Switch Mode Power Supply Repair, SMPS

hi there and welcome to mr. Carlson's lab today we're going to diagnose and repair a switch-mode power supply so let's get started before I get started repairing the switch mode power supply I've had many people ask in the past what's the difference between a linear power supply and a switched-mode power supply and why does the linear power supply have a such a large transformer inside it whereas the switch mode power supplies got such a small transformer yet they provide roughly the same amount of current and I'll attempt to explain it here on the whiteboard so this is a simplified version of a switch mode power supply and we'll say that it's running at 100 kilohertz this is where the line plugs in we have a switch and a fuse and that runs into a bridge rectifier so we get direct current here so basically it's just taking the line and changing that to DC we have a filter across that which is sometimes classified as a bulk capacitor all right the positive goes into the switching transformer here on the bottom half we have a switching transistor and the emitter is connected through a sense resistor back up to the negative half of the bridge rectifier now since this is running out 100 kilohertz here this IC right here is telling this transistor to turn on and off at 100,000 cycles per second or 100 kilohertz now the reason that these transformers are so small is because as the operating frequency goes up the core size goes down so because over here in North America our mains are line frequency of 60 cycles per second we've got some pretty large transformers alright now in a linear power supply because they have to operate at that frequency because this is operating at 100 kilohertz we have a much much smaller core size the higher the frequency goes the smaller the core size gets alright so that's why switch mode power supplies in such a small light package you get a lot of current but there are some downsides to this as well and I'll get into that here in just a moment so what's happening is this IC is telling this transistor to turn on and off at 100 kilohertz here this is a sense resistor so basically what's going to happen here is if you pull too much current on this side the secondary side of the transformer this is going to tell this IC right here because of the voltage rise across this resistor to either fold back or to go into current limit depending on the circuit and how it's hooked up all right so really what that's doing it's just protecting the circuit so that you know things don't you know you don't overload components in there the secondary side here is what supplies the voltage that you're using say 5 volts or something like that we also have a diode and a filter capacitor here you'll notice that the filter capacitor values are much much smaller than in linear power supplies and that's because again they're running at a different frequency all right we have an opto isolator and a voltage regulation network in here that I haven't drawn I've just kind of drawn that as a block all right and what that does is that isolates the secondary side from the primary side and the reason that we want that is because this whole primary side is really referenced to the main the mains voltage here and if this was somehow connected to the secondary side that would pose a risk of shock all right so we have an opto isolator which is basically passing the information from the secondary side back to this IC here through a feedback and that's telling this thing how to regulate that's really how simple this is so really what an opto isolator is really is if you wanted to simplify it it'd be like an LED and a photo resistor in a sealed block all right so the brighter the LED gets all right that changes the resistor in the photo resistor basically and gives this I see just a little bit different information and tells it to correct on that side all right that's really how a switch mode power supply works now this I see here is not going to be running at you know rectified mains voltage so we have a resistor and a Zener diode to drop that and the Zener diode just acts as voltage regulation to keep the voltage low so that we have no damage to the IC so this would be the positive supply and the negative would just connect to the negative side of the rectifier there that's really how simple these things are so since this again is running at 100 kilohertz that's a much much higher frequency and of course this is getting turned on and off rather abruptly all right in that creates a lot of noise and that's the downside to switch mode power supplies they need a lot of filtering to get rid of that noise now if your ham radio operator or you're using a power supply like this in a really sensitive environment most receivers you know receive down into the micro volts right this thing here is going to pick up noise you know and harmonic and all sorts of you know content from that 100 kilohertz switching frequency and that's why linear power supplies are preferred for radios and stuff like that it really takes quite elaborate filtering to get all of that noise out especially when you have a hook to receiver that's that incredibly sensitive so next we'll take a look at a linear power supply this is a simplified linear power supply so we have a line coming in here we have a switch and a fuse and that runs directly into the primary side of the transformer if this was a 50 amp power supply that would be that really big transformer inside there this is the secondary side we have a bridge rectifier here so we have AC to DC conversion we have direct current on this side here this is the filter capacitor usually a relatively large filter can for something that's about 50 amps pretty big can size capacitor in there we'll have a whole bunch of pass elements I've only drawn one in here but for a 50 amp power supply usually see a bunch of heat sinks on the sides of the power supply or inside with a bunch of these pass elements on them so I've only drawn one here and a pretty standard IC that controls these is in LM 723 alright that's one of the most common ones there is and really what's happening here is we have the positive voltage from the bridge rectifier on the collector of the transistor here this transistor is just acting like a big