SDG #305 How to Wind a Flyback Transformer for SMPSU

hi in the last video we completed our flyback converter PCB design and I'm just about to order all of the components on it but one of the items that we can't order is the flyback Transformer itself now if you're clever with the tool you can actually get it to a design with one of the Transformers that you can buy off the shelf but in this particular example we do actually need to wind our own and there's quite a few different approaches to it just to remind you what the Transformer is that we're trying to wind it's this one that's Illustrated in the design here on the pi expert website so 204 turns on the primary and then we've got 51 terms for the bias feedback winding and then just 10 and four turns for the center-tapped secondary so this is what we're going to be winding today but before we do that a quick word from our sponsor for this video PCB way so PCB way is your One-Stop shop for all your project needs as you know they offer a wide range of PCB manufacturing capabilities including very cheap prototype pcbs production level boards all the way up to 60 layers and also rigid Flex pcbs they also offer ptb assembly Services where you can get your entire PCB assembled with the components onto both sides of the PCB as well as CNC fabrication this month PCB way are celebrating their ninth anniversary and so there are some coupons on the promotion page as well as promotional pricing on a lot of their services here and there's also a lucky draw so if you sign in you can participate in the Lucky Draw and potentially win one of the prizes so don't forget to visit pcbway.com and we've got the materials that we need we've got some 38 gauge Magnet Wire and I need to just dig out the secondary windings as well but I've got this tool here now this one isn't really set up for a Transformer bobbin of this size this is quite small but I have successfully used in the past by just modifying it with this little thing on the end here now this does actually make it a little bit unwieldy to use because when you're turning it here this does have a tendency to wobble about a little bit but this is basically just allowing us to stick an M4 Bolt through a Transformer bobbin like this then we could hold it in place with a couple of nuts and then that actually screws into the end of this because this is just a PCB standoff but that's a nice easy way of just getting this to sit on here I think ideally given that I'm never really going to wind anything of this size I might just chop this off a bit smaller and make something on the 3D printer that holds the bobbins that we're going to use so I'm going to mount this onto here first of all I have marked which one is pin one so that we don't start getting confused and then we can get winding this Transformer right so first of all we're going to start winding the primary and it needs to begin on pin two and it's important we get this correct because if we look at the diagram for the Transformer you can see the little dots which indicate the phase of each of the windings the primary is opposite to the others so we need to make sure that although we wind in the same direction the start and ends are connected to the right pins so I've got some enamel copper wire here I've just put a little bit of Teflon tubing over it just to protect it where it goes from the pin down to the layer of the bobbin because there's going to be some secondary windings that run nearby so we'll put this on pin two and just use some tweezers to help get that into place here we go we've got the Teflon tubing and then that takes us down to where the bobbin is now the idea is that we try and do this without overlapping any of the windings we want it a nice and flat as far as possible however this is going to be quite unlikely to actually be able to happen because this coil former sorry this coil Winder does wobble about all over the place so we'll do our best basically we need to put 204 windings on this in a clockwise Direction so let's do our best here so probably lay down like 10 to start with and then we'll just nudge these back to the end but with uh why are this fine it's actually going to be quite a difficult job to keep this completely flat without a automatic coil winding machine which basically feeds the wire in and moves it across in the right direction and so that's 204 windings finish that on pin one and then we need to do the bias winding next it calls for some Transformer tape now unfortunately I've not got any that's quite the correct width for this bobbin so we're gonna have to use some that's a little bit thinner so that's the primary complete let's do the bias winding next so the bias winding starts on pin four so we'll just wrap this round here again same procedure as before try and keep it neat without overlapping and there we go 51 windings it's not exactly need but I think it will do the job so we'll just scrape a bit of the enamel off ready for soldering and then between these primary layers it asks for three layers of insulation tape so let's try and do that now next up we've got the secondaries and this wants to start on pin seven so let's wrap that round there and this is just 10 turns around the core evenly distributed so that's 10 and it finishes on pin six and then the final part the secondary starting on pin five reset the counter and this one is just four layers and this one ends on pin seven now that we've round our Transformer the next thing we need to do is Gap the core to set the primary inductance now if we look at the tool once again it tells us here that our primary inductance should be about 3844 when all of the windings are open and we're testing out the switching frequency so the LCR 45 which we've got here from Peak electronic Design This one unfortunately you can't test at the switching frequency because I think