Programmable power supply(design and prototyp) - Page 1
Forum section: electronics / projects

author: vonB
author_rank: Newbie
timestamp: July 14, 2022, 10:12:33 am
content: Hello all!!!I'm facing need for replacing my old power supply (Thurlby Thandar TS3022) for something programmable and more compact due to lack of space.So the goal is to design and build something new and fancyWhat i need:-At least 2 isolated channels (preferably 4)-40V/2A per channel looks ok-USB/Ethernet control-1mV and 1mA resolution should be fine-Stability over temp range 10℃-35℃-Low noise is important because im working with sensitive analog things quit oftenSo lets then start looking for ideas:-buck preregulator based on LTC7891 should be ok-Linear main regulator-Each channel as separate brick controlled via optoisolation-Separate 48V input for channelsPPSU-1.pdf(28.25 kB - downloaded 245 times.)My first questions:1. What DACs will be good?2. What ADCs will be good?

author: blackdog
author_rank: Frequent Contributor
timestamp: July 14, 2022, 12:37:55 pm
content: Hi vonB,Just some tipsWithout thorough knowledge of SMPS power supplies and especially the ones operating at 1MHz and above,it is better that you do not apply SMPS power supplies in linear Power Supply's that must have and low noise/interference level, is is realy a bitch!It is somewhat difficult to find transformers with taps to keep the power to be dissipated low, this is even more difficult for toroidal transformers.However, the solution is quite simple, buy two toroidal transformers.For a 40V Power Supply, you can then buy two transformers of 2x12V at 3.3-Ampere, this is about a 80-Watt or higher transformer for each transformer.You switch the taps with a couple of relays that you control via comperators, or if you have the knowledge you control them from an Arduino or other controller that you have experience with.DACFor a not-too-expensive DAC check this one out: AD5662This is a 16-Bit DAC, one bit is about 0.6mV if the full range is 40VBut read the datasheet of the DAC you wil use about the linearity! for best linearity use software compensation.If you are using 5V logic levels, choose an LT1021 in 5V as the voltage reference for this DAC.At lower levels for your logic and your DAC voltage say 3.3V then you can use e.g. an LTC6655-3.3V.HeatKeep components that produce heat away from the voltage reference and DAC, place the voltage reference in a corner of the PCB, see why in the voltage reference datasheets.This has to do with stress in the PCB material due to mounting forces and temperature changes.Power LevelThink carefully if you really need a power supply that can do 40V at 2-Ampere and a low interference level, maybe smaler is better. :-)Do not try to design one size fits all Power Supply,You are scaling the reference voltage from 3.3V or 5V to a maximum of 40V so that means that a good portion of the noise from your reference is also scaled up.This is a point to consider with a Low Noise Power Supply.WiringKeep all wires that carry a lot of current away from the control electronics.Twist the wiring from your transformers, twist the wiring to and from your bridge cell.Twist the wires to and from your buffer capacitor, this depends a bit on the construction you are going to choose.ResumeTest the linear circuit, usually one opamp for voltage and one opamp for current according to the Harrison/HP linear power supply concept.Without sufficient knowledge and good measuring equipment, using multiple opamps in sequence is a No-No!Don't start a circuit board until your linear electronics are doing an excellent job working!Test the circuit with dynamic loads in a good measurement setup.This is really the most important part of the power supply you're going to design.Kind regarts,Bram

author: vonB
author_rank: Newbie
timestamp: July 14, 2022, 01:12:07 pm
content: Hi Bram, thanks for your replay.I think I will still go for SMPS psu to produce 48V main rail. Reason for this is lack of space and think is I have couple of meanwell 48V PSU lying around.Heat:Thats shouldn't be e problem becouse i have step-down preregulator that will take 48V down to output voltage + couple of volts for linear regulator.I'm planning to have PDiss=5W for linear regulator stageSimilar concept I saw on R&S PSU teardown video so that should be good way

author: blackdog
author_rank: Frequent Contributor
timestamp: July 14, 2022, 02:27:54 pm
content: Hi vonB,I can tell you that R&S power supplies are not Low Noise, always look at the P.P. spec. voor the P.A.R.D.Commonmode interference is a very big problem to solve.Especially if you're going to be working with some larger powers and everything has to be built compactly.And then R&S has many years of experience in building Prof. measurement equipment and also they do not get the interference noise distance below 0,1mV inside 20MHz bandwidth.Your Meanwell SMPS power supply are definitely not Low Noise, and cause strong commonmode interference.I do not mean to say that Meanwell does not make good power supplies, but for a LAB Power Supply that must have low P.A.R.D. is totally unsuitable.When I make measurements that involve the noise level, almost everything here on the test bench has to be off.Almost always I use a linear old fashioned Power Supply such as the indicated Delta or any other that satisfies and never at all a Power Supply that has an SMPS.Some of the measurements I do are even done with battery power to make sure I am not going to have any comonmode problems.I tell you all this because you yourself indicate that the power supply should be Low Noise, SMPS + Low Noise is like water and fire.Without the knowledge and materials, you cannot make a Low Noise LAB power supply which also contains an SMPS .But, if you want to gain a lot of knowledge and are a go-getter, you're going to learn a lot, if you want to do it right.Above all, don't let me stop you, it's a very good way to gain insights and a lot of fun!Measuring equipment1eYou need at least a 100MHz scope(for everiting that switches), you need to learn good probe techniques, look for this in the Application Notes of Linear Technology, Jim Williams bring this up several times.2eA decent function generator like the Siglent SDG1032X its a "Low Cost" model with good specs.3eWide band (20MHz) 20/40dB, low noise measuring amplifier with a protected input, to measure your power supply output at low, medium and high load.4eBuild a passive 20MHz low pas filter, ore use the 20MHz filter that is available in most scoops.5eThink carefully about your wiring technique!Some information on how to measure P.A.R.D.https://www.prodigit.com/show/how-to-test-noise-from-the-output-of-power-supply.htmHave fun!Bram

