My bench PSU attempt  - Page 1
Forum section: electronics / projects

author: Berni
author_rank: Super Contributor
timestamp: December 26, 2011, 09:57:16 pm
content: I had a PSU designed schematic wise for some time but now that Dave did a blog about bench PSUs i decided to finally make a working implementation of it, i had the few boards i manged to shoehorn in some spare room of a PCB order for work.This PSU had some special design features first is MCU controll so the current and voltage is set and read by a small voltage of about 0-2V then it goes down to 0V (Turned out it even goes a tad below 0V if it wants) and the last thing is something nearly no PSU does, it can sink current as well.I plan to use it as a lab supply and one whopsie that can let the magic smoke out is accidentally shorting say the 12V and 3.3V rail together. With almost all supplys the 3.3V will get pulled up to 12V while that 3.3V supply is happy thinking its running without a load.So i built up the thing and it did regulate the output voltage, but then the more i tested it the more i found out how true it is when Dave said that this thing is difficult to make stable(See photo of what i mean).So then as i was tinkering with it i kept soldering caps and resistors in various spots until it was stable, but the overcurent protection was still osculating when you show it something near a short(A few Ohm) Later the solution to it was doing it similar to Dave by a transistor to pull the voltage down since messing with the voltage regulators feedback loop was looking to be a bad idea for stability. Now the over current protection still osculates but only when you try to limit current very low. But i did find something interesting happen with it when the current limit hits very hard (Here it was the supply set to 10V 400mA and then an 1Ohm resistor put across it) It actually slams down in to 0V and then recovers, at first i tried to solve it but then after some thought i started to like it since the current limit kicks in super fast and it doesn't really take that long for it to come back on (Also if my MCU is very fast it can shut down the supply before it starts rising again)Then when i got to trying the sinking current bit things didn't go well. I tested it by connecting various power resistors between 12V and its output and in almost all cases it caused it so start oscillating about 1V around its set voltage.Thats still much better than just letting its output go to 12V but id like it to regulate like that too.So you guys have any tips on how to improve on this design?Oh and btw that -0.7V supply comes from a capacitor charge pump that has its input wired directly on the transformers output and gets regulates trough a forward biased diode, this gets around the op amp output not reaching 0V problemscope_22.png shows the output current in to a 1Ohm resistor(so 1V = 1A) when hitting the current limit hardscope_21.png shows the output voltage(Yellow) and the output of the opamp(green) when giving it a load between its output and 12V

author: Bored@Work
author_rank: Super Contributor
timestamp: December 26, 2011, 10:59:06 pm
content: Look at TI's OPA548 or Linear's LT1970 (a really crazy IC) for the power stage.And in general, give this HP app. note a look for some trusted and tried principleshttp://www.wfp76364.w1.com/HP-AN90B.pdfincluding occasional notes on stability.

author: Berni
author_rank: Super Contributor
timestamp: December 26, 2011, 11:50:09 pm
content: That chip is quite a good idea, i didn't think op amps went to these sort of currents.I asked LT for some samples of that little chip. I don't plan to use it in the final version because the package is a pain to heatsink but i can see what sort of performance these high current opamps give I imagine its near perfect apart from the output not reaching 0V problem but i can live with that if this thing really works as good as i imagine it to.

author: RJSC
author_rank: Regular Contributor
timestamp: December 27, 2011, 12:53:43 am
content: If the problem is that you think there is not enough current capacity to drive the pass transistor accordingly, try to use an n-channel mosfet, they only require some gate current when changing the voltage (the gate acts as a small capacitor).I'm experimenting just with the voltage regulator by now, but it is surprisingly stable with capacitive loads, motors, etc.It also features an extremely low dropout. It will require a charge pump to supply the op-amps driving the mosfet.The power dissipation will be minimum since I will be using a tracking pre-regulator (i' thinking of a ready made adjustable laptop power supply) of a regular transformer with a circuit that only charges the filter capacitor up to 2V or 3V above the output.I'm using an IRFZ44 mosfet now ( TO-220, 100W max power dissipation), and an OP07 precision op-amp, which is availaiable locally at 0.99€ each, but I might need to change it since it is only capable of 30V on the power rails, and the outputs only go to 3V below the rails.I'd like the PSU to go up to 30V at least, so, the op-amp would have to handle at least 41V on its rails (30V output + 5V Mosfet Vt + 3V positive rail drop + 3V negative rail drop).Does any one know of readily available op-amps that can have 60V on their rails such as those rare OPA548?I could have the op-amp supply floating, hanging on the output like many commercial power supplies, but that would require an additional transformer.

author: caroper
author_rank: Regular Contributor
timestamp: December 27, 2011, 09:19:17 am
content: Quote from: BoredAtWork on December 26, 2011, 10:59:06 pmThanks,I have been searching for something like this.I look forward to reading it in detail.CheersChris

author: Berni
author_rank: Super Contributor
timestamp: December 27, 2011, 09:33:13 am
content: Yes that opamp has a pretty wimpy output current of about 5mA when sourcing i think, but then again that darlington only needs 0.5mA to conduct 2A over it. I also meshed its dropout voltage and its about 2.8V at 3A so im pretty happy with that as i expected much worse with the opamp not really being rail to rail output.I also found this regulator pretty much stable as long as you take current from it and since i have some 300V DC to 24V switch mode modules with a adjust pin im planing on making it tracking aswell and i might up the current rating above 2A too if it turns out to work well.One of the explanations i came up with by poking around the cirucit is that it takes a lot of voltage swing to get the bottom transistor to turn on as the base voltage of each darlington is 1.4V that means when its in normal operation the opamps output is 1.4V above the supply but when it suddenly needs to sink the output of the opamp has to fall all the way down to the output and then additionally fall 1.4V below the output to push current in to the bottom transistor and turn that on. One idea i came up with but not yet tested is to have a diode develop that needed 2.4V drop over a pulldown resistor meaning the opamp doesn't have to swing, alto that might push the tiny output current capacity of the opamp a bit too much.Or i might try getting a higher slew rate opamp (But that might be stability issues again).If you need a high voltage opamp you could try the OPA454 it goes up to 100V, but might be tough to find locally, also TI has a high voltage category for opamps, meaby some of these will suit youhttp://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=analog&familyId=1482&uiTemplateId=NODE_STRY_PGE_TIm personally trying to stay away from mosfets as without a charge pump im going to get a huge dropout voltage out of it.When operating in linear mode there is not that much of a advantage to use one anyway. Im also personally not going for this much voltage because i plan to have 4 of these in one case that are isolated so i can series them in case i need more voltage and not many railsAlso yeah i find that appnote interesting reading.

author: PhilipGilmore
author_rank: Newbie
timestamp: January 01, 2012, 12:51:13 pm
content: Quote from: RJSC on December 27, 2011, 12:53:43 amThere is another way. The op-amp doesn't need to drive the MOSFET gate directly.I am designing a power supply myself, and found a way that lets me use 5V components in the regulation circuit (and i want 30V or more output). Just use a bipolar NPN (or maybe N-Channel MOSFET) to pull the regulating MOSFET's gate low, and pull it high with a resistor. You'll have to invert the output op-amp logic, because op-amp high means low output and vice versa, but that's not a problem.You still need the charge pump to get the gate above output.

author: gxti
author_rank: Frequent Contributor
timestamp: January 01, 2012, 05:53:31 pm
content: Here's a simple 70 volt regulator using high-side pMOS, no charge pump required:http://mackys.livejournal.com/980741.html