Flyback transformer construction

I don't understand how the coils are wound in a flyback transformer.
The most confusing is the feedback coil.
Can someone post an image or explain.

This is how ordinary transformers are wound. Simple to understand - a coil on primary side and a coil on secondary side. The feedback coils are usually shown on the primary side in flyback transformers. I don't get it.
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"Flyback" transformers don't generally have a feedback winding. Transformers dedicated for flyback converter mainly have an air cap, reducing the main inductance to store more energy with a particular core flux. This is required by the flyback operation principle of storing the energy in one half cycle and releasing it in the other.

The main difference between flyback transformers and normal transformers is that flybacks are meant to store energy in their cores, like inductors do. To prevent saturation, flyback transformer cores are normally gapped, and the gap is usually the parameter that trips people up when designing them. Other than that, there's nothing really different about how they're constructed. However, minimizing leakage inductance is relatively important for flybacks, so you want to wind them in a way that gives good coupling.

FvM said:

"Flyback" transformers don't generally have a feedback winding.

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No, feedback windings are still pretty common in flyback supplies.

feedback windings are still pretty common in flyback supplies

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Yes, of course, at least partly. But please read my post exactly, when referring to it. I just mentioned, that feedback windings are not an essential feature of flyback transformers, as the original poster apparently assumed. It's not clear, by the way, if oscillator feedback or a DC sense feedback is meant here. Both kinds of feedback can be found with flyback designs.

P.S.: Reviewing the flyback transformer portfolio of a major manufacturer , I find the second primary winding dedicated for Aux/Bias rather than feedback.

mtwieg said:

The main difference between flyback transformers and normal transformers is that flybacks are meant to store energy in their cores, like inductors do. To prevent saturation, flyback transformer cores are normally gapped, and the gap is usually the parameter that trips people up when designing them. Other than that, there's nothing really different about how they're constructed. However, minimizing leakage inductance is relatively important for flybacks, so you want to wind them in a way that gives good coupling.

No, feedback windings are still pretty common in flyback supplies.

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A diagram or a photo would be great. Where exactly is the gap.
Are bias and feedback winding the same?

For mostly used EE cores, the air gap is provided by grinding the center pole. You can refer to the product manuals of major manufacturers, e.g. Ferroxcube or Epcos for details. They have also formulas or design tools for inductor and transformer dimensioning, that show the effect of gair gaps.

Are bias and feedback winding the same?

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The purpose of bias winding is to power the switching controller of a high voltage input (off-mains) SMPS with better efficiency. Can you clarify, which kind of feedback you're referring to?

FvM said:

For mostly used EE cores, the air gap is provided by grinding the center pole. You can refer to the product manuals of major manufacturers, e.g. Ferroxcube or Epcos for details. They have also formulas or design tools for inductor and transformer dimensioning, that show the effect of gair gaps.


The purpose of bias winding is to power the switching controller of a high voltage input (off-mains) SMPS with better efficiency. Can you clarify, which kind of feedback you're referring to?

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I am talking about the optocoupler feedback used in flyback smps.

O.K., it won't need a particular winding to operate it. Some HV switcher ICs, e.g. from Powerint are deriving the supply voltage for the optocoupler transistor from an auxilary winding for ease of design. It's a special design to reduce the IC pin count, no feedback action of the winding itself is involved. A DC supply can do too.

Some simple switcher designs are using a feedback winding to sense the feedback voltage, relying on its proportionalilty to the output. They are working without any optocoupler.

FvM said:

O.K., it won't need a particular winding to operate it. Some HV switcher ICs, e.g. from Powerint are deriving the supply voltage for the optocoupler transistor from an auxilary winding for ease of design. It's a special design to reduce the IC pin count, no feedback action of the winding itself is involved. A DC supply can do too.

.

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I was also referring to powering flyback circuits.
So where is this auxilary winding wound? Is it on the primary side or secondary side?
Powerint circuit diagrams indicate it on the primary side.

hithesh123 said:

A diagram or a photo would be great. Where exactly is the gap.

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The "gap" can be anywhere. It's purpose is to increase the effective reluctance of the core. Anything that does this can be considered a gap (drilling holes, putting spacers between core halves, etc).

