Benchtop supply: VR, op amp, or uC? - Page 1
Forum section: electronics / projects

author: ratio
author_rank: Regular Contributor
timestamp: May 25, 2017, 02:15:46 am
content: I'm in the planning stage of a benchtop power supply, roughly based onthis. Requirements are: output down to zero volts, adjustable current limit, output in the 15-20A range. Maximum voltage can be sacrificed for an easy-to-acquire (for me) 24VAC xfrmr of an appropriate rating, nothing else is fixed.I've only seen a few VR ICs that are commonly recommended, and most of the stuff I've read indicates that they are either not terribly precise, old, or both.Question: should I look into op amps, or perhaps a microcontroller? An op amp looks like it would be the most precise, but I don't know spit about them. A microcontroller would open up a lot of options but would be even more complicated than an op amp, and can it really control as. I think. I dunno.Suggestions?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 25, 2017, 07:35:49 am
content: There is essentially 1 regulator chip useful for a bench-top supply, the LM723 (and variations from other manufacturers, like µA723 ). This chip is old and the internal current limit is not really stable, it is more a limit for protection, not an accurate regulation. The voltage regulation is reasonably good, though not high precision.So the way to go for a lab supply is using OPs for the regulation. A µC is not really helping in the regulation. It would be more a thing to replace the potentiometers and maybe do the voltage / current display and maybe control a fan.For the first try, I would strongly suggest not to go for a high current range, but start with a moderate power, like 1 A at up to maybe 20 or 30 V.In the 15-20 A range, there will be a lot waste heat with linear regulation, unless you go for really low voltages, like 0-5 V. So this power range is usually using switched mode regulation or at least some kind of pre-regulation. This makes this a much more complicated task.

author: David Hess
author_rank: Super Contributor
timestamp: May 25, 2017, 01:59:51 pm
content: 15 to 20 amps and variable output? Ouch.For a variable output power supply at that current range, I would use an operational amplifier to drive an integrated regulator which drives the many parallel power transistors (at least 4) which will be required to handle the maximum power dissipation. This way with proper design, the integrated regulator's fast current limit, safe operating area protection, and thermal protection will protect the power transistors as well.National LB-28 below shows an example of how the operational amplifiers control the output current and voltage but I would probably use a different configuration which is more like the Tektronix PS501 and PS503 power supplies. HP (Harris?) power supply designs of that era are worth studying also.

author: Seekonk
author_rank: Super Contributor
timestamp: May 25, 2017, 02:17:03 pm
content: Champagne appetite and beer pocketbook. There is this belief that you can build something cheaper and better than you can buy it. Recently I bought a current model Xantrex 30V 10A supply for $46 shipped. Not that I needed it, but I can never pass up Xantrex stuff. Sure it was broken. If you are going to build the said power supply, you better have some talent. A repair like this may take some time for a beginner, less than building something. Certainly less cost. May take a little patience. I've bought all my electronics this way.

author: ratio
author_rank: Regular Contributor
timestamp: May 30, 2017, 03:35:43 pm
content: Thanks for the replies, everyone, & sorry for the Friday in responding. I know it's high current, and maybe I'll have to revisit that, but my background calls a 1200 amp circuit middling large-intellectually I know that 1 amp is plenty, but...This is, of course, a learning experience, so I'm not opposed to biting off a big hunk. I just want it to be useful when I'm done and not end up with an expensive useless toy. Perhaps I will look around for a quality but broken supply to repair, but that seems to me like it would be harder than designing my own.Thanks again for your input!

author: Kleinstein
author_rank: Super Contributor
timestamp: May 30, 2017, 03:48:05 pm
content: There is a lot of background behind designing and understanding a good lab power supply, even if only small. So repairing an old one is not that difficult in comparison: for the old ones there are often circuit diagrams available. Even if not, the circuits are usually relatively simple and thus reverse engineering quite possible. However there are a few that used the now obsolete chip MC1466. Even if a direct repair is not possible the case, transformer and heat sink can often be reused.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: June 05, 2017, 10:54:03 pm
content: It is quite amusing that a LPS topics pops up and draws the same people all the time. Well, for me it is a topic in which I'm interested since about 35yrs.I would always prefer an analogue controller in a LPS, no matter whats around it to make it comfortable.Quote from: David Hess on May 25, 2017, 01:59:51 pmEspecially, as you can see in them the techniques needed to normalize the setting and measured values. They are typically normalized to 0...10V (that is what all the professional devices do) and this is where digital functions can interface cleanly.HP also had a very nice special document explaining a lot about this concepts and also other topics related to Lab Power Supplies, such as pre-regulators: 'DC Power Supply Handbook' which I have in Print, but maybe it is available on some website.When you have your setvals and actual values available in a normalized range, your digital interface can do everything like ramping, data logging and providing remote control. Of course, you can also use it as an analogue control interface.But heed the warning of colleague Kleinstein: a Lab PS in this current range is an advanced topic. My HP6274B (60V 18A) has the weight of a small pig and it handles just like that, with the main transformer giving it an assymetric center of gravity. And I will never forget the amount of LM337s I killed (no problem building a 5A PS now, as this three pin regulator does everything, just like the smaller ones, No?) before I learned about proper thermal design..Those beasts cost near to 50DM back then!You will need lots of metal, not only in yout XFMR but as a heatsink too as well as for wiring and traces.

