Author Topic: Choosing off the shelf transformers for flyback converters  (Read 586 times)

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Online Arznei

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Choosing off the shelf transformers for flyback converters
« on: December 12, 2018, 09:45:31 pm »
Hello,

I am currently trying to learn a bit about switch-mode power supplies and DC/DC converters in general. I am however having a real hard time actually finding off the shelf transformers. Let's say I plan on having a 24VDC input for my circuit and I need 3 seperate 5VDC rails in my design, 2 of which need to be isolated from the 24VDC input. How would I go about finding a flyback transformer that suits those criteria?

I tried looking through the webpages of coilcraft and the likes, but I barely ever find a transformer that has 3 "secondary" windings and much less one that is suited for 24DC input and happens to have 3 secondary windings all specified for 5VDC output.

Is there some site where I could go about a parametric search for transformers? Or is the only way out a custom transformer?

Thank you for any help!
 

Offline Benta

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Re: Choosing off the shelf transformers for flyback converters
« Reply #1 on: December 12, 2018, 11:05:12 pm »
Unless it's a very, very simple transformer, it's custom every time. Ferrite grade, saturation curves... you name it.

Have fun in the wonderful world of magnetics. Been there, done that. 

 
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Offline TimNJ

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Re: Choosing off the shelf transformers for flyback converters
« Reply #2 on: December 12, 2018, 11:24:39 pm »
As Benta said, almost always custom. Transformer design varies a lot between power supplies. Depends on the topology used, controller, input/output voltage and current, physical form-factor, etc. There many be some off-the-shelf offerings but they probably won't be just right.

Making your own transformer actually isn't all that hard. Just need magnet wire, maybe some triple-insulated wire (TIW), 3M 1350 polyester tape, and core/bobbin set.
 

Online coppice

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Re: Choosing off the shelf transformers for flyback converters
« Reply #3 on: December 12, 2018, 11:31:12 pm »
If you can find an application note or EVM from a silicon vendor which matches your needs, you often find that the transformer in it was made by a well known winder of coils. Often, the coil winder has given this a part number, and made it available to order.
 

Offline spec

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Re: Choosing off the shelf transformers for flyback converters
« Reply #4 on: December 13, 2018, 12:26:00 am »
Hi Arznei,

As the other members say for a special application you would need to have the fly back transformer made to suit. But there are hundreds of 'standard' fly back transformers available, one of which may suit your purpose which is fairly standard.

There are also a load of standard fly back circuits, both on the net generally and in manufacturers chip datasheets and application reports.

The other thing you could do is modify an existing SMPS, especially computer PSUs which are powerful and cheap.

https://www.google.co.uk/search?q=flyback+transformer&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj4m7zJwpvfAhVdSBUIHfEJBZUQ_AUIDigB&biw=1333&bih=602&dpr=1.2#imgrc=_

https://www.digikey.co.uk/products/en/transformers/switching-converter-smps-transformers/168?k=flyback%20transformer

https://katalog.we-online.com/en/ctm/switch/mid_power_magnetics/mid_dc_dc_flyback_transformers

« Last Edit: December 13, 2018, 12:29:12 am by spec »
 

Online Arznei

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Re: Choosing off the shelf transformers for flyback converters
« Reply #5 on: December 13, 2018, 10:45:57 am »
Well thank you for all your replies, that is what I already feared. I have actually already wound a first "transfomer" - albeit it's probably a pretty s*itty one, but it works so far for just testing the basic principles.

The only problem is: if I ever wanted to actually produce a small quantity product with it winding your own transformer seems like a bad idea and it's probably going to blow the BOM cost to have a custom transformer made for just 10-20 boards i guess.

However, for future reference and for whomever might stumble upon this post, I have found a pretty nice "smart transformer selector" by W├╝rth Elektronik. I was actually able to find quite some transformers that could potentially suit the described needs of my first post. It can be found here:

https://www.we-online.com/sts/app/web/en/

Now back to winding my own transformers: are there any nice tricks to keep the wires nice and tight while winding the transformer? It always works pretty nicely while I am doing the primary, but as soon as I start the secondary the primary starts to "unwind" a little because I can not hold it in place anymore. Is it okay to glue it in place? Are there any common tricks I should know?
 

Offline Kilrah

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Re: Choosing off the shelf transformers for flyback converters
« Reply #6 on: December 13, 2018, 11:20:36 am »
Tape?
 

