Help rewinding a small 50kHz transformer in a blown Chinese grid tie inverter - Page 1
Forum section: electronics / repair

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 25, 2022, 09:16:03 pm
content: I got a broken 1000CW (that's Chinese Watt) solar power inverter for a 24 Volt system or two 19 Volt panels in series. The input side is blown out but I got all the semiconductors fixed already.The problem with these is when they blow, the current from the solar panel keeps flowing through the shorted MOSFET which over loads the transformer.I got the right size magnet wire and rewound the transformer the same way as best I could. I used the 3M yellow tape instead of the transparent brown tape that it had in it. I used the tape in all the same places. I used heat to separate the top and bottom of the core, and then put a tiny bit of super glue between the top and bottom and pressed the pieces together. All seemed great.When I tested it with the high sides of the transformers disconnected and the heat sink removed, the two MOSFETs on the transformer that I repaired were warm, and the rest created no noticeable heat. I measured the transformer inputs with a scope and found the problem. There is huge ringing on the transformer. On the others there is a nice square wave that goes up to about 32V at the top of some resonating and then it settles down to 25V which is my test Voltage. On the transformer that I did it jumps up to over 40V and there is huge resonating that lasts a lot longer.What in the world did I do to add so much stray capacitance or whatever it is to my transformer?I'll try to post a picture if needed. I'm on Tor and I'm not sure if my account will get banned if it rotates to a Tor IP that is banned on the forum. It does so when viewing the forum as "Guest". Guest continues to be banned on all IP addresses until the cookie is cleared. I'll make a very similar account if needed.The attached picture shows the output side of the transformers. There are two windings (4 wires) on that side because they're usable in both the 120V / 240V version by connecting them in parallel or series.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: October 25, 2022, 09:40:33 pm
content: Maybe YOU did not do anything. Maybe that is the failure mode that caused your problem.Maybe try some ferrite beads near the transformer?? Or on the transistor leads?

author: jonpaul
author_rank: Super Contributor
timestamp: October 25, 2022, 10:09:34 pm
content: bonjour cher MonsieurNormalement les inverters chinois utilize les FETS très bas qualitéI would replace all FETS, and check the drivers to the FETSThe transformers are unlikely to be damaged, fuses will blow first.Difficult to troubleshoot without schematic, pcb layout and transformer winding sheet.Bon courageJon

author: wasedadoc
author_rank: Super Contributor
timestamp: October 25, 2022, 10:17:39 pm
content: Did you check the components in the snubber network?

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 25, 2022, 10:34:24 pm
content: I think it failed because one of the 24 Volt fans shorted. This made the regulator transistor fail. The 24 Volt portion over volted and broke the 12 Volt regulator that is 35 Volts maximum input. The 12 Volt PWM controller broke and so did some of the MOSFETs.But maybe not? A MOSFET could have failed. That could have taken out the PWM controller IC. The PWM IC could have shorted and caused the LM7812 12V regulator to go in to over temperature reduced Voltage output, while limiting the output to <1.5A. They have the 24 Volt regulator transistor referenced to the output of the LM7812, so it would drag down the 24V supply with it, increasing the Voltage drop and heat on the NPN transistor used for 24V regulation. That could have caused the 24V regulator transistor failure. This scenario seems unlikely though because a failed MOSFET would just short out the solar input and shut off the whole inverter.Bonjour!QuoteIt's fused at 90 Amps. 1000W solar = 35 Volts * 28 Amps. Who knows how many solar panels this thing was hooked up to though. I obtained it broken. The transformer on the right of the one with the yellow tape that I worked on has a damaged primary winding as well. So two MOSFETs shorted on and all 28+ Amps was left going through the primaries of 2 transformers, without fans operating. Half of the primary is 5 turns of 3x 22 AWG copper magnet wires in parallel (10 turns total).The board is fairly easy to figure out without a schematic. Everything is repaired except for this transformer issue.QuoteWhat's that? Is that what you call the diodes where the anodes are connected to the low side transformer / MOSFET connections that take excess Voltage away in to some capacitors and resistors? I've seen those before but this inverter doesn't have that.The resonant frequency of the transformer that I rewound is as low as half of what the originals are!

author: strawberry
author_rank: Super Contributor
timestamp: October 25, 2022, 11:02:23 pm
content: there must be 100A fuse somewhere

author: TheMG
author_rank: Frequent Contributor
timestamp: October 27, 2022, 02:21:04 am
content: Quote from: strawberry on October 25, 2022, 11:02:23 pmFuses would not usually protect against the type of failure described here. This is because PV panels behave like a constant current source. The short-circuit current is only slightly higher by about 5-10% than the nominal current at MPP. It's not like a battery or AC mains where a short draws huge current and quickly trips a breaker or blows a fuse.

