References on flyback transformer construction

Dear All,

Please could you suggest references for flyback transformer construction.

I have two doubts

a) Whether one should fix air gap first (say at 0.1mm) and then calculate turns based on AL value (as most of the data sheet gives AL value with specific gaps such as 0.1mm onwards but not less). However, in these conditions I am getting operating B at around 0.1T , whereas most commonly its suggested to use at 0.3T.
c) Whether high voltage coil should be wound first and then low voltage or vice versa and what should be pattern of winding if there are number of layers. What are the issues.

Are ther any design tools which can give winding construction details.

Thanks and regards

Hi,
*Determine the turns ratio
*Determine primary inductance
*Determine Area Product or core geometric constant
*Select core
*Determine airgap
*Determine number of turns
*Determine wire sizes
...

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There are a lot of articles on the internet on how to wind for efficiency. You may need to look them up.

sabu31 said:

Dear All,

Please could you suggest references for flyback transformer construction.

I have two doubts

a) Whether one should fix air gap first (say at 0.1mm) and then calculate turns based on AL value (as most of the data sheet gives AL value with specific gaps such as 0.1mm onwards but not less). However, in these conditions I am getting operating B at around 0.1T , whereas most commonly its suggested to use at 0.3T.
c) Whether high voltage coil should be wound first and then low voltage or vice versa and what should be pattern of winding if there are number of layers. What are the issues.

Are ther any design tools which can give winding construction details.

Thanks and regards

Click to expand...

1)Flyback t/f does not only provide isolation but also it stores energy. Most of the energy is stored inside the airgap as the core stores very little. So don't fix airgap first. Select appropriate core based on volume requirements. You can then select airgap based on how much targetted inductance & stored energy. You can select no of primary turns to achieve a particular flux density. 0.3 T is the maximum value of saturation flux density which is dependent on certain things such as temperature. Keep sufficient margin between Bmax & Boperating.

2)You can keep the high voltage winding as the innermost layer. You can follow split primary with interleaving(Half Primary- Secondary- Half Primary- Bias winding) to get better coupling between primary & secondary.

You will also get lower leakage which will cause lesser power loss in RCD clamps

if you are only getting 0.1T peak, then it suggests you can go down a size in transformer.
The primary, for an offline flyback is usually split, as BiswAITH says.........some people put the switching node half at the base of the former, so as to kind of give more shielding of the switching node.

You usually wind to get full layers,......and sometimes have to use parallel strands to get this.......full layers generally means less leakage L...but also its easy to wind the next layer on top of a full layer

found this excellent article on linkedin:

**broken link removed**

Slobodan Cuk's corrective commentary, which follows the initial article, is very correct and should be considered. There is often a gap between theory and practice, especially in terms of optimizing a Flyback design.

Prof Cuk is indeed an expert on his own creations - however I once queried him on optimal design for a flyback - and found his ideas "interesting" to say the least - but not connected to real engineering - I imagine he has never designed one.

I do like his work on resonant input PFC, near zero ripple Cuk converters and the other constructs he has created - however the control bandwidth is often very limited for a Cuk based power circuit of any size ( i.e. > 1.5kW say ... )

Hello sabu31,

In the design of a Flyback transformer there are several criteria to consider before any decision on the options that will be taken.
We don't make the same decisions depending on whether you have a low voltage, a high voltage, several outputs, the power level.
Likewise, depending on the nature of the load to supply: a simple ohmic load, a light bulb, a battery charger, etc.
One of the objectives is also to limit the leakage inductance as much as possible, by winding the secondary between two semi-primary and by choosing the best transformation ratio and the number of turns in primary, the type of conductors for the windings, the choice of magnetic core, etc..
Also, there is a difference in approach between manufacturing a single transformer for a personal need and hundreds of thousands at the best price. You would have to specify the framework of your request better because this subject is very broad.

Regards.

AN 18 from power integrations is a good app note for flyback transformer design.

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How to pick number of turns in transformer
Your question about the actual numbers of turns is a very common one.
Turn it round, and tell yourself, you can have as many , or as few turns as you like as long as…

1…..The number of turns for a coil generally fits across the bobbin length (even if you do multiple layers) …you generally get better coupling if you use the full bobbin width.
2…you can get your turns ratio……for example, if you want a turns ratio of 3.5, then you cant pick a single turn primary because you cant have a 3.5 turn secondary.
3….You don’t saturate the core
4….you don’t suffer too high core losses
5…the turns fit on the bobbin space
6…you can terminate a coil to the former pin (eg a 3mm thick wire wont be easily solderable to a small former pin)
7…you dont have too many layers, as if you do , you may get proximity effect problems.
8…you dont get too much winding loss
9….you get leakage inductance reduced to your requirement
10….the turns fit on the bobbin…turns being litz or double coil etc
……………..
So make out a spreadsheet and carry it through….when you see you violate some condition, just change it till it converges to the right solution.
If you don’t know what number of turns to start with…pick any number out of your head and try and work it through…if its not viable your spreadsheet calculations will soon flag this up to you and you will be able to see if you need to reduce or increase it.