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If it is a HV flyback with primary and secondary, protect primary with TVS diode to limit the voltage spike from here. Say, critical voltage over secondary is 10kV and turn ratio is 1:100. Then put 100V rated bidirectional TVS diode in parallel with primary. Heatsink required.
Registered Member #5374
Joined: Mon Jun 18 2012, 06:54PM
Location:
Posts: 10
I wasn't sure if it was immediately obvious what the transformer was in the picture, so I added another one - it's the little green toroid. You can just about see a bit of arcing in the top picture.
Is it not because my secondary winding only has one layer? Should I use more layers and insulate between them?
Here is a secondary I wound once with 0.15mm wire over the same kind of toroid:
It is a part of 800V flyback converter, so did not need to insulate anything except that i put a tape over it before winding the primary - the epoxy coating, as datasheet says, is 1000V guaranteed. If you get kV, then you need to add additional insulation between core and secondary. If you don't have an output cap and feedback Vctrl circuit, then your voltage spike is not limited by anything and you will end up either with the arc over secondary or blown mosfet. To wind easily, make a cut through the toroid using the diamond disc and rotary tool. For each turn you add pass a wire through the gap. Flyback need an air gap anyway. The one on a picture was wound without the gap initially - it was added later. But I had many others wound through the gap.
^^ thats cool ,I thought about it but never did it because I thought they were too brittle to cut. I got the ferrite non conductive ones, I thought the colored ones were conductive
They all somewhat conductive - for HV it is surely an issue, even low permeability ones. So additional few layers of insulating tape over the core before winding is a right idea. Diamond disc (~$10) goes through ferrite like a knife through the butter and I cut half a dozen of cores without problems - disc is still perfect. Here is how it looks like:
Be careful with cut cores though - if dropped on hard surface, surely brakes on halves.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Toroids are a bad idea for really high voltage transformers. Too much ferrite, not enough winding window to cram in the insulation you need. Try an E core set with a plastic bobbin.
Technically, a flyback transformer would be any transformer used in a flyback converter. But in order for a transformer to work in a flyback circuit, the magnetic circuit must be designed to store energy. Either ferrite with an air gap, or iron powder or MPP.
You're using a ZVS, which is a forward converter, not a flyback converter.
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Have I just built a flyback transformer, and how can I stop it arcing internally?