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 27, 2022, 04:54:45 pm
content: Quote from: jonpaul on October 25, 2022, 10:09:34 pmEt les transformateures aussi?Here are some pictures of taking the second transformer apart. The tape that they used is clear. Their transformer wire is a light yellow color, and mine is red. I wonder if there is a difference with capacitance? The coating on their transformer wire is flaking off, and at least one winding was shorted.

author: strawberry
author_rank: Super Contributor
timestamp: October 27, 2022, 05:12:09 pm
content: fuse is to protect wires and connectionshad one 500W cotton candy machine with immortal 10A 6x20mm fuse that tripped junction box 16A C class fuse

author: jonpaul
author_rank: Super Contributor
timestamp: October 27, 2022, 08:24:54 pm
content: bien sûr le transformateur chinois le pire qualité fabrication et matériels.Core is Chinese knockoffs of TDK PQ, pix show poor core face finishBobbin is Chinese knockoff of TDK, plastic and pins may be inférieurWinding is sloppy, termination sloppyMagnet wire is made in single, double ( heavy) triple or quad insulation buildDye Colors available red, green, clear, natural. no relation to build or insulation typeSolderable insulation is Nylease, Solderease, polynylonClass 155, or higher insulation is HPTZ, heavy polythemaleaseConsult Belden magnet wire catalog.Je suis certain que les Chinois utilize le bas coût fil.Fail perhaps from overheat or winding or termination defects.Bon courageJon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 27, 2022, 09:25:10 pm
content: FYI, clear yellow tape is polyimide (Kapton(R)), much higher temp rating than polyester (Mylar(R)). May or may not matter with respect to other materials in construction.There are many subtleties that you may not've picked up on, that need to be replicated. Transformers aren't just wraps of wire around a core, the relative position and order also matters. At a guess, the primary is centertapped (push-pull), and the secondary is... whatever (say, FWB forward converter?). Ringing depends on leakage inductance and node capacitances; if your MOSFET replacements weren't exact, they may ring different from the originals. (Or the originals were nasty to begin with; how does it compare to the neighboring channels?) Likewise if the leakage inductance (winding spacing and interleaving) is different. And insulation type and thickness has some effect on self-resonance, and coupling at high frequencies (ultimately, an EMI concern).It should also be varnished (preferably by vacuum impregnation), which improves thermal conductivity of the build.In any case, the original windings, being that they apparently used solid wire, must not be very good either; even at low frequencies (<50kHz?), that looks big enough that it won't be fully utilized. This might not matter for fault conditions (excess DC current will do it in, regardless), but would be a big factor in failure particularly up near rated power level.And if it depends on a fan to keep it cool, there should be thermal cutout(s) monitoring all critical heat-generating components -- usually heatsink(s) (one each), often transformers as well.If this is an installation, I would strongly encourage replacing the part with a known good OEM part. The insulation amount and type is part of system safety, and insurance will not be happy to see a bespoke replacement of a critical part like this, should the unlikely occur.The best option, it would seem, is to discard this junk and buy an equivalent from a well known and reputable brand. It will be (should be!?) more expensive in the short term, and much less bother overall -- particularly in terms of safety. As a minor bonus, the higher efficiency will be nice (at least, I'm assuming this thing is trash on efficiency, if it's running transformers this hot).Tim

author: 2N2222A
author_rank: Regular Contributor
timestamp: October 28, 2022, 05:29:01 am
content: Quote from: jonpaul on October 27, 2022, 08:24:54 pmTranslation: Good courage or you can do it!QuoteTranlation: Give it up and buy a new one!Anyway, what does safety have to do with this? It's enclosed in a metal box. It failed because what should be 35 Amps from shorted solar panels went through the primaries of two transformers for a long time with no fans operating. If solar power is meant to save money, then having to buy a $500 grid tie inverter instead of $150 one kind of defeats the point. If saving 10% on efficiency costs $350 more, consider that an 11th solar panel instead of 10 only costs $120 more.These devices are mostly inefficient because they're a single stage design that doesn't actually do proper maximum power point tracking. The input ripple is huge with the small capacitors that they come with. The expensive ones boost a constant input Voltage up to higher Voltage that is stored in capacitors and converted in to AC from there. But that additional boost stage adds loss too just like it does in an APFC power supply.Is it the different tape, a different coating on the magnet wire, or something to do with winding that has created such a big difference between the transformer that I rewound and the originals? Or something completely different? They're all using the same MOSFETs. I filed smooth the rough surface on the core that joins together on the transformer that I already rewound.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2022, 06:27:01 am
content: Why buy a $50 DMM when a $15 DMM will do?Why buy a $5 000 car when a $1 500 car will do?If saving 10% on fuel economy costs that much, why not just......okay fine, the analogy breaks down there.But exactly here, and the two previous points are relevant.Mind, if an additional panel to cover the efficiency difference is indeed that price, that's fine -- but that still doesn't say anything about reliability and safety.Or.If it costs $150 to buy one per year (ballpark), why not just do that? Granted, you might not know when during the year it's going to fail.Or if buying one for $500 lasts ten years, why not expand the array with the balance $100/yr (assuming you get enough excess capacity to do this), meanwhile you're profiting -- what, $300/yr (I forget what electricity runs over there, I know it's usually higher than US rates but I'm not sure what they're paying for solar there) from generation? Or offsetting your own consumption the same amount. But then that amount can grow year-on-year, and you can reinvest that money in your system, and so on. And you'd be making, what, about half that with the unreliable one, and no growth, nevermind the cost of catastrophic failure should the unexpected happen (destruction of property, insurance declines to pay -- assuming they investigate the affected unit and conclude as such)?Not that these are very ponderous numbers; like, I spend $150 at Digi-Key just because I need a couple things, and, that's good enough for business. Obviously, pricing weighs different in development; but if you were talking industrial amounts of electricity, you'd not be talking peanuts like this. Like, casual gamblers spend more; I spend about as much on my bar tab; etc. These are discretionary costs for a lot of people. Or nuisances, and occasionally even setbacks, for others. No, I don't know where you fall on this scale here.Or, to take another angle -- if it's just a 1kW system, why not get a couple car batteries and do an automotive sort of system, ditch the inverter altogether -- or instead of---Wait, is this a grid-tie thing, or a stand-alone inverter for exactly that kind of setup? Maybe I'm about to assume it's more than it is; you haven't really said.Well, suffice it to say, if it's more than a curiosity -- these are tiny costs in the grand scheme of things. I understand if you might not have much to spare -- if this is a hobby installation, you might have little to budget for it -- but that is, alas, how things go in a capitalist economy, it takes money to make money, and if you don't have money, you simply don't get to play (whether that's playing to make money, or just to have a little fun entirely for its own sake). Changing the latter system is, obviously, a rather more daunting task, but you also aren't the only person to have made this set of realizations if that's the case.Tim

