What is the proper way to design a dual output ( ~14V, 5V ) supply circuit? - Page 1
Forum section: electronics / beginners

author: MS | ECE
author_rank: Newbie
timestamp: February 11, 2012, 03:32:16 am
content: Hi EEVblog,I thought since Dave is doing a series of videos on power supply design there would be a good amount of information rolling around these forums. I tried getting help on Adafruit forms without much success and I'm stuck at the moment. EEVblog to the rescue?Objective: Design linear dual output power supply. Specifically, it should have 3 terminals, 14V, 5V, and Ground. It should be rated for at least 2 amps.The reason I need 14V is that I already designed a board of LED's to be driven on 12V through MOSFETs and need to account for the voltage drop across drain / sink, so that it is actually 12V at the LED's. If this fails I'll just go convert an ATX power supply and deal with it, but I thought it would be cool to learn to design my own power supplies for my projects so they can be integrated and look more professional. ( Also, I'm definitely looking forward to building a PSU like Daves whenever he finishes it, maybe he'll partner with some site and set kits )Problem:I've already built up the circuit on a breadboard and my initial design ( the top circuit in the eagle schematic attached below ) appeared to work so I built up a board and soldered everything and for some reason the 14V rail using the LM317 no longer works. If it turn it on it seems to hold voltage for about 20 seconds and then slowly drops until there is about 2V or so. This is my first PSU build so I don't have much experience in troubleshooting but it looked like the regular's internal thermal protection circuitry was kicking in to drop the voltage, but I have both of the regulators fitted with double heatsinks and they didn't seem too hot so I didn't know what to do. Since I invested all that time and it didn't work I decided I need to make sure my circuit is correct before I go etch another board and spend more money on parts.EagleCAD Schematic explantaion:I have some notes in there to help. Again, the top design was the one I breadboarded and produced on PCB as you can see in the other pictures and after realizing problems with this circuit I began looking online more and saw some other designs which suggested the regulators should go in series instead of parallel. I originally chose them to be in parallel so that the LM7805 would only be dissipating 8V off the center tap which is more efficient than 14V from the output of the LM317 if it were in series. However, by tying the grounds together it seems that the voltage is shared across the first two filter caps ( 2200uF ) so instad of the top one having full 16V and the bottom having 8V I think they both have about 20 V for some reason. Which is causing the 805 to run hot but shouldn't effect the LM317.Anyways, in I basically just wanted someone to tell me the proper way to design a dual output power supply with one transformer and if they're feeling extra generous maybe give some insight as to what problems I may have with regulators in parallel, and suggest proper capacitor values.If I missed any background information you need please ask.Ok, thanks EEVblog.- Matt

author: Rerouter
author_rank: Super Contributor
timestamp: February 11, 2012, 05:12:51 am
content: certain your transformer is rated for the wattage?

author: mariush
author_rank: Super Contributor
timestamp: February 11, 2012, 05:22:56 am
content: I would probably keep it simple and get 2x LT1083... it does 7.5A max..http://cds.linear.com/docs/Datasheet/108345fg.pdf1.3v dropout voltage at 7.5A load, much less at lower A guarantees you'll get 14v out of that 16v transformer.Connect the second LT1083 at the output of the first one. 5v @ 2A as you require should be relatively easy to heatsink.Of course, why linear when you can go with switching... Something like AP1506 springs to mind, it even has basic circuits for 5v and adjustable versions in the datasheet:http://www.diodes.com/datasheets/AP1506.pdfps. there's also solutions like the Tinyswitch chips, for example TNY380... they're used in phone chargers with success:http://www.powerint.com/sites/default/files/product-docs/tnypk_family_datasheet.pdfDo a 14v @ 2-2.5A (35 watts @ 85-230v, 45w @ 230v) and then go from there with your 5v...

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 06:43:10 am
content: I made a simulation of the rectifier and filtering part of your circuit to show what it does. As you can see it doesn't quite do what you hope. Both capacitors get charged up to the full voltage of the transformer.With a center tapped transformer the two windings are connected and not independent. Therefore you can't try to put two separate and independent rectifier circuits on them like this. There is a way you can get two stacked voltages, but it needs a different arrangement of diodes and capacitors. Maybe you can think about how to rearrange a the circuit so it works?It's not unusual for a design to fail to work first time round. It becomes a case of "back to the drawing board" -- or breadboard in this case.