author: 2N3055
author_rank: Super Contributor
timestamp: June 06, 2017, 08:38:23 am
content: Quote from: Neomys Sapiens on June 05, 2017, 10:54:03 pmYes there is ..http://literature.cdn.keysight.com/litweb/pdf/5952-4020.pdf

author: technix
author_rank: Super Contributor
timestamp: June 06, 2017, 10:03:48 am
content: At this power level you are more likely than not stuck with a switch-mode design (or hybrid-mode). You can try implement one directly in a MCU like STM32F103CBT6 and use one of its advanced control timer to implement a digitally controlled digital feedback three-phase synchronized rectifying buck converter. Additional parts required in this setup is an auxiliary rail regulator (LM2596-12 and TPS562020 maybe, keeping on the switch-mode hype) to power the MCU itself, MOSFET gate drivers (e.g. IR2101) and power MOSFETs (e.g. 6x IRF540) to switch the power, and inductors and capacitors. You can throw in a few Schottky diodes to help the low side transistors' body diodes too, as this may increase your efficiency.

author: David Hess
author_rank: Super Contributor
timestamp: June 06, 2017, 03:07:22 pm
content: Quote from: technix on June 06, 2017, 10:03:48 amI disagree but when novices consider a design like this, they almost always underestimate what is going to be required to handle the heat dissipation and just how physically large and heavy the final result will be. Multiple parallel output transistors, like 8 or 12 or 16, are needed to get enough case to heat sink surface area. The transformer is going to be massive. Based on previous experience, I would build it as part of a cart with wheels and use the top as a shelf so the power supply is more like a piece of furniture. Forget building anything this large and heavy which you will have to lift or putting it on a table or workbench; it can *become* an instrument cart or small table.My big Astron power supply has 8 2N3771 (150 watt 30 amp) power transistors in parallel and it only supports a 50 amp output or 4 amps per 30 amp transistor and about 5 volts of voltage drop for 250 watts total dissipation or 30 watts per 150 watt transistor. Operating down to zero volts would increase the power dissipation requirements by 2.5 times. If forced air cooling made up for that, then a 20 volt 20 amp design which operated down to zero volts would still be the same size and this is about the largest and heaviest design I would want to lift or put on workbench.This also points out why I like using current boosted integrated voltage regulator. If the circuit design and thermal design are correctly done, then the integrated regulator's thermal and safe operating area protection will protect the output transistors. You do not need safe operating area protection in a fixed output design because foldback current limiting (which the big Astron power supplies have) is sufficient but in a variable output design, the voltage drop across the transistors is variable so the foldback current limiting needs to be variable also and you end up with safe operating area protection. The integrated regulator implements this and does so taking into account the operating temperature as well. It also includes peak current limiting.

author: technix
author_rank: Super Contributor
timestamp: June 06, 2017, 04:08:08 pm
content: Quote from: David Hess on June 06, 2017, 03:07:22 pmThe point behind me mentioning this switch mode design is that the loss in power elements, thus heat generated by the power supply, is minimized. This in turn helps minimizing the bulk of the resulting power supply. Using a multi-phase system allows the SMPS to operate with an upstream power supply with less output current but higher output voltage (like how the buck converter on a the dual Xeon motherboard of mine converts 12V 20A into 1V 210A.) And the suggestion of a phase number of three comes from the fact that one advanced control timer in STM32F1 series contains three complimentary outputs which can be built into a three-phase synchronous rectifying buck converter.

author: Robert763
author_rank: Super Contributor
timestamp: June 07, 2017, 09:25:49 pm
content: A linear bench power suply is a good design project. The one thing that can complicate the design (if using a packaged regulator IC) and is not really needed for 99% high current supply uses, is the ability to go down to zero volts. I personally like the LM723 controllers used with MOSFETs. Two tricks here are to run the 723 (and thus gate drive) from a separate low current supply at several volts (10 for a conventional MOSFET, 5 for "logic level" ones) above the desired max output voltage (this does limit the max output to about 30V with conventional MOSFETs).Also use a resistor in series with the gate and a zener between gate and soure to prevent the mak Vgs being exceed if the output is shorted.You can always keep the power DC components and try different control circuits.If you want a microprocesor "front-end", build a linear regultor that will take a 0-5V DC contol signal and then generate the control voltage using PWM. For example most PICs have have 10 bit hardware PWM modules built in which will give 29mV step resolution on a 30V supply.G8RPI.

author: CopperCone
author_rank: Super Contributor
timestamp: June 08, 2017, 03:32:47 am
content: If you want to make something useful that you can't buy easily, then put a ultra low noise adjustable output on it with no current limiting, for signal level voltages (i.e. <100mA).. some really nice options exist for linear regulators that do this (and you can experiment with filters and gyrators and whatnot).If you are building for yourself you might as well give it options that are not on the market place.Simple, but very useful. I typically use a voltage calibrator for this purpose, but you get ground loops and stuff (its not ideal for laboratory use in circuits, I would much rather prefer something that could share the ground without a mess of banana cables. This should have a ten turn pot. For biasing prototypes and stuff.In addition to your other outputs I mean. This way you won't go "damn I spent 40 hours designing something I can buy for 150$ on ebay" It will have something unique and relatively easy to implement.