It doesn't really make sense to think of a winding being on the primary or secondary side. All the windings are mutually coupled together, so what we recognize as a "primary" or "secondary" doesn't really matter. Just wind it such that all the windings are strongly coupled together and it should work.

The only exception to this might be when you are dealing with very high isolation voltages between windings, in which case you would want to space them far apart, but then you'd have to deal with more leakage inductance.

Hi
Fly-back transformers are works just like a inductor.
During the switch on time, there wont be any power conversion from primary to secondary side,since secondary side diode will be reverse biased. hence energy is stored in the inductor itself. Inorder to store the energy in magnetic field we use airgap in the inductor. but if it is too large, leakage inductance problem will occur. So provide airgap with minmum as per calculation obtained.
When switch is off, the stored energy in the inductor(primary) will be transfered to the secondary. Through diode the capacitor will get charged. and this stored energy in the capacitor will be discharged when switch is in ON. and cycle repeats.
There is a difference between line/high frequency transfermer and transfermer which we use in fly-back converter, see the polarity of the transformer, you will find the difference, and period at which energy transfer in secondary.
Normally transformers are not energy storing device(there won't be any air gap in the core). It converts the instantanious power from primary to secondary. But fly-back transformers are energy storing device(provided with airgap).

I hope this information will be usefull to you

Powerint circuit diagrams indicate it on the primary side.

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In these application circuits, primary or secondary means nothing but on one or the other side of the high voltage isolation barrier, as mentioned by mtwieg.

for transformer detail there is good application note on powerint.com website...........

they also have nice software. .........you put in PSU spec, and it makes the transformer for you.....and give you nice drawing..........


i didnt know that making the airgap larger increases the leakage flux............at least i know that it does increase it but i thought not by very much (?)

anyway, its not the airgap which causes the leakage flux, that i know for sure.

.....if it was , then everyone would use forward converters all the time as they have very small gap...or often none at all.


the secret of an SMPS is in the winding of the transformer......................whether it was "sandwiched" or not...etc etc.

..and most SMPS companies will not let the juniors near the transformer winding procedure...because they know that they would then be empowered with the trade's biggest secret.

there are all kinds of secrets about how to reduce leakage by winding technique.................and for those who try and reverse engineer by trying to rip the wound transformer apart...............you will not manage becasue they are tightly taped up and glued and varnished so that prying eyes cannot see the winding secrets that it contains.

heres one for you to look into.......................

should you put the bias winding..................

(a) closest to the core?
(b) on the outside of all the windings?
(c) just after the first primary layer
(d) just before the second primary layer
(e) in between a single primary layer and a single secondary layer?


..you see there are so many ways to do it.........and each gives a different coupling.....

and how do you handle layers which dont take up the whole bobbin layer area?

.......there are the secrets of the trade, and as we speak , its gauranteed that engineers are sweating as they try desperately to **** apart wound transformers from competitors switch mode power supplies......the secrets of which are glued up to high heaven.....

i didnt know that making the airgap larger increases the leakage flux............at least i know that it does increase it but i thought not by very much (?)
anyway, its not the airgap which causes the leakage flux, that i know for sure.

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I stumbled upon the same statement, but a certain increase of leak inductance with air gaps can be expected for usual core shapes. You can visualize leak inductance (respectively k < 1) as field lines generated by one winding, bypassing the other. If you imagine an air gap in the core center, then part of the field lines "emitted" from it crosses the inner winding. They represent a certain amount of leak inductance. As a general effect, by reducing the effective µr of the core, the share of magnetic field outside the core increases. You can use a magnetostatic solver, e.g. Quickfield Student Edition to explore this effect.

Thanks for all the replies. If any of you come across an image or a photo of flyback winding please post it.

The feedback winding is used to detect when the transformer magnetism goes to zero so you can start the next power cycle.

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hithesh123 said:

I don't understand how the coils are wound in a flyback transformer.
The most confusing is the feedback coil.
Can someone post an image or explain.

This is how ordinary transformers are wound. Simple to understand - a coil on primary side and a coil on secondary side. The feedback coils are usually shown on the primary side in flyback transformers. I don't get it.
Google Image Result for http://www.constructionknowledge.net/electrical/images_electrical/transformer.jpg

Click to expand...

I wire all three windings in phase with each other. I lay the primary first covered in transformer tape for insulation.
Then the secondary winding covered in tape.
Finally the feedback winding.