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Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
I am considering making a few flyback transformers for a very high power experimental flyback system, and also to simply practice winding transformers. However, most flyback transformers I have seen use a rectangular ferrite core wtih an air gap, with the primary and secondary on two opposite sides of the core. I cannot seem to find this type of core, so I am wondering if it be possible to use an Amidon iron powder ferrite toroid as a core instead. I have read that iron powder cores have energy storage capabilities, just like gapped ferrite cores. Is the energy storage of one of these toroids enough for a flyback? And if it is possible to use one of these cores, should the windings be on separate sides of the core to make intristically safe, or is it better to spread the windings over the whole core to contain the magnetic flux?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Well, if you want to run bery high powers, then you want to run it from a bridge so the core must store the least possible energy. Powedered iron cores are completely unsuitable for the task. And you didn't mention how would you wind hundreds of turns around a toroid, not even talking about insulation
Registered Member #1819
Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137
Why can't flyback transformers be run at high power? I have never seen anywhere else a reason to not use high power flyback transformers.
As for winding the transformers, winding hundreds of turns is not a problem with thin wire, because all I need is time. I also can make my own Litz wire for the primary.
For the insulation, would 3 layers of polyimide tape work? It has 7kV breakdown characteristics, but is this not enough for high power flybacks?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Flyback transformer cores can be - and are - fabricated from gapped ferrites, dust-iron, Fe-Si-Al powder, 'Permalloy' powder, and even gapped laminations.
Ferrites saturate more readily than dust-iron cores, so dust-iron is often chosen for high-current, low frequency (<50kHz) discontinuous inductor current mode flybacks, when the instantaneous ampere-turns (summed in all windings) are at zero for a part of each switching cycle.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
A flyback transformer is the DC-DC type driven in flyback mode, not just a generic high voltage transformer. Due to limitations which you can read about here, flybacks aren't used over a few hundred watts. However that doesn't prevent flyback transformers being modified and abused. As Jan said, for high power you should use a bridge and ferrite core to get the best efficiency and fewest problems. The info in this thread should be of some use.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
killah573 wrote ...
Why can't flyback transformers be run at high power? I have never seen anywhere else a reason to not use high power flyback transformers.
I didn't say that. I said that powdered iron cores are not suitable for high power throughput using a bridge converter.
killah573 wrote ...
As for winding the transformers, winding hundreds of turns is not a problem with thin wire, because all I need is time. I also can make my own Litz wire for the primary.
I thought you were talking about toroidal core.
killah573 wrote ...
For the insulation, would 3 layers of polyimide tape work? It has 7kV breakdown characteristics, but is this not enough for high power flybacks?
Use a heat-resistant material, breakdown voltage really does not play a big role in HF transformers, what matters is dielectric loss. If I were to make a high power transformer (and I'm planning to), I would simply use a (waxed) paper. If you use anything made of non-heat resistant plastic, you will limit the maximum power of your transformer because when it heats up too much, it melts the insulation.
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Flyback Transformer Design Question