rheostat like a big variable resistor alright so what happens is when you draw current on the output here this LM 723 just turns a transistor on a little bit harder and basically turns that rheostat up a little bit now since all these transistors here are running in the linear region they're acting as variable resistors they get pretty hot and again that's the reason for those relatively large heat sinks that you see on linear power supplies very simple this is the sense resistor here and it just basically tells that LM 7:23 whether to you know crank out the current er not really his health it's doing that's how simple a linear power supply is now linear power supplies have their advantages they're very very quiet all right switch mode power supplies make a lot of noise because you know the switching is that's going on inside them so linear power supplies are preferred by ham radio operators and in quiet environments radios and receivers they peer down into the microvolt region and linear power supplies are relatively dead silent all right whereas if you have a switch mode power supply you have an oscillator in there that's oscillating at state for example 100 kilohertz we have a square wave getting fed into that transistor or FET or whatever and that creates lots of harmonic energy and you need a lot of filtering to really filter noise out of a switch mode power supply whereas in a linear power supply it really is almost as simple as this it's not much more to it than this here whatsoever so they're you know relatively easy to design and put together it's just that you know the downside is you need that really really heavy big transformer and they're a big block of iron whereas the upside to a switch mode power supplies you get a lot of current out of a relatively small package here's our switch mode power supply candidate for today this was at the bottom of the box I got a Tektronix oscilloscope somebody pulled it completely apart put all the board's in there this was laying at the bottom as well as these plug into there and this was also laying in the bottom of the box which appears to plug in over here like so so somebody had taken this thing all apart already these two pins here on each side are pretty short I don't know whether the person broke these off or whether you know these are just cut short for size constraint maybe with the width the power supply is installed and in the scope I really don't know so you can see all the capacitors here still silicone to the back and you can see that they're all physically leaking pretty bad you see that around the pins here they're leaking out all of their electrolyte so these are all bad these all have to go these here are the bulk capacitors and these rarely fail so since this is my power supply and it's just going to sit on a shelf chances are I'm just going to reinstall these I'll test them to make sure that they're okay I'm pretty positive that they are though and I'll just use these again if I was going to be doing this for somebody else all of these would get replaced hands-down but again you know this thing is just going to sit on a shelf and white order up and replace these expensive capacitors just to have them set so I'll do that this is a single sided board so it's relatively easy to damage and damage has happened on the bottom side here this solder is a really dull gray color and you know originally I'm thinking well maybe some of that you know electrolyte got down here and you know caused some issues with the soldering but I really doubt that it looks more like somebody at some point in time has used acid core solder on here you can see it's taking the coating off the circuit board and it's bare in some areas here so I'll have to be careful when I'm soldiering this area don't want to ruin the tip on my iron another thing I noticed on this side here is there's a a chunk of trace missing right here so it looks like it's been actually pulled off you can see it's got a cut over here I really don't know what's going on there so I'll have to figure all that out so already this power supply is looking like a challenge and I'm up for the challenge so let's bring this old power supply back to life the first thing I want to do is check the transistor here I'm pretty sure it's okay usually when these things go parts in this area it'll go bad you see chunks of resistors missing and things like that and it looks pretty solid in here so I'm pretty sure that this transistor is fine so what I'll do is I'll do solder one pin on the bottom here and we'll test the transistor out make sure that's okay and if that's okay then chances are this hybrid is okay as a hybrid in here that controls this transistor so if this transistor goes bad usually it takes these guys with them and then of course if it did that we would have to D soldier this hybrid and carve the top coating off and remove the components from the hybrid and replace them so that's what I'm going to do next I'll just heat up my soldering tool or desoldering tool and all D solder some of the legs and I'll be right back now what I'm going to do is just D solder this pin of the transistor here I've already added some fresh solder I wasn't going to do this on camera but I figure I'll show you what I'm doing here so what I'll do is I'll just desalter this pin will pull the solder off and this is the base this would be the collector and this would be the emitter this is an NPN transistor on here so not using a FET in this supply so what I'll do is I'll just pull the solder off of here really gently alright it's looking pretty good a little bit stuck on the side there still all right and what I'll do now is just test it to make sure the transistor is okay I'm pretty sure it is usually there's pretty extensive damage if this is gone so since this is an NPN transistor we want to put the positive of our diode checker on here I've got it on beep so it'll beep when I'm testing a diode here so again NPN transistors basically test us two diodes so we'll PNP s but NPN s here you put the positive lead it's pretty easy to remember if you're testing an NPN transistor with your diode checker and P and they put the positive lead on the base or PNP put the negative lead on the base so this is an NPN so the positive lead goes to the base here and should get a beep no