author: strawberry
author_rank: Super Contributor
timestamp: October 28, 2022, 07:11:41 am
content: Quote from: T3sl4co1l on October 27, 2022, 09:25:10 pm100ohm gate resistor is not particularly fast switching for ringing to happen. dv/dt is 48V/>1us

author: jonpaul
author_rank: Super Contributor
timestamp: October 28, 2022, 08:21:42 am
content: Rebonjour 2N222A, BRAVO for the efforts! Rewinding the transformer is a super learning experience,The cost is that of your own time.1/ tips on a rewind:Solid wire is very common in low cost production transformers , the losses are mainly core loss and Pcu resistive. At 50 kHz ~ 1KVA, in these sizes the Litz/Bunched/solid wire skin and prox losses are not significant.I would carefully check the wire gauge, select a solderable SS or HS wire to rewind. A hot iron or solder pot will burn off the insulation.Mylar tape 2.2 mils (3M56) is commonly used interlayer.Kapton is great in very high temp apps but VERY prone to tears and hard to fab. We have boxes of 5" wide Kapton, unused due to the issues.We used to embed a #36 ga type K TC at the hotspot for Trise measurement.2/ FET gate RFor the FET gate R, probe (with case!) the gate waveforms with a low wideband scope and 10X probe especially take care of the probe return lead length.Look for the ringing while the unit is a max P.Would start with 100 Ohm, decrease R gate till the ringing is too high.Bon Courage....Be brave (for your problem..)Amicalement (Friendly!)JonPS: 2n2222A... Peut-être sommes-nous voisins ? J'habite à Paris 6eme

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 28, 2022, 08:01:45 pm
content: Quote from: strawberry on October 28, 2022, 07:11:41 amWho says they're 100 ohms? dV/dt can still be quite high, with modern parts, even in large sizes. I haven't seen any claim that this won't be the case.Tim

author: 2N2222A
author_rank: Regular Contributor
timestamp: November 24, 2022, 09:43:24 pm
content: The 4 scope pictures are of the yellow tape transformer that I did versus the original working ones. The yellow tape one has a lot more resonating going on.The 4 pictures of the transformer are of me winding the other transformer that was bad. I used the right kind of tape. It works just like the others now. There is no longer a resonating problem.I redid the yellow tape transformer. You can see it in the last picture on the left. It is a lot better but still has more resonating than the others. Maybe it has to do with how it is wound. Or maybe because the core is cracked on a corner and I glued it back together. Both were filed smooth and pressed together with tape. Nothing was put in between. Some JB weld is on the outside to hold the tops and bottoms together.So it was put back together and it worked for a little while. At about 20 Amps at 36 Volts while testing the mosfets on the worst transformer failed. It must be because I used mosfets with a single pulse avalanche energy of 1/10 what the originals are. There are no snubber diodes to take the excess voltage away. The original mosfets are only rated at 75 Volts! When the junction warms up that should go up to 85 to 90 Volts. Still device operates at 44 Volts normally. The mosfets will see 88 Volts not counting any ringing from the transformers! The winter open circuit voltage can go near 50 Volts!QuoteMalheureusement nous ne sommes pas.

author: fzabkar
author_rank: Super Contributor
timestamp: November 25, 2022, 03:31:16 am
content: You can see the RC snubber across the transformer's primary winding in this 1000W inverter schematic:https://elektrotanya.com/suoer_1000w_solar_inverter_sch.zip/download.htmlI would think that the ringing would be damped by the R component of the snubber.

author: jonpaul
author_rank: Super Contributor
timestamp: November 25, 2022, 03:07:57 pm
content: Rebonjour ringingleakage inductance may be higher, depending on the radial build and winding techniques eg interleaved P...S.Also check your probes, have seen such ringing from probe ground return lead.Jon