Online Arznei

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Re: Choosing off the shelf transformers for flyback converters
« Reply #7 on: December 13, 2018, 11:36:50 am »
Surely, but what kind of tape would be applicable and would I just wind the primary, put tape around it and then wind the secondaries on top of the tape? I just don't know what the "proper" way of doing things is.
 

Offline chris_leyson

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Re: Choosing off the shelf transformers for flyback converters
« Reply #8 on: December 13, 2018, 01:13:06 pm »
Wind first primary layer, apply three layers of polyester film tape, wind secondary layers, apply three layers of polyester film tape, wind second primary layer, apply three layers of polyester tape if you want a transformer with an interleaved primary winding. AN-1024 from Infineon is a good read https://www.infineon.com/dgdl/an-1024.pdf?fileId=5546d462533600a401535591115e0f6d
If using polyester film tape it's a good idea to stick some to a piece of glass and cut it to the right width and length with a scalpel, makes it easier to apply.
« Last Edit: December 13, 2018, 01:15:41 pm by chris_leyson »
 
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Offline TimNJ

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Re: Choosing off the shelf transformers for flyback converters
« Reply #9 on: December 13, 2018, 03:00:24 pm »
Surely, but what kind of tape would be applicable and would I just wind the primary, put tape around it and then wind the secondaries on top of the tape? I just don't know what the "proper" way of doing things is.

As mentioned, 3M 1350 tape (or equivalent) is what just about every SMPS transformer uses. There's lots of Chinese alternatives. Usually a light yellow color. If you're curious about construction techniques, I think the easiest way to understand how its done, would be to disassemble an old transformer or buy one of those Wurth ones and take it apart.
 
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Offline Buriedcode

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Re: Choosing off the shelf transformers for flyback converters
« Reply #10 on: December 13, 2018, 03:54:32 pm »
For your specific example, these may eb an off-the-shelf solution:
http://www.cooperindustries.com/content/public/en/bussmann/electronics/products/eaton-magnetics/eaton-transformers/eaton-versa-pac-inductorsandtransformers.html

I realize yours was just an example, but for low turns ratios/low voltage the above can be handy - especially handy for creating bipolar supplies with just a boost converter.
 

Offline T3sl4co1l

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Re: Choosing off the shelf transformers for flyback converters
« Reply #11 on: December 13, 2018, 05:23:13 pm »
Thing is, if your application needs a standard transformer, you can do everything so many times cheaper with a standard PSU module -- that already carries agency approvals.  |O

Transformers aren't all that cheap from distributors, either:
https://www.digikey.com/products/en/transformers/switching-converter-smps-transformers/168

And obviously(?), nothing is more expensive than the one it takes an hour to wind, by yourself (unless your time is discounted to ~zero).  Designing and winding transformers is a valuable skill, and the two go hand-in-hand for designing manufacturable transformers as well; but winding more than a few for prototypes?  Geez...

Tim
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Offline blueskull

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Re: Choosing off the shelf transformers for flyback converters
« Reply #12 on: December 13, 2018, 05:28:43 pm »
Flyback is not the best idea. Depending on your input voltage range, I would consider push-pull first, then half bridge forward.
My take is that I will go with PP, then use a post-regulator to get the desired voltage.

Power loss of flyback is largely contributed by leakage inductance, and as we know E=1/2*I^2L, the lower the voltage, the higher the current, the higher the loss.
Thus, flyback is not very favorable for low voltage applications, unless you have a way to recover the leakage inductance loss.

I might be biased, but most of the time I mess with flyback, I get some bad losses. Bifilar winding can help, but then you need to trade with EMI and SRF.
 

Offline T3sl4co1l

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Re: Choosing off the shelf transformers for flyback converters
« Reply #13 on: December 13, 2018, 05:37:39 pm »
I might be biased, but most of the time I mess with flyback, I get some bad losses. Bifilar winding can help, but then you need to trade with EMI and SRF.

The last one I did, I wound the transformer as twisted pairs -- following TLT theory.  Damn good performance, the capacitance was low (switch and diodes dominate) and leakage was exactly as expected: low enough to just barely need R+Cs across diodes.

PITA to wind, though.  In a TLT, primary and secondary come out together, on the same side of the bobbin.  Easy to run out of pins, for a multi-winding transformer.  It's also a lot of wires connected in parallel (the primary from every pair gets wired in parallel), loading up the pins a lot.

It was still just a bit more complex than that, because I made a 2:1 (Guanella type) section, because the overall converter was step-down.

So that was fun.  Fun to realize a design of high optimization.  I don't recommend trying to buy them from a coil winder. :P

Tim
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Electronic design, from concept to prototype.
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