problems and test the emitter here no problems so transistor is absolutely fine no problems of this and that tells me that chances are this hybrid here is going to be ok so usually when bad things happen they blow holes in these hybrids and you know resistors are burnt out of this thing and it gets pretty ugly at that point but it looks all-in-all like this thing is in relatively good condition looks a dark down here on the hybrid hopefully that's nothing nothing too bad so if the hybrid did go bad I basically just have to take the coating off of it and replace the components inside the hybrid that work faulty so hopefully it's not going to go that far maybe this single just require caps and a cleanup and fixing some traces and this thing I'll go so we'll see how see how this goes I'm ready to test the vault capacitors here so there are 250 volts 680 micro farad they're hooked up like that I'll just press GO and we're at 616 mics it's good enough point one three nine for the ESR absolutely fine so I'll just switch to the other cap here so cut it on the other side now same cap 688 250 634 0.14 absolutely fine no problems again if this was somebody else's power supply these things would get replaced in a heartbeat but for this test is absolutely fine in fact these things will probably last for a very long time yet okay I'm ready to put these ball caps back in they still got the silicone on them they're going to only go on one way here this is positive marked here and positive marked over here so definitely want to be sure when you put capacitors in like this that just fits right in that they're put in the right way if you put them in the wrong way bad things happen really bad things so I'll grab my iron here and just flood this kind of doing this around the tripod so mixing a little bit difficult nice big pads and they're back in so next I'm going to replace all of these right here I've poked all the capacitors through and oddly enough on this side here they haven't marked the polarity so it was nice that these are all stuck to this goop so I could get all the polarity correct not it would take quite a bit of tracing to figure out what goes where now they're nice enough to put the you know the voltages and stuff over here so and the pins so the corresponding pins I don't know if you can see that in the camera kind of bright so there and then the corresponding pins that they're the voltages are on so wouldn't be too hard to figure out what goes where and all that but it was still kind of nice so that makes things a little bit easier so whenever you're working on a board like this you want to mark the polarity of the caps before you pull them out sometimes they're the board's aren't marked just like this one so these are all in there all dangly and so what I'm going to do now is flip this over hold them tight and solder them up and the next thing I look at is this little circuit board here I've got all the capacitors installed along here now I've left the resin on the bottom here having cleaned that off yet so I still have a lot of soldering to do here in the end I'll clean this all up so that's all done so now the next thing I'm going to do is focus on the small board here now I plug this in and yes these are too short they just line up exactly with the with the bottom of the with the circuit board there so I'm gonna have to remove these and I'll just put some wires in and jumper it to the other board now there's a lot of time on the traces on the bottom side of this board and I really don't want to take this in and out anymore so I don't know if the previous person has zapped these ICS so I'm not going to take any chances and I'm just going to replace them if one of these ICS is bad or just one part of one of these ICS is bad I can't get to it because the backside of the board is you know up against the capacitors like this so then I have to D soldier this entire board and pull it out again and it's putting a lot more time on the traces on the bottom side of the board so I'll change these pins I'll change these two ICS I'm going to change this capacitor here and I'll just through and test a few of the diodes and some of the things on this board just to make sure that this is okay and then at that point I'll reinstall this and then I'm on to the next part all right I removed two of these pins on this side and two of the pins on this side fortunately this one took part of the trace with it what a pain these things are pressed in here very very hard and they just do not want to come out and of course they're just you know taking the traces with them when they leave so what a pain that is oh well there's a vo right here I can just put the wire through the via and run it down not fun to get these things out that's for sure boy they are just in there like crazy so I've already replaced this IC here so what I'm going to do is replace this IC right here so really all that requires is for me to add some solder to these pins and I'll just do that right now and then I'll do solder it so it's kind of necessary to do this just because there's usually the solder on here there's not enough and if you don't add solder it makes desoldering very difficult so yes that's part of the fun of desoldering is you have to solder 2d solder much of the time very rarely do you get to not do that so really all I'm doing is just flooding the area not I don't really care if I'm bridging pins or doing anything at this point clean that all up in the end I just want to add solder and that's really what I'm doing so that I'm going to do is do solar these now you can see that I'm waiting for just a moment and the reason that I'm doing that so that the solder melts on both sides this is a double sided board if you don't give it just a little bit of time so that it melts to the other side it means that you have to desalter this all over again meaning I'd have to add solder again and do all of that so with this it really is quite a balance these boards are very very tidily you mean you see this one here didn't get through to the other side so what I mean is is that you know they're very finicky a lot of the time at all so what plays a big role is the size of the trace on the other side of the board now it isn't a good idea to go prying with the tip of your soldering tool hot but