.Even general purpose operational amplifiers usually have better noise and precision than a reference so the requirements are not demanding.I have used this circuit many times with a low noise buried zener reference, low noise precision operational amplifier like the LT1007, and a 7805 or 317 regulator. With Kelvin sensing the load regulator can be better than 10 microvolts going from 0 to 1 amp of load current.

author: TimFox
author_rank: Super Contributor
timestamp: June 19, 2023, 04:59:50 pm
content: Usually, the initial DC offset voltage is not so important.Noise and drift are probably more important to this application.

author: wraper
author_rank: Unknown
timestamp: June 19, 2023, 08:02:34 pm
content: Quote from: MasterT on June 19, 2023, 04:32:38 pmThere are 22uF caps from "GND" terminals of 78/7905 to actual GND, so I fail to see how this improves PSRR.

author: MasterT
author_rank: Frequent Contributor
timestamp: June 19, 2023, 09:22:16 pm
content: Quote from: wraper on June 19, 2023, 08:02:34 pm2.2 uF x 300 --> 240 Hz

author: moffy
author_rank: Super Contributor
timestamp: June 19, 2023, 11:14:26 pm
content: One thing I don't like is C130 after R111C and the reference, leakage currents can be relatively high and temp dependent, which would introduce drift and possibly thermal noise.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 01:02:51 am
content: Quote from: moffy on June 19, 2023, 11:14:26 pmI would have left C130 out unless the reference is really noisy. C130 could be bootstrapped easily in this case to reduce its leakage.

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 20, 2023, 10:13:46 am
content: C130 is a tantalum chip - should be pretty well stable over time. And anyway, the whole point of the design is not to be aprecisionsupply but to be astableone. For me it just seemed to be a good example of a supply well designed to serve the needs of the equipment it was powering - elegant in its simplicity perhaps. I'm one of those that will overengineer something to a fault so it was good to be being reminded that, most of the time, simple is better."Perfect is the enemy of Good Enough."Hal

author: moffy
author_rank: Super Contributor
timestamp: June 20, 2023, 11:06:13 am
content: Quote from: HalFoster on June 20, 2023, 10:13:46 amThanks for the info. I had a look at an article on tantalum leakage currents:https://www.avx.com/docs/techinfo/1_Analysis.pdf,and they look reasonably significant and temp, time dependent. Personally I would possibly use something like a 1u Polyester in place of the tantalum.

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 20, 2023, 01:01:34 pm
content: The tant is one of the Kyocera TPS series - low ESR (doesn't really matter here) but should be very stable over time. Worst case leakage looks to be only a uA or thereabouts so again any variation in voltage is lost in the noise of the other tolerances. And, dropping the value down to 1uF raises the fc an order of magnitude to ~15 Hz from 1.5 Hz; IMO the 10K/10uF combination is a good compromise for reducing the reference noise. At the end of the day it is a stable power supply - not a voltage reference.Hal

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 20, 2023, 01:08:35 pm
content: Just to be clear, I'm not trying to be argumentative - about the first thing I did was redesign the thing into effectively a voltage reference... with a significant increase in the BOM and cost. So I mostly agree with all the comments geared towards increasing the performance - if nothing else were I to build it today I would choose a different reference and a lower offset/drift OA just because they are available now for less, or about the same, as the existing ones. I'd keep C130 as is, though - usedappropriatelyI am a pretty big fan of tantalums.Hal

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 01:24:03 pm
content: The capacitor leakage combined with the 10k source impedance creates an error 10 times larger than the temperature coefficient of the reference. Filtering noise below the corner frequency is useless when even more low frequency noise is added.To improve the situation, double the size of the capacitor, place two capacitors in series, and tie the new node to the reference output through a higher value resistor to bootstrap the top capacitor. Now the leakage is reduced many times.This design is more stable than the regulator alone, but the example I gave is even more stable at low frequencies. Stability ultimately depends on the reference and filtering if the filtering introduces low frequency errors.