author: mawyatt
author_rank: Super Contributor
timestamp: July 14, 2022, 03:37:58 pm
content: Many of us have "Been There Done That", sometimes over and over. You'll never get a SMPS to give anywhere near as low a noise as a simple linear supply can achieve and with much less design effort. Likely the SMPS is not as good at current and voltage settings and stability as well.Listen to everythingblackdogmentioned, can save you lots of $ and time unless of course you want to learn from the experience which many of us already haveAnother good point is once you've become a "Seasoned Veteran" (meaning having many teeth marks on your butt from "Murphy's Bites"), you'll understand the importance of a quality lab supply. Things like low levels and accurate current limit settings, low output capacitance and no overshoot on turn on or off, and of course low noise help save you on many occasions, especially in developments with very expensive ICsGood luck,

author: sahko123
author_rank: Frequent Contributor
timestamp: July 15, 2022, 11:32:28 am
content: A main supply of 48V with a switching pre-regulator followed by a linear regulator is a good way of getting a reasonably low noise supply with good power. (not really necessary but if thats what you want who's to stop you?).If you are worried about reference noise then you can use an external reference rater than the one in the DAC/ADC itself. Something like an adr445/adr4550 you wouldnt need anything better than that unless you were designing an smu.If budget isnt an issue then most of this stuff is easily achievable with not too much effort. You just have to make sure you're putting your money where it matters shilling out a few euro for a reference might not make a difference if something else is causing more noise.Make sure you account for headroom in the different stages. Also to consider is the voltage headroom between the output regulator and the pre-regulator. For a Darlington output I would use a 2V Headroom. So for a 40V output at 2A you would only need to dissipate 4W.

author: blackdog
author_rank: Frequent Contributor
timestamp: July 15, 2022, 03:07:28 pm
content: Hi,It is very difficult as I have already described that a Power Supply that must have a low noise/interference level be equipped with some form of SMPS.These are the two main problems: common mode and the EM field.And of course a lot of knowledge, time and money to tackle this properly.Reference noise is possibly only a minor problem if you compare it to the problems you will have by using one or more SMPS power supplies (horror)The best Low Noise power supplies have an EI core, with separate chambers for the primary and secondary windings with sometimes multiple screens.But then those are the extreme examples.I am not against SMPS power supply, we all benefit a lot from it in this world, especially now with the high energy prices.But wanting to build a low noise power supply based on one or more SMPS power supplies is really a No-No.As I mentioned, use transformers with taps or two transformers.Use power transistors with low thermal resistance on e.g. cooling elements used in the computer industry.The dropout voltage depends on your configuration, prefer not to use power MOSFets but good power transitors designed for audio applications as a suggestion.Do not use cheep China crap!The dropout voltage can be between 1,5 and 3V depending on your circuit design and how the circuit reacts to fast dynamic loading.Kind regarts,BramPSI would like to endorse mawyatt's last line.This along with my starting point for power supplies, first, the circuit used must be "sweet, caring" to itself, only then can he take good care of the d.u.t.!Itself will never generate unwanted signals, it itself will never go beyond its limits, outside of the normal current limiter there will be a peak current limiter which will provide additional protection to the power stage.There will be no in-and-out phenomena when switching on the mains power.There will be an enable switch that switches the power supply "smooth" on or off.Use a current source on the output of about 5% of the maximum current that power supply can deliver.This will ensure better dynamic behavior and faster depletion of buffer elcos present in the d.u.t. when using the enable switch.If a schematic for a linear power supply is properly designed then you can use between 22 and 47uF per ampere of output current depending on the construction and speed of the loops.But start with 47uF, with tuned circuits and very fast opamps I got the output voltage deviation very good with 22uF across the output per Ampere output current.Good another tip. :-)Build it as if it were an HF amplifier, short wires that have been well thought out, modern power transistors can have an Ft of over 30MHz.Test setups as if you are testing an Arduino circuit is a No-No, no spaghetti wiring!Thinking you have solved the problem when you are testing with the current and voltage loop of your circuit by taking higher and higher value for the compensation capacitors, then you know you are not doing a good job.Sorry, its a bit long.