Design and construction of a power supply - Page 1
Forum section: electronics / beginners

author: alxnik
author_rank: Regular Contributor
timestamp: October 26, 2013, 09:58:18 pm
content: For some time now, I'm gathering information and knowledge on building a linear power supply. This is a learning experience as much as a need for a power supply, so I decided to not go the easiest possible way in order to learn as much as I am comfortable with at this point.Now I am at the point of I might need some help from you guys. Although specific questions follow in the end, I'd like some sanity check on the whole thing too.So here is the deal. Initial specs vary according to my mood but in general it is as follows:1. Descrete design. I would learn nothing if I just used some linear regulators2. Pass element must be a Mosfet, mainly because I have lots of them in stock and I want to learn more about them3. Has to go down to 0V or pretty close to that4. Max output should be close to 10A, altough in practice it will go as far as I am comfortable of not setting my house on fire. Probably 5-6A? I guess it will depend on the out voltage too.5. Try to make it as good as it can be without pissing away money on expensive components.6. Initial power will come from 2 mains transformers ripped from UPSes wired in series. These are center tapped 16V each so I have ~40V of rectified voltage to start with along with many taps to lower the voltage according to the output voltage. As the transformers are the most expensive part of the circuit, and I already have them, I'm not willing to buy new ones just for this.7. Given 6, I'd like to have ~33-35V max voltageI decided to start with a ready made schematic since I am not experienced enough to build my own from scratch, and mod it according to the route I want to follow.The initial circuit comes fromhttp://www.electronics-lab.com/projects/power/001/index.html(schematic attached), which someone else suggested in another thread here about power supplies.Now on to the questions:1. R7 is the shunt resistor. I'd love to have decent accuracy without spending too much money on an ultra low TCR resistor. So how low can I go without going to exotically low TCR? I found some 10W resistors for less than 2 euros each with 100ppm TCR, which I feel is on the high side for this application. Would it help that I have already planned for 2 80cm fans from a computer psu to cool the whole thing down?2. The design has the opamp drive a darlington configuration. Given that I want to drive a mosfet, what problems should I expect and how can I rectify them? I am suspecting some instability due to the capacitance of the mosfet, although I would like to know more on this. In the past I saw some configurations to fix instability on electronic loads in the forum. Would these work here too?2. The design calls for TL081 opamps. I would like to upgrade these to increase stability and response time, however I have no idea on what to look on the specsheet. I suspect slew rate is an obvious one, but what are the traps for the young player?Is the TL081 already the best option for this?3. The design calls for 3 descrete opamps. Would it increase noise if I used an IC with 4 opamps?4. Realistically what output ripple should I expect? I breadboarded the thing (without the current limiter) and I get 160mVpp, which is kinda appalling. Understandably it's on a breadboard and probably I need much more fine tuning to get it down, but how low can I go on jelly bean parts and not much experience?5. I will upgrade the filter cap to 10000uF to be able to get the output closer to the input voltage. Is there some capactiance after which it would be pointless to add more?

author: peteroakes
author_rank: Contributor
timestamp: October 27, 2013, 05:14:35 am
content: not going to answer all the questions but a few.getting zero volts out should not be an issue as the circuit includes a charge pump to provide a negative rail for the amps, this will enable most op amps to output down to zero or less voltsif you must use a FET though, the output stage will need significant re-design, you should look to find a ready available circuit already designed to use one, I think Dave has a video or two on this already, go have a watch10 Amps is a lot of current, even the input rectifiers will be getting very hot at full output, 600mV at 10 amps = 6W and that's just one diode in the bridge so it will also need a heat sink1. Shunt resistor: you don't need a super accurate one, even 5% will do, but you do need it to be stable, capable of handling the power and the ability to measure it accurately (0.1ohm 10W or 0.05ohms 5W kind of range), if you know its resistance and know it is stable then your good, you can adjust the gain of op amps to compensate3. look at OPA2277, there low offset and pretty fast for your needs, should not be too expensive5. be careful not to up the output capacitors too high, it will negate any current limiting you may put in place, the caps after rectification should be high but not the final output ones (After the regulator), ripple can be caused by several factors but common causes are insufficient capacity of the transformer, insufficient pre regulator caps, high internal resistance in the caps (Cheap Chineese onces for instance) or operating too close to the rectified and smoothed input volts to the regulator, causing sag in the supply output (the ripple will show at 50 or 60 Hz depending on where you live

author: Thor-Arne
author_rank: Unknown
timestamp: October 27, 2013, 07:54:41 am
content: You should also check outthis link, it's a followup on that design.