it jumps from 10 kilohertz up to 200 kilohertz and there's no in between uh the switching frequency for the link switch 562 is about 66 kilohertz now I do have an LCR meter on the bench that can do that but to demonstrate what's actually going on we can still use this it it displays it perfectly so we connect up the LCR meter to the primary windings here and it shows as an inductance of about 200 micro henries or so now if I put the inductor core into the Transformer and clamp it together tightly you can see the inductance shoots up to about 40 milliamp results so and as we change the gap between these two halves of the Transformer you'll notice the inductance goes up and down so if I increase the Gap you can see the inductance going down down down down and if I change the Gap so it's tighter the inductance goes up and basically what we need to do is we need to try and shim the Transformer core so you can use things like paper and introduce it to both sides of the core and keep testing it basically until when you press it tightly together you get the inductance we're after and so obviously if the inductance is too high we try and increase the Gap and if the inductance is too low we try and decrease the Gap and once you've got the right combination of materials usually paper is a good start you can also use some tape as well obviously nothing magnetic but once you get it right then you tightly bind the Transformer cores with either the clamp that you can buy for some of these unfortunately this Transformer doesn't have that so in this case we'd wrap Transformer tape around the outside and then the final thing to do is to dip the transformer in varnish which keeps all that in nice solid formation so as you can see we're pretty much there obviously at the wrong frequency here but 3084 on the LCR meter to the side of the bench and I've taped the Transformer halves together so the only part remaining now is to dip this Transformer in some Transformer varnish let it dry and then we can use it on our PCB design so you might be able to see the odd bubble coming out interestingly the instructions say don't vacuum impregnate so although we've got a vacuum chamber that we could get it into every Gap in the Transformer it tells us not to do that so we'll leave it in here just for the bubbles to dissipate and then we'll leave it to dry the Transformer varnish is now cured it's the next day and as you can see we've got a bit of a Sheen to the Transformer the varnish will help keep it all together try and reduce vibrations in the windings which might manifest itself as an audible frequency that we can hear at certain switching frequencies but the primary thing there is to try and improve the insulation resistance of the windings so what we'll do just quickly is make sure that this is still assembled properly and nothing changed overnight so we'll just check the inductance of the primary which should be about 3844 micro henris and it's about three eight oh two Michael Henry's a test frequency of 15 kilohertz so slightly off um but that looks like everything is in order so the Transformer is all good now we could drop this into the design if we had the PCB and we could test it but I think what we need to do first of all is just test the insulation resistance between the primary and the secondary because this is our isolation barrier between the mains and our low voltage output so if we've got a problem in the Transformer a short between the primary and secondary then that's immediately an electrocution risk so we should test this now ideally we'd have a high potential tester a high pot tester that could test us at 3000 volts unfortunately I don't have one of those but I do have a multi-function tester for electrical installations that will test at 1000 volts and what we will do is test between the primary and the secondary windings and technically there should be basically infinite impedance between those two up until the point where the voltage goes high enough that it flashes over and breaks down the windings now we shouldn't do that at a thousand volts but let's just test it so yeah greater than 50 Mega ohms let's change that to 100 volts greater than 100 Mega ohms then 250 volts and still good 500 volts and that's still good and our final test voltage on here a thousand volts and there we go so even at a thousand volts we've got um an impedance bigger than what we can read on this multi-function tester so I think that points towards this Transformer being okay to use in our PCB design so that's the process of winding a very small flyback transformer and as you can see it's a little bit fiddly but it is achievable now we weren't able to properly achieve some of the specifications with the Transformer winding because it suggested we might be able to get that primary done in just three layers and there's absolutely no way that was happening by hand if you've got an automatic machine it will move the wire along as it rotates the Transformer we get very nice uniform layers a DIY attempt I think is never going to be like that unless you spend absolutely ages winding it but this one should be good enough we'll be able to test its performance when we connect it up to the flyback driver circuit and see if we actually achieve a decent level of efficiency but that will be in the next video when we get the pcbs in from our sponsor for this video PCB way so if you've got any thoughts or comments don't forget to leave them in the comments section down below hope you enjoyed the video and until next time thanks for watching

Channel: SDG Electronics

Views: 12,776

Keywords: smpsu, power integrations, electronics, power supply, ac-dc, converter, soldering, oscilloscope, DC, linkswitch, lnk, flyback, winding, transformer, coil, inductor, inductance

Id: fQmyrBEPsek

Length: 15min 8sec (908 seconds)

Published: Sun Jun 25 2023