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Which of these common mode chokes has the highest attenuation?

treez

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crutschow

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Since the Wurth device doesn't state the test conditions, it's difficult to make a direct comparison (I think it may be with a 50 ohm load).
But since the Bourns device shows a more constant attenuation to higher frequencies, that might be the better device.
 

treez

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Thanks, holding the CM chokes, the wurth one appears to have at least double the size and cross section of ferrite material.
 

treez

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scopeprobe

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treez

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Hi,
Just doing an Offline Flyback transformer for 13.7V and 3A output. VAC input is 240VAC.
We are doing it sandwiched [Pri1 – Bias - Shield1 – SEC – Shield2 -Pri2]
I found it impossible to wind the shield without “gaps” (as attached) between the coils, due to the uneven winding surface. (due to my club-handed winding) Do you think that it will still be OK? The shields are connected to Primary side (post rectifier bridge) GND.

You can see the blue tape between the coil turns due to the gaps
All coils are using Enamelled copper wire as we cant source TEX-ELZ (TIW) cheaply and at short lead time.
 

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FvM

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An effective shield would be made of copper foil. Not clear what you are doing here.
 

scopeprobe

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Hi,
Just doing an Offline Flyback transformer for 13.7V and 3A output. VAC input is 240VAC.
We are doing it sandwiched [Pri1 – Bias - Shield1 – SEC – Shield2 -Pri2]
I found it impossible to wind the shield without “gaps” (as attached) between the coils, due to the uneven winding surface. (due to my club-handed winding) Do you think that it will still be OK? The shields are connected to Primary side (post rectifier bridge) GND.

You can see the blue tape between the coil turns due to the gaps
All coils are using Enamelled copper wire as we cant source TEX-ELZ (TIW) cheaply and at short lead time.

Could try an off the shelf transformer, parallel the 3 secondaries to give your increased current. https://www.we-online.com/katalog/en/MID-OLSLIM
 

FvM

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..Top of page 8 of this..
They also show how to make a real shield in figure 11. The application note talks about utilizing the bias winding as shield, but it should have a high fill factor. Not explicitely mentioned, but obvious by nature of a bias winding, it should have only few turns. Your photo looks like your "shield winding" has many turns, thus it doesn't shield effectively even with a high fill factor due to the voltage induced in the winding.
 

    treez

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treez

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Your photo looks like your "shield winding" has many turns, thus it doesn't shield effectively even with a high fill factor due to the voltage induced in the winding.

Thanks, The AN-18 doc states that 0.25mm ECW is common for shield windings, and doesnt say anything about not having lots of turns in a shield winding. Then wire is good for shields because it doesnt take up much vertical room in the bobbin.
I agree the induced voltage looks like being a problem, but is not discussed anywhere, neither in AN-18

There's no stipulation about phasing of the shield winding in the doc either, which i thought was a little unusual.
 

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They say AWG 24 to 27, 0.5 down to 0.36 mm, nevertheless I'd prefer a foil shield.
 

treez

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Thanks, so would i, but its cheaper just to have a coil of turns, so we have to put up with it.
--- Updated ---

Hi,
Above what frequency is noise said to jump around common mode chokes? That is, you have to have the input filter away from the switching stage or else the noise will simply jump round the common mode choke.
Does this only happen above say 10MHz?
 
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scopeprobe

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Thanks, so would i, but its cheaper just to have a coil of turns, so we have to put up with it.
--- Updated ---

Hi,
Above what frequency is noise said to jump around common mode chokes? That is, you have to have the input filter away from the switching stage or else the noise will simply jump round the common mode choke.
Does this only happen above say 10MHz?

Noise can propagate at any frequency, Typically from your switching frequency and all subequent harmonics. Are you sure its Common Mode and not differential mode as a CM Choke will have limited effectiveness against DM Noise.
 

treez

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Hi,
If we have a offline flyback with shield windings as follows……

Pri1---Bias-----shield1---sec----shield2------pri2

Do you agree that Shield1 dot should be at same end of bobbin as pri1 dot, also Shield2 dot should be at same end of bobbin as pri2 dot?
This will reduce the voltage between the primary and shield windings and reduce dv/dt
 

treez

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Typically from your switching frequency and all subequent harmonics. Are you sure its Common Mode and not differential mode as a CM Choke will have limited effectiveness against DM Noise.
We have problems above 100MHz....and as you know, this cannot really be DM noise?...only CM noise
 

scopeprobe

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We have problems above 100MHz....and as you know, this cannot really be DM noise?...only CM noise

If you are generating it then it can propagate through common mode or differential mode. The type of noise is just dependant on how the noise is coupling. Flyback converters are broadband generators of noise so it can go up into 100Mhz no problem. Thats where decoupling, loop area's, ground planes and filtering are required BUT you have to know what your filtering.

To determine if is common mode put a RF current probe oround the power and reurn lines together, doing this should cancel out the differential emissions and leave you with 2x the CM emissions.

To measure the diffential emission use a RF current probe around the power lines such the go opposite directions. This will subtact the CM Mode noise and show you the 2x differential mode emissions.

Once you know what you've got then you can look at filtering.
 

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