0 - 30 v 0 to 5-10 amp Lab Bench Power supply design and questions - Page 1
Forum section: electronics / projects

author: Hiemal
author_rank: Regular Contributor
timestamp: December 15, 2018, 03:26:32 am
content: Hi All!I've been brainstorming a method to create an easy lab bench power supply using some common, relatively "cheap" parts.I bought a server 960 watt power supply for about $15 off of eBay, brand new. 80 plus platinum, a pretty damn good deal considering the amount of power it can push out.So, my thought process goes something like thisI bought a buck-boost converter, one of these:https://i.snag.gy/kzlhTZ.jpgI'm planning on swapping out the fets and diodes with TO-220 package ones, and set each of them on separate heatsinks, for increased power handling. The original fets arehttp://www.irf.com/product-info/datasheets/data/irl3803s.pdfones, which are already pretty decent ones. The new fets arehttp://www.farnell.com/datasheets/1955477.pdf?_ga=2.248090887.1659718664.1544680552-1337621676.1538812331, which are pretty similar to the originals, except a slightly lower on state resistance, and they can handle higher voltage. The gate capacitance is marginally larger, but I don't think it should be a big issue.The original schottky diode is ahttps://www.onsemi.com/pub/Collateral/MBRB20100CT-D.PDF, a pretty damn good one. I couldn't find anything necessarily "better", but still needed a TO-220 package. I chose this as a replacementhttp://www.farnell.com/datasheets/2175827.pdf?_ga=2.17478713.1659718664.1544680552-1337621676.1538812331.I'm also going to swap out the inductor with a better, higher power one. I have a few large "transformer" style ones from Coilcraft that should work.For regulation, I'm planning on adding a linear regulator after the buck boost converter. Using an arduino, and a digital potentiometer system I'm thinking to track the buck/boost and keep it just above the linear regulator's dropout voltage, to add the smoothness of linear regulation but keep it efficient. For the linear regulator I've spent a bit of time researching what to use. There are a number of solutions out there but I think an LM723 fits the bill pretty well; by adding a pass transistor, one can achieve some pretty ridiculous power outputs. The pass transistor is gonna behttps://www.onsemi.com/pub/Collateral/2N5684-D.PDF, from an NOS heatsink set I got from eBay. They're big transistors, with really low thermal resistance junction to case (0.584 C /W!!) , so I thought they'd be a good choice for this. Either PNP or NPN, but from what I understand a PNP transistor works better since it has a lower Vcesat ?I'm hoping to make the voltage tracking all digital, using a keypad and the arduino to dial in voltages. The CC will be handled using discretes (differential op amp, current sense resistor) I think... or I may try to use digital control for that too, I'm unsure. I have parts for both, so either is an option.A lot of this is still in the planning stages, but overall, a few questions:1. Does this sound feasible? Is there anything specific I should try to watch out for, or keep in mind when designing/putting all of these parts together?2. With the LM723, it has a built in zener reference, but it's 7 volts; to keep the dropout voltage more reasonable, would it be okay to use an external reference, like a TL431 with a voltage divider to get a 1.25 vref? Or is that a bad idea? I didn't find any information on it but I'm not sure if the error amp or something wouldn't like that.3. I know that there's going to be issues with oscillations/loops. Is there a general guideline for how to approach and solve them? I've been reading up on how to design linear supplies, and found a lot of different information but wasn't sure if there are more general rules and stuff that people have learned from doing it themselves here.4. Is 10 amps asking a lot for this? I have a nasty habit of slowly increasing a project's scope until it becomes unreasonable. Would it be better to shoot for a lower current rating? Or is this more something I just have to test and try out once it's built? The 2N568x transistor's SOA shows that with a Vce of under 6, it can do it's full 50 amp rating, so I would think that it'll be okay.5. Is using a linear regulator after warranted in the first place? What benefits would it give? From my understanding it helps with noise, but only to a certain extent, and also that they are "faster", in that they respond to transients and load changes quicker, or am i mistaken?Please let me know your thoughts on this undertaking! I'm excited to try and build something like this least to say, since it'll be the first true electronics project I've taken on in quite some time.