these pins are so light that they just move nice thing about these tips too is they have an insert in here so it's almost like a piece of pipe that they've put in here and it's really really tough so these tips last a long time as long as you don't overheat them I'm sure they could last a lot longer but then they wouldn't get sales right all right so that looks like that's pretty much ready to come out this onto this side here and the way it comes and there it is the old LM 324 and grab the new one here pop it in and resold RIT still have one more road to go here want to get back I have this installed and we'll be moving on to the next part the board is now reinstalled see I put the wire jumpers in here in here and that turned out just fine so now I've got to get in here and replace some capacitors it'll replace this guy here this little one here there's another one back there and there's also one hiding right down in there so I'll get rid of those and then just before testing I'll replace that little open trace area down here with a small piece of Kiner wire so really it's just going from here to this pad right here the power supply is finished and ready to be tested all these capacitors here have been replaced one capacitor on this vertical board has been replaced as well as the two ICS the bulk caps are still original but okay this capacitor down here has been replaced this cap has been replaced this cap and this cap has been replaced on the bottom side of the board I've repaired that broken trace with a piece of kynar wire so it's just taking place of the trace that's missing ivory solder this row of pins here that leads out to this connector whenever you're working on a single sided circuit board it's always really good to Reese alter or reflow all of the pins just because any kind of movement makes them crusty and they break open and this one here was pretty much loose there's one other one close to the end here that was very loose as well so if you're going to put something back into service that would turn into an intermittent problem quite shortly so you want to make sure that you Reese alter all the pins now if you remember correctly they included the pin notes on the top here it's kind of hard to see with a light included the pin out with all the voltages here basically a legend for all of these pins so what I've done is I've written the voltages on the circuit board zoom on in here just a little bit I've written the voltages here on the board just to make it a little bit easier to test so we got positive 5 volts negative 5 volts here as a whole conglomeration of pins for that because this is a high current supply here there's a positive 15 volts negative 6.4 a common and a negative 15 volt supply and this common is just for the positive 15 the negative 6.4 and the negative 15 the 5 volt supply has its own positive and negative so it's pretty much ready to test at this point I haven't cleaned the resin off of these pins here yet I'll do that in the end and it'll take all of this writing off with it as well so I didn't want to do that just yet I did clean this row of pins on the vertical board just because they're so close together I didn't want to have any connections between them these ones here you can see that you know there's enough spacing between them you can see that there are going to be no bridges and on this end of the board here I haven't cleaned the resin off of the ball caps either yep no big deal I'll do that in the end so I'll just hook some voltage up to this thing and we'll see if it fires up the power supply is now finished and I'm ready to test it so I have the power supply hooked up to an isolation transformer and a current limited main supply if you're ever working on a switch mode power supply like this and you feel the need to put mains voltage on it you should have an isolation transformer and some sort of a fused supply or current limited supply just keep in mind if you're following along you're doing so at your own risk if you're ever working on any of these power supplies when there's mains voltage applied to them those bulk capacitors hold a nasty charge you could get very very badly shocked so just take care so I'm just going to turn on the main supply now I'm not sure if the 5 volt supply on this needs a load many of these power supplies won't start unless they're loaded so what I'm going to do is just give it a shot and see if it'll start without it so I've turned on the main supply right now and I'm just going to come in here and see if we have voltage and we - there's five volts it's alive so let's test out the other supplies positive 15 here there is a negative six point four volt supply here and that's moving around that might need a load we'll find that out in just a moment here and we'll test out the negative fifteen volt supply and it's absolutely fine so what I'm going to do is switch the power supply off here for a moment I have a small load over here which is basically just a resistor so I'll clip this onto here have this ready just in case this thing needs a load put it on here should always do this with the power supply off if you do it with a power supply on and your bridge pins bad things will happen so you don't want to do that all right so I've got a load on the 6.4 volt supply here it's actually a negative 6.4 volt supply so we'll test it see if it's stable and there it is absolutely stable no problems whatsoever so this power supply is finished and it works very very well so this could be reinstalled in that oscilloscope in order to test it out but unfortunately that thing is in a box and a million pieces the guy completely disassembled it so maybe in a future video I'll rebuild the oscilloscope thanks for stopping by the lab hope you enjoyed this video if you did you can let me know by giving me a big thumbs up and hang around there'll be many more videos just like this in the very near future so until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 698,320

Keywords: creativedesigncomponents.com, Creative Design Components, power supply capacitors, fix a power supply, switching power supply repair, switch mode power supply fix, leaky capacitors, bad capacitors, power supply test, TDS 420 power supply

Id: NoGl7oeh7eU

Length: 29min 45sec (1785 seconds)

Published: Wed Apr 13 2016