author: dietert1
author_rank: Super Contributor
timestamp: June 20, 2023, 01:37:17 pm
content: Quote from: David Hess on June 19, 2023, 04:59:42 pmIn your schematics the opamp gets its supply from the output voltage. This detail was implicit in the Boonton schematic, yet important for precision. Don't know whether one can call it "control from below". One way to do it is a darlington made of a n-channel JFET and a npn bipolar transistor. I am using that for some voltage references.Regards, Dieter

author: iMo
author_rank: Super Contributor
timestamp: June 20, 2023, 01:49:11 pm
content: A power supply without a current limiting capability calls for trouble when used as a lab psu or for powering some precious prototypes.I smoked many expensive chips while I used 7805/317 based kind of stuff.Therefore I built myself a 723 based psu with 2 floating voltages (fixed 2x15V) and an 50mA foldback (easy to do with 723, just 2 resistors). And it is "stable and quiet" as well.PS: it works such that after the current reaches set 50mA, for example, the current drops to set 5mA, for example. The <50mA would not smoke devices most of the time. You can go lower to say 20mA for the initial smoke test. The 78xx/317 and alike will shoot 1.5A (at 15V) through your wrongly wired chip easily.

author: wraper
author_rank: Unknown
timestamp: June 20, 2023, 02:37:19 pm
content: Quote from: HalFoster on June 20, 2023, 01:01:34 pm1uA leakage current through the cap would mean 10mV voltage shift.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 02:45:12 pm
content: Quote from: dietert1 on June 20, 2023, 01:37:17 pmPrecision operational amplifiers have such high power supply and common mode rejection that they could be powered directly from the unregulated supply, however when using a 7805 the operational amplifier's output is right in the middle of a 10 volt output and any reference lower than the output may be used so why not take advantage of the output to power the operational amplifier?My schematic shows it directly, but another aspect helping precision is that the voltage dividers and reference can be connected directly at the load to support remote sensing. So variation in load current through the power supply wiring does not result in variations in voltage. The load regulation of this power supply design can be incredibly good, like microvolts from zero to full load or better.Quote from: iMo on June 20, 2023, 01:49:11 pmThese designs rely on the integrated voltage regulator to implement current and thermal limiting to protect the power supply, but this may not be enough for sensitive loads. Adjustable current limiting could be added. I would also work to reduce the output capacitance.

author: iMo
author_rank: Super Contributor
timestamp: June 20, 2023, 03:02:56 pm
content: Yep, the bulky 78xx/79xx/317 etc. are good for standalone fixed power supplies with known loads (with their 1.5+A current limit). When planning a current limiting and a design with smaller capacitors at the output a discrete solution with opamps+transistors would be a better approach, imho. Anyhow, that has been discussed many times here..

author: MasterT
author_rank: Frequent Contributor
timestamp: June 20, 2023, 03:25:52 pm
content: There are regulators with adjustable small current limiting NCV47701, NCV47551

author: iMo
author_rank: Super Contributor
timestamp: June 20, 2023, 03:40:07 pm
content: Quote from: MasterT on June 20, 2023, 03:25:52 pmThat is a good candidate, current limit adjustable from 10mA to 350mA via an ext resistor.Set the output to 5V (its min), and put it into the op-amp loop with a low noise reference as in the above schematics..The floating I_limit resistor Rcso (a pot) will adjust the max current.Perhaps it would work that way.. Try it..PS: fixed the output cap.

author: moffy
author_rank: Super Contributor
timestamp: June 20, 2023, 11:06:47 pm
content: Quote from: David Hess on June 20, 2023, 01:24:03 pmThat is a nice solution, I'll tuck the idea away.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2023, 11:44:30 pm
content: Quote from: moffy on June 20, 2023, 11:06:47 pmI would have linked an example but could not find one. I think Intersil discusses it in one of their integrating converter datasheets.

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 21, 2023, 10:07:31 am
content: This is from a Renesas datasheet but I am pretty sure that it is not the original source. For some reason I think it was from one of the papers written by Bonnie Baker, but it could have been Bob Pease originally.Hal

author: HalFoster
author_rank: Frequent Contributor
timestamp: June 30, 2023, 01:05:23 pm
content: To give credit where it is due, the first reference to the idea I could find was an article written by Walt Jung:https://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Jung_W/Build_Ultra_Low_Noise_Voltage_Reference.pdf