Winding flyback transformers?

[gxti]

Anyone wind their own transformers for flyback converters? I keep running into cases where I want to build an isolated supply for use with mains or Ethernet or the like, but it seems that every converter requires a unique transformer, and of course custom transformers in small quantities aren't cheap.

Specifically, I'm looking to make transformers for use in switching converters at <50W, so these are pretty small. Where in the U.S. can I buy cores, bobbins, wire, and anything else I'd need?

[Zero999]

What isolation voltage do you need?

If it's really high you might need a double section bobbin.

[gxti]

Coilcraft seems to have a good selection but I don't like how they hide their quotes. I don't have a specific price in mind, but paying $50 for a few pieces is too much. There's also no telling how much stock they actually have.

Isolation needs to be good enough for typical mains applications, although the project I'm pondering at the moment is a power-over-ethernet tap which needs 1.5kV for compliance. Is that difficult to achieve?

[gxti]

I actually did abandon a mains switching PSU project because it was just too expensive (> $20 each in parts) to bother building it, even for fun, but the particular thing that prompted me to make this post is a less readily available component. I want to make small on-board power supplies for use with Power over Ethernet (802.3af) devices.

Coilcraft looks promising assuming I can conform my project to the parts they have. After flailing around for 5 minutes even after you pointed out what I needed to click on I finally figured it out. You can never trust an industrial supplier to make a functional web store. Looks like I can get 13W PoE transformers for just over $2, so assuming they don't have large minimum orders or ridiculous shipping fees this could be cheap enough to convince me not to wind my own. I still barely have a grasp on the parameters that go into choosing/building the transformer, so I'll need to get that figured out and this should all get much easier.

And no, none of this is commercial. Thankfully. I'd still like to learn how to wind my own though, so I may try it anyway.

[Zero999]

Isolation needs to be good enough for typical mains applications, although the project I'm pondering at the moment is a power-over-ethernet tap which needs 1.5kV for compliance. Is that difficult to achieve?

That's not really enough information to go on.

It depends on the mains voltage and the class of insulation. From memory, for reinforced insulation (safe to touch without being connected to protective earth/ground) on 230VAC, the insulation should be able to take an 8kV spike. If this is the requirement you'll need to use seveal layers of insulating tape.

[AdShea]

I've had good luck with Elna Magnetics for the stuff I do at work/school.

[benjius]

Hello gxti,
I've been studying flyback converters for some time now, last year I wanted to do exactly the same thing you do, power up my device with an isolated flyback converter I think they are awesome and everyone that is serious about electronics should know about them.
I’m just a rookie but my experience tells me that you should first identify a few other components for your power supply, the PWM IC, the mosfet and the output diode rectifier. Because the transformer construction and physical characteristics are closely related to these devices, and many other variables trust me, the math behind flyback converters is awful.
A while back I posted a similar thread and a guy named "AcHmed99" give me the best advice, he pointed me to “power integrations”, they specialize in switching devices and have a great software called “pi expert design software”  you put it the parameters of your design, like input signal, output power and voltage and the software will  tell you exactly how the transformer should be constructed.
Check their site: http://www.powerint.com

Check my previous thread in this forum: https://www.eevblog.com/forum/index.php?topic=1956.0

Regards.
Ben.