author: MS | ECE
author_rank: Newbie
timestamp: February 11, 2012, 06:55:23 am
content: Quote from: Rerouter on February 11, 2012, 05:12:51 amYes, I'm certain the transformer will work for that amperage. I had the breadboard circuit working stably and ran it for about 10 mins and nothing was too hot. The sticker on it says rated for 3.5 A and I will be way under that.Quote from: mariush on February 11, 2012, 05:22:56 amOk, I will look in to those. I thought LM78XX and LM317's were the most basic of the regulators. I'm a little shaky on op-amps so sometimes I can't follow Dave's psu tutorials as well.1.3v dropout voltage at 7.5A load, much less at lower A guarantees you'll get 14v out of that 16v transformer.Connect the second LT1083 at the output of the first one. 5v @ 2A as you require should be relatively easy to heatsink.Quote from: mariush on February 11, 2012, 05:22:56 amThe datasheet you linked looked like all of those chips are accepting DC input which confused me. I thought switch-mode power supplies get rid of the transformer and rectifier and take the mains AC signal directly and step it down by controlling pulses so that the RMS on the output was at the desired level and appears DC.Quote from: mariush on February 11, 2012, 05:22:56 amThe circuit for the Tinyswitch looks rather complicated and still requires a transformer so I'll need to do some more research about SM power supplies I guess.I would like to learn SM power supplies since they're more efficient but I'm not sure I'm at that level yet.Quote from: IanB on February 11, 2012, 06:43:10 amYes, I didn't know how to explain it though. I think it has something to do with the voltage across two capacitors in series has to be equal because whichever capacitor has more charge will flow down to the other. When i was first designing the circuit I actually kept the grounds separate all the way through so both rectifiers had separate negative paths as well as each regulator and i believe this eliminated the capacitor issue but it's not practical because then I had two "grounds" at different potentials and couldn't connect them to a circuit unless I tied them together re-introducing the problem.Quote from: IanB on February 11, 2012, 06:43:10 amYea, I just always get discouraged when I mess up because I feel like I wasted a bunch of time accomplishing nothing. That simulation is really cool though. I've wanted to learn how to do that so I didn't have to make mistakes on the breadboard and not be able to see what's happening. I did some Agilent Pspice stuff in school but haven't done any since. I heard there is some program called LTspice that is free so maybe I can do my next attempt on that first.Hopefully tomorrow I can at least make a schematic of the 2x LT1083 as you suggested and hopefully get that reviewed.I really appreciate the help.- Matt

author: amspire
author_rank: Super Contributor
timestamp: February 11, 2012, 06:59:30 am
content: The rectifier arrangement is not right, but it would actually probably work in a wierd sort of way.What you want to do is remove the middle rectifier totally, and use the transformer center tap directly as the supply for the 5V supply. So the transformer center tap goes straight to the IN pin of IC1. The existing rectifier will only have one of the four diodes conducting, and will be acting as a half wave rectifier with double the output that you want.When you have a full wave rectified on a center tapped transformer, the center tap is at a DC level half way between the other two windings. That is what you want.The rectifiers for the two 14V supplies are correct.The 5V regulator circuit for the second supply is a bit weird. Is that a work in progress?Don't forget to add reversed biased power diodes across each supply output - they are very important, particularly if you are going to connect the two supplies together to make a +/- supply.Richard

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 07:06:52 am
content: Quote from: MS | ECE on February 11, 2012, 06:55:23 amYes, I used LTspice for that simulation. It comes with a standard component library largely consisting of Linear Technology devices, but it is very useful nonetheless.

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 07:21:15 am
content: Quote from: amspire on February 11, 2012, 06:59:30 amHere is the circuit suggested by Richard. It looks "odd", but it works

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 11, 2012, 07:47:39 am
content: Well, that didn't turn out too bad. Based on what Richard and Ian said it sounds like you can salvage your current board simply by de-soldering the bridge rectifier for the 5V and adding a few jumpers. Not sure how easy it will be to add the power diodes as Richard suggested, however.

author: MS | ECE
author_rank: Newbie
timestamp: February 11, 2012, 10:35:10 pm
content: Ok, so I downloaded LTspice and decided to simulate some LT1083's per suggestion of mariush. However, the first LT1083 in the series which is intended to output the ~14V is not very stable and I'm not sure why. I could probably fix it by just raising capacitor values but I would like to know the actual reason.I have attached the LT spice simulations with the circuit I think mariush was suggesting. Please let me know if I should change something. The first simulation was with standard 10uF caps as suggested in the datasheet which has bad ripple and my altered version with 100uF caps showing marginal improvements.---Also, I could not find the equation to calculate the resistor values on the adjustment pin. Does anyone know this equation? I assume it is similar to the LM317. When designing the circuit I just narrowed it down by trial and error in LT spice with the 10 / 1000 Ohm to get roughly 13V.Lastly, does anyone else reading this know more information about the workings / advantages of Switch Mode powersupplies. I would be willing to work with the chips Marish suggested if they are truly worth the time investment. I'm just not familiar with switch mode circuits yet. Although when I was downloading LTspice I saw they have LTpowerCAD which looks like it helps design them. Perhaps IanB is familiar with this process since he uses LTspice.One more thing, is there an easier way to embed attached images in the post than creating the post then copying urls and modifying? Maybe put them inline during post creation?- Matt