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2018, 04:30:10 am
content: Quote from: Hiemal on December 15, 2018, 03:26:32 amI suspect the buck-boost switching regulator will be the most difficult part. If you can get more of the inductor cores used on it, then you can stack them. The input and output capacitors will likely need to be replaced for a higher ripple current rating.A more reliable and interesting (1) way might be to work out the control circuits to stack them in parallel.QuoteThe only thing preventing the 723 from generating an output to ground is that the error amplifier cannot operate to its negative supply. Unfortunately its voltage reference is referenced from the negative supply so just having a negative bias supply is not enough to fix this.I suspect it is easier to use the 723 just for its voltage reference with separate operational amplifiers for the voltage and current control loops. At that point maybe it is better to just not use the 723 unless its error amplifier fulfills an auxiliary function.QuoteKeep things simple. Do not stack too many operational amplifier stages.QuoteThat is a big transistor but a single transistor also means dissipating a lot of heat from one spot. It might be better to use two transistors even if smaller just to spread the heat out on the heat sink. Of course with a preregulator, power dissipation might be only 30 watts anyway which is no problem at all and that big 2N5684 or 2N5686 provides plenty of gain at 10 amps. The commonly used 2N3055 runs out of gain starting at 5 amps so two would likely be needed even though one could handle the power.QuoteMost linear regulators do not filter out switching noise as effectively as a passive LCR filter.(1) Interesting in the sense of catastrophic failure if you get it wrong but I think that is likely at least once anyway.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 15, 2018, 04:41:38 am
content: Im sorry but a buck boost converter has a problem it wont provide an good isolation in case of problems.But replacing this part and this part to boost the buck converter power may create more problems.Just add or build an linear power supply based on lm723, 3 legged to-220 regulators etc ... you overthink things too much, adding an linear regulator after the buck boost is bad, since you take the power from an server psu, everything is in there for power and load regulation, you may even have a very good pfc circuit in there. It is already an smps psu with ultra low noise on the output, since its a server grade psu.Why oh why adding a buck boost would help you in anyway ??You have very good and simple linear regulator circuits, you have lm338 5 amp to-3 case regulators who can be very efficient and can be adapted to an arduino. Theres some arduino projects flying around.And provide active cooling on the server psu if it doesnt have any fans, i've blown 1k server psu's because they are placed in a airflow section with tons of fans sometimes.

author: Hiemal
author_rank: Regular Contributor
timestamp: December 15, 2018, 05:00:27 am
content: Quote from: coromonadalix on December 15, 2018, 04:41:38 amAdding the buck boost converter allows for fully variable voltage, where the original server psu only has 5 and 12 volt supply; I could try modifying the server psu too, but I'd rather not mess with mains supplied voltage and end up screwing something up catastrophically.Quote from: David Hess on December 15, 2018, 04:30:10 amThe inductor cores I have from coilcraft are pretty large ones, they'rehttps://www.mouser.com/ds/2/597/agp4233-774569.pdf, the 68 uH ones... so I think they'll be okay for the current I want out of it. Unless I'm told otherwise of course.QuoteGotcha. So, probably would be best to nix the 723 and use discrete op amps, since the vref is referenced from negative. I was thinking to use the Lm723 since it's purpose built for linear supplies, but if it doesn't offer any substantial benefits that's not a problem.QuoteNo overdoing op amp stages. Got it!QuoteRight, that's what I've been told... but like I said, is there any real benefit to using a linear regulator in the first place? What purpose/benefits would it give over just using the buck/boost regulator?I appreciate the response and your advice!P.S. Also, are there any op amp specifications or specific op amps that would be better suited for this? I have some TL072's on the way here, but if there's a better, relatively inexpensive op amp, please let me know.

author: coromonadalix
author_rank: Super Contributor
timestamp: December 15, 2018, 05:11:07 am
content: Arduino controlled lab variable power suplyhttps://forum.arduino.cc/index.php?topic=66916.0

author: coromonadalix
author_rank: Super Contributor
timestamp: December 15, 2018, 05:25:04 am
content: Well the advantage is : all the job has been made by the server psu, the 12vdc will be very clean, adding an boost buck converter to go up to 30-40 vdc may add some switching noises at the output. By the way you have very powerful buck boost who dont cost too much ?My 2 cents : you're going too far to design your psu, you do know that your buck boost will consume way more current to work and give you an stable output. Does it have over voltage, over current, short circuit and thermal protectionsWhat is the server psu model ?Ex : you take 12vdc, boost it to 36 vdc, say 10 amps, it is 360 watts of power, your server psu will need to deliver 30 amps to do so .... since the server psu need to deliver 360 watts too36vdc x 10amps = 360 vs 12v at 30amps is 360 wattsYou see the problems ?? you'll have to build or modify the buck boost to handle very high currents.Finally your buck converter photo seems pretty China cheap design.Sorry I would never ever connect any sensitive electronics design / projects on your psu.

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2018, 01:50:16 pm
content: Quote from: Hiemal on December 15, 2018, 05:00:27 amThe 723 is great for linear power supplies with an output down to about 2 volts. Going lower requires machinations which make other parts and designs more desirable. (1) (3)I might still use it as a reference with its error amplifier and output stage handing some auxiliary function.QuoteAn NPN output stage will probably be easier to use and have better performance for a positive regulator.QuoteSwitching regulators can be trickier to use compared to linear regulators when the output has a wide voltage and current range. Linear regulators are more straightforward.QuoteNothing special is required as far as performance. The TL071 series is useful for high side current sense amplifiers because its input common mode range includes the positive supply. (2) In the distant past, the LM301A was used for this and it is still sometimes is. Today of course there are lots of suitable parts.For operation down to ground, a single supply amplifier like the LM358 dual is more suitable unless you want to include an auxiliary negative supply for the control circuits.(1) It would be a fun challenge to design a lab grade variable power supply using the 723. Use two with one controlling the voltage and one controlling the current from the low side. A negative bias supply allows operation down to zero volts but that still leaves fixing the reference somehow.(2) Barely maybe. The LM301A or TL051 is better. If there is a more positive bias supply, then any part can be used. If you have a preregulator maintaining a minimum voltage across the linear regulator, then that can be the positive bias supply.(3) There actuallyis a way to do itwithout a negative supply; the error amplifier is used in inverting mode instead of non-inverting mode.