Flyback Transformer with sandwich winding and shielding with copper foil

[strape81]

Hi all,

I am currently designing a flyback converter with sandwich winding and I want to reduce the parasitic capacitances between the windings.
In picture 1 you can see the design without copper foil shielding and in picture 2 you can see where I have put them.




Taking into consideration these two configurations I have the following problem:

with the copper foils between the windings I cannot get the proper primary winding inductance (440uH instead of 930uH). Without the foils the right value is there.
How does the copper foil affect the inductance at the primary winding???
What do I do or understand wrong???
I measure the inductance using a LCR bridge.

Thanks a lot in advance for helping me with this problem!!!

 

[FvM]

I presume you know that the foil layer must no be shorted?

Larger primary to secondary spacing will increase leak inductance, but main (magnetizing) inductance won't be affected.

 

[strape81]

No, the copper foils are only ground shorted.For example the two foils close to primary winding are shorted to the primary ground and the other two with the secondary ground

- - - Updated - - -

I would like to give you the source of my information...please look at pages 6 and 7

http://www.fairchildsemi.com/an/AN/AN-4140.pdf

thanks a lot for your answer

 

[mtwieg]

I believe when FvM refers to the screens being "shorted" he's referring to them being shorted transformer turns. That is, the ends of the screen should overlap in order to completely cover the windings, but insulation must be put between the overlap to prevent the screen from conducting forming a closed loop around the core. If the loop is closed/shorted, the shield will conduct currents due to the EMF that results. This will apparently manifest itself as drastically reduced magnetizing inductance (since the shield effectively acts as a shorted secondary winding). It may explain your observation, but I would actually expect a much greater reduction in Lp (like 75% at least) from a shorted shield.

Also to be clear, the shields do not reduce capacitances, but merely gives the currents induced by the capacitance another path which reduces EMC and EMI.

 

[FvM]

I believe when FvM refers to the screens being "shorted" he's referring to them being shorted transformer turns.

Exactly.

It may explain your observation, but I would actually expect a much greater reduction in Lp (like 75% at least) from a shorted shield.

Yes, that's true. But the only alternative explanations would be wrong number of turns or different air gap or core material.

Not directly related to the inductance variation problem, but I wonder what's your exact motivation to use a dual screen. It could be considered if you want to avoid injecting any switch frequent current between primary and secondary. But in this case, you won't place a primary auxilary winding between secondary.

 

[strape81]

Not directly related to the inductance variation problem, but I wonder what's your exact motivation to use a dual screen. It could be considered if you want to avoid injecting any switch frequent current between primary and secondary. But in this case, you won't place a primary auxilary winding between secondary.

At the beginning I have put the primary auxiliary winding after the primary winding and I could not have the right voltage for my PWM Controller. Playing with the turns of winding could not give the desired result. Afterwards I have put it between the two secondary windings and the voltage was correct...between 12V and 16V (max voltage of PWM controller). In this way the coupling between auxiliary winding and secondary windings improved.
About the dual shields...I believe your are right. There is no reason being there because their effect is neutrilized from the presence of the auxiliary winding between the secondary windings.
Could you give me a tip how I could improve the coupling between the auxiliary and the secondary windings?

- - - Updated - - -

I believe when FvM refers to the screens being "shorted" he's referring to them being shorted transformer turns. That is, the ends of the screen should overlap in order to completely cover the windings, but insulation must be put between the overlap to prevent the screen from conducting forming a closed loop around the core. If the loop is closed/shorted, the shield will conduct currents due to the EMF that results. This will apparently manifest itself as drastically reduced magnetizing inductance (since the shield effectively acts as a shorted secondary winding). It may explain your observation, but I would actually expect a much greater reduction in Lp (like 75% at least) from a shorted shield.

Thanks a lot for clarifying the problem.

 

[szhighstar]

there is a layer of tape between primary and foil, it is not enough, same, only a layer of tape between secondary and foil, it is not also eough. you maybe can overlap tape on the foil for insulation.

 

[FvM]

If the auxilary winding is intended for output voltage feedback, the present position is very good. I don't know how to improve it.

 

[Orson Cart]

make sure the ends of the Cu foil do not touch - or you will have a shorted turn which will affect things (for the worse) greatly.

Having a (non shorted) shield should not affect the Lpri by more than 2%.

We have designed many many flyback transformers with shields to limit CM propagation....