author: alxnik
author_rank: Regular Contributor
timestamp: October 27, 2013, 08:43:49 am
content: Quote from: peteroakes on October 27, 2013, 05:14:35 amIndeed, already achieved 0 voltsQuote10A is the ideal target. I already have a bridge rated for 10A and given the heatsink and airflow I have in place, I was able to comfortably push it to 8A which is good enough. (just the rectifier not the whole psu...)QuoteNot afraid of accuracy of the resistor but the temperature coefficient. In my trials with an electronic load I found that with run of the mill resistors, I could variate the output current wildly by moving a fan over the resistors, and that's not good.QuoteFirst of all, I was mistaken in my original post, it's not ripple as in an oscillation but more noise like. The transformers are 100VA each and I always monitor the input/output through the oscilloscope so the problem is not on the input stage. I think it has to do either 1. with the way it's built (breadboarded), 2. with the way I measure 3. with the opamp.@Thor-ArneCool! It seems that this design is much more popular than I thought

author: mariush
author_rank: Super Contributor
timestamp: October 27, 2013, 10:05:24 am
content: You may want to look into hall effect sensors for currents over... let's say 3 amps.Maybe something like this:http://www.digikey.com/product-detail/en/ACS713ELCTR-20A-T/620-1192-1-ND/1284609It's only 1.5% accurate but that may be good enough at 3A+ ... I think it's an error of +/- 10mA or something like that. Use a resistor for under 3a, short it out when going over and switch over to the hall effect sensor output. This way you won't need a high wattage resistor that would drift as it heats up and so on...And I'd suggest using a bridge rectifier rated for much more than the planned current.. don't use a 10A rectifier if you plan to use up to 8A... use a 15-20a rectifier, with a heatsink.

author: Thor-Arne
author_rank: Unknown
timestamp: October 27, 2013, 10:10:26 am
content: Quote from: alxnik on October 27, 2013, 08:43:49 amA lot of people having issues with the design at least.I've had some issues with it to, but I put it aside for now as I found a couple of more interesting designs.These are work in progress as of now, but they look promising.Amspire's general psu designandivan747's psubased on it.I've also played around with this a bit, but I've been too busy with work to get anywhere with it.And there isGerry Sweeny'smodular psu which looks very promising.

author: alxnik
author_rank: Regular Contributor
timestamp: October 27, 2013, 10:27:51 am
content: Quote from: Thor-Arne on October 27, 2013, 10:10:26 amNow why did you do that? I was happy in my own little world, with my little design that I analysed and was able to understand. Now I have to start all over again!On the serious side, there are probably better designs but I would love to first understand its limitations before moving on. Nevertheless, thanks, this is invaluable information.Quote from: mariush on October 27, 2013, 10:05:24 amFair enough. But why? I already have 2 honywell sensors which are totally non-invasive, and would probably fit the bill, but they are bipolar so 0A is at 2.5V (0-5V full range) so I will have to add some more circuitry to have proper feedback. But at the price these things cost, I might be able to find a 10W 0.05ohm resistor with low temperature coefficient so are there any benefits after all?

author: Thor-Arne
author_rank: Unknown
timestamp: October 27, 2013, 10:40:30 am
content: Quote from: alxnik on October 27, 2013, 10:27:51 amSorry...Well, I was happy in my little own world to... I failed you might say...Anyway, the thread with Amspire's design is full of useful information.I learned more from that single thread than everything else put together.

author: ThomasDK
author_rank: Regular Contributor
timestamp: October 27, 2013, 02:21:46 pm
content: Quote from: alxnik on October 26, 2013, 09:58:18 pmOne trap: The supply voltage for the opamps. They are powered by the full output of your rectifier, + the charge pump voltage.If your transformers are 32V center tapped, your peak voltage will be 32*sqrt(2) = 45.3V. Your mains voltage can vary +/- 10%, so on a bad day you might get 45.3 * 1.1 = 49.8V. Bye bye magic smoke

author: Thor-Arne
author_rank: Unknown
timestamp: October 27, 2013, 02:40:05 pm
content: Quote from: ThomasDK on October 27, 2013, 02:21:46 pmOne of the schematics describe using the TLE2141 which is good for up to 44V.I had some issues with this as my peak voltage got above 45V at one point with no load on the output.However the opamps got very hot but they survived.

author: alxnik
author_rank: Regular Contributor
timestamp: October 27, 2013, 03:10:16 pm
content: Quote from: Thor-Arne on October 27, 2013, 02:40:05 pmI have thought about this. Right now I am experimenting with only one of the 2 transformers so it's no issue yet. However eventually I will add some protection/voltage regulation of the supply of the opamps. In the grant scheme of things, the output opamp doesn't need to have this supply voltage. The mosfet needs at most 10V out in order to be fully on, so if I play around with the feedback circuitry, I might be able to get away with much lower supply voltages for the opampEdit: This was kind of a brain fart. The Vgs must be 5V for a logic level mosfet, which means that in order to be fully on, it must be 5V over the output voltage. So probably I will need some kind of driving circuit and not drive the circuit straight from the opamp

author: Thor-Arne
author_rank: Unknown
timestamp: October 27, 2013, 03:22:58 pm
content: Yes, that's true.Also, you can see on the v.2 schematic that he uses +33V/-3V to supply the opamps.In that design it limits the maximum output of the psu, but it also allow for a bigger "head-room" for the pass transistors to regulate.I think this will make the ripple smaller when the psu is at max output?When it comes to using mosfets in psu designs i have no clue, haven't tried that yet.If I remember correctly there was a discussion about pros/cons between npn/pnp/mosfet in one of the threads I mentioned.

author: ThomasDK
author_rank: Regular Contributor
timestamp: October 27, 2013, 10:37:53 pm
content: Quote from: Thor-Arne on October 27, 2013, 03:22:58 pmIt's certainly betterThe maximum is limited by the simplicity of the design.A 44V Rail to Rail out opamp (I'm sure they are out there) supplied with +43V/-1V could get you near 40V out. Not bad