Current Sense resistor burning up uc3483 flyback - Page 1
Forum section: electronics / beginners

author: craftyjam
author_rank: Newbie
timestamp: September 10, 2021, 10:08:14 pm
content: Hello, first post on the forum.I'm having trouble getting this high voltage flyback converter circuit from seventransistorlabs.com working.The output voltage will appear normal with no load, and then drop from 250v to 30 with a 50mA load.Also, upon conecting the load the current sense shunt resistor rappily heats and the mosfet supsequently melts.I've included a picture of the gate terminal of the mosfet with respect to ground, there are clearly large spikes (.5v per division, 2microsec per division).Im using a salvaged transformer from a known working flyback converter, also used in a 12 to 250v design.I'm fairly inexperienced with the flyback topology and do not know where to begin with this problem. Any ideas would be greatly appreciated!

author: james_s
author_rank: Super Contributor
timestamp: September 10, 2021, 10:56:40 pm
content: Can you post a picture of your physical layout? I don't know how much experience you have with SMPS's in general but layout is critical. I had a lot of problems in my first few attempts, sometimes I'd build one and it would work great, then I'd build the same circuit in another project and it wouldn't perform right at all. Eventually I learned just how important layout is, particularly grounding and keeping the high current sections away from the feedback loop.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2021, 11:09:42 pm
content: Curious where you got the image from, it's a nasty JPEG. The original is much clearer!Anyway, might you have the transformer wired backwards?A note that I don't think I put on that when it was posted, the 6.3V supply should be monitored as well. To do this, simply add a resistor from 6.3V to the FB pin. You'll have to adjust its value, and the Vadj divider, to get desired output voltage.In the intended application (vacuum tube power), the 6.3V load is very high at startup, while 200V is unloaded; with regulation only on the 200V output it just sits there doing nothing. With joint regulation, the 6.3V rail is pulled down modestly, while the 200V pushes up a bit higher to compensate. Once both outputs are loaded normally, they balance out.Also, a 100k in series with the 0.01 COMP cap is a good idea. (I wasn't as keen with compensation, back in, whatever that was, 2010 or so.) Exact values of these two components, depend on load capacitance and stuff; try nearby values (big steps are okay, like try values of +/-50%) while testing load step response, adjust for fastest settling output with least overshoot.Tim

author: floobydust
author_rank: Super Contributor
timestamp: September 11, 2021, 12:21:28 am
content: To me the 1uF snubber cap seems big, how can it flyback when the cap absorbs so much.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2021, 04:23:47 am
content: It charges up (it's discharged by the 1k).Exact values of snubber components depend on the transformer. Evidently that wasn't a very good one...Tim

author: craftyjam
author_rank: Newbie
timestamp: September 11, 2021, 06:48:23 am
content: Thank you everyone for taking the time to respond!Quote from: james_s on September 10, 2021, 10:56:40 pmI actually have the setup on a breadboard right now, which is probably a major confounding variable.Quote from: T3sl4co1l on September 10, 2021, 11:09:42 pmI had to compress the image to make it small enough to postQuote from: T3sl4co1l on September 10, 2021, 11:09:42 pmI am quite sure that the transformer is the correct way around since I marked the pinout when I was tracing the circuit I ripped it from.I will try adding the resistor in series with the comp cap and will mess around with the compensation when I get some time tomorrow.Thank you for providing so much great free info on your website, it's been a great help for me.Quote from: T3sl4co1l on September 11, 2021, 04:23:47 amI will mess with the snubber too, the resistor was getting quite hot while the circuit was active.Is it sufficient that I am using parallel 1ohm 1% metal film resistors for the current sense shunt resistor, or will that induce too much parasitic inductance?Also, is it bad to tie together the grounds on the secondary side of the flyback with the primary side even if the control circuitry is not being damaged?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 11, 2021, 10:23:00 am
content: Even at 24-bit bitmap I think it'll still upload, but I mean, the original was 3.24kB:https://www.seventransistorlabs.com/tmoranwms/Circuits_2010/Tube_HV_Supply.pngShrug, anyways.Quote from: craftyjam on September 11, 2021, 06:48:23 amThe currents aren't particularly high in this circuit, nor the speed especially fast; I probably breadboarded it at some point. You do need to pay attention to current paths, and keep loops small. No using ginormous fly leads! Short trimmed jumpers, solid core or tinned stranded, will do.QuoteYou're welcome!Oh, also note that it's backwards if it was a forward converter transformer. Unlikely at this power level [it's not practical at low power], but I guess it could happen. (Such a transformer will probably have too high inductance to be useful this way anyway.)QuoteYes, in fact that reduces inductance by widening the current path.QuoteIt's a non-isolated supply, and feedback requires common ground. It's only flyback because that's a handy way to get a large ratio transformer. (Direct boost is terrible at this. Tapped boost is nice:https://www.seventransistorlabs.com/Images/Deadbug_Sch.pngbut hard to find off-the-shelf parts for.)Tim