author: IanB
author_rank: Super Contributor
timestamp: February 11, 2012, 11:08:08 pm
content: Quote from: MS | ECE on February 11, 2012, 10:35:10 pmActually it is quite stableIf it were unstable you would be seeing a lot of high frequency noise messing up your tracesQuoteWhat is happening is your filter capacitor after the rectifier is far too small. You need a value something like 5000 uF or even 10,000 uF, depending on how much current you intend to draw.The 10 uF in the datasheet is not a recommendation for smoothing, it is a recommendation for stability of the IC. The IC won't produce a regulated output at the voltage you desire unless you give it an adequately smoothed DC supply voltage.

author: Bloch
author_rank: Unknown
timestamp: February 11, 2012, 11:27:08 pm
content: Quote from: MS | ECE on February 11, 2012, 10:35:10 pmhttp://www.linear.com/product/LT1083>http://cds.linear.com/docs/Datasheet/108345fg.pdf> page 11As a starter you can use for C2 = 1000uF per 1 Amp.

author: mariush
author_rank: Super Contributor
timestamp: February 12, 2012, 12:21:26 am
content: Leave the 10-22uF capacitor at input and output and add a second capacitor at output, 1000uF to 3300uF would be good. And please read the datasheets, as it explains there on page 11:QuoteSo at input, don't mess with the decoupling capacitor value. At the output, you can knock yourself out.At the same page 11, the formula is on the right side:The equation is Vout = 1.25 ( 1 + R2/R1) + IadjR2But since Iadj is generally very small, about 50uA, the IadjR2 is usually ignored. And, as it says about R1 should be as small as possible, preferred to be 100-120 ohm or less.Your Vout was 1.25 ( 1 + 500/5) = 1.25 * 101 = 126.25v? That may explain the first regulator's behavior.The benefit of that switching power chip is that it generates much less heat so you save on heatsinks and your device becomes smaller. The transformer is also much smaller - maybe not cheaper (for 1-2 pieces, not talking volume) as it needs several auxiliary windings but it's won't be as heavy as a transformer needed for linear voltage regulators. Picture for example a computer power supply - they need to provide 20-40A yet they don't have 20 Kg transformers inside.For your needs, to obtain 5v @ 2A from 12-14v, you'd need a pretty good heatsink, maybe you'll even need a fan to move some air - after all the difference is dissipated as heat.As for learning about switching, I'd suggest a good start here, the Linear applications notes:http://www.linear.com/designtools/app_notes.phpIn particular: AN25, AN29, AN30, AN35 are all about switching regulators and should all be a very good read.The chip I suggested may not be the perfect one, but I picked it after checking the datashet and seeing it explains and shows some typical circuits (so it would help you understand how it works)

author: IanB
author_rank: Super Contributor
timestamp: February 12, 2012, 03:01:10 am
content: Hang on, don't confuse the poor guy. In the circuit above the regulator is the LT1086 low dropout linear regulator. Where does switching come in to it?The OP needs to provide the regulator with a smoothed input voltage with a minimum value that exceeds the dropout voltage of the regulator for the conditions demanded. That means placing sufficiently large filter capacitors between the rectifier and the regulator.

author: mariush
author_rank: Super Contributor
timestamp: February 12, 2012, 03:55:32 am
content: The part about switching comes from his last paragraph: "Lastly, does anyone else reading this know more information about the workings / advantages of Switch Mode powersupplies."I'm sure he's smart enough to understand and he knows LT1083is linear (I mentioned LT1083, he chose to test with LT1086).Another thing that I remembered ... he said the transformer was 16v but I'm not sure he meant without anything connected to it or under load and I'm not sure if that's rectified or not? Because under load, the output voltage would drop.So, if 16v was the output already rectified and keeping in mind these linear regulators have a voltage drop of 1v at low loads to 1.5v at high loads, it may be possible that 14v output won't work.

author: IanB
author_rank: Super Contributor
timestamp: February 12, 2012, 05:02:26 am
content: Here's an example simulation giving 14 V output from a nominal 16 V transformer:Note how the input voltage (green trace) must remain at least 1 V above the output voltage to keep the LT1086 in regulation. This is what happens if the filter capacitor is too small: