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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
I'm building a ZVS flyback converter. I have most the components except I need to find some good wire for my primary, and I need to find/make a current feed inductor.
Ive heard that for the inductor in a ZVS circuit, to keep it from saturating, you should use a powdered iron core. However, I'm having difficulty finding decently sized powdered iron cores. did however get some ferrite E-cores. the two outside legs touch, and the inside ones do not touch at all when you put them together. However if needed, I can mill them at work easily, with a cnc mill. With a core that large, could I get away with using it for the current feed inductor? I'm not looking to make a high powered device, probably only 100w at the most for now. Between gapping the core, using a high value on that inductor, to use a small current, will that core be ok to use?
Also, where does everyone source inductor cores from usually? I got mine from electronics goldmine, though they don't always have the best selection. Digikey doesnt have much in the way of cores at all, and mouser probably doesn't either.
Ok, next question, when winding secondaries, what kind of tape should I use between layers? I was thinking teflon plumbers tape, since its thin, forms well to threads, and teflon is a good hf insulator. But is there some reason that plumbers teflon tape cant be used? I.E. quality of teflon too low for our purpose, or there are small perforations in it that we cant see, etc... All the transformer winding information Ive found online has mainly been for audio output transformers, just using paper between the layers.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
For 100W you could use those cores for either the dc inductor/rfc OR the main invertor transformer.
Plumbers teflon tape is ok but not very tidy (multiple wraps per layer required) I like to use mylar tape just a little wider than the winding area on the bobbin. Most plastic packing/adhesive tapes are mow made of polypropylene, which is also good for hv/hf
If you measure the cores accurately (the area of the centre limb and the airgap length) you can estimate maximum turns/volt at required frequency and inductance fairly accurately. (For good sinewaves using typical power ferrites I'd use Bpk = 0.2T @ 20 kHz and Bpk = 0.1T @ 100 kHz) You can also calculate maximum amp.turns for the inductor. (Bpk = 0.3T)
Registered Member #192
Joined: Fri Feb 17 2006, 03:08AM
Location: Canada
Posts: 44
You don't need a core for you inductor. I'm using a air core inductor so that there is no saturation problems. I used a bottle of 1.6 inches diameter as the form, a spool of 18 guage magnet wire and use this web site to calculate you value.
My coil was 128 uH with 47 turns. I've been told by others on this forum to use a larger value inductor so that is my advise to you too. I measured my air coil with an inductance meter and it works out perfectly.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
ok, after a few orders around, I have more then enough parts to build it now, I wound up a secondary for it, 5 layers, 50 turns or so each layer, probably a bit less. I'll etch a PCB this weekend. I ended up using some LDPE bagging I had around for my insulation between each layer, and a plastic tube for the form. I used a bolt and some fender washers for my winding rig...
I'm allready eyeing up the stepper motors I have in my electronics pile, and thinking of what would make a good fairlead :p
edit: I put 5 turns of speaker wire around the core, and flicked it across the leads of an old PC power supply, I couldnt feel anything on the secondary with 3.3v arcing at the primary. Nothing for 5 either though I had trouble arcing it more then once or twice. I couldnt even arc the 12v without tripping the overcurrent protection. Coil measures 1.5 ohms or so for the secondary... dont think there is a short then... though my multimeter probably sucks for down that low.
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Few general questions about resonant half bridge drivers...
