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[SOLVED] Leakage inductance of common mode choke?

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treez

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Hello,
We are thinking of using the SS11VL-R22020 common mode choke (2mH) for our offline flyback’s input filter. Please could you advise on what is the leakage inductance of this choke?

(ie, what inductance [roughly] would be measured if we shorted one coil and measured the inductance of the other?…the datasheet does not say)

Common mode choke: SS11VL-R22020
http://www.kemet.com/Lists/ProductCatalog/Attachments/429/KEM_LF0023_SS11V-H-CH.pdf
 

FvM

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I see two options:
- ask the manufacturer
- measure the parameter yourself

From the photo (shows separate windings), I would expect a larger leakage amount, e.g. 10 - 25 % of the main inductance.
 
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Orson Cart

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With this construction about 3% leakage, so 15 + 15uH or around this...
 
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Thanks, obviously common mode chokes don't saturate with the differential mode current, (since the fields of each coil cancel each other out) ..however, they will saturate on the magnetising current of this....eventually. When saturated, the leakage inductance will also drop to around zero.......what, do you believe is the differential current that will saturate this common mode choke?............the datasheet says "2.2 Amps" is the rated current, but this is not the saturation current, this is just the rms current that the part can withstand, whether saturated for short intervals or not. Do you know what is the saturation current?...I mean, our 60W flyback has mains input current peaks of some 6 Amps, and this could saturate it, do you agree?
 

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It is not the leakage inductance rather the coupling capacitance that causes common-mode current. This impedance depends on the harmonics of the pulses and primary frequencies.

It is a very common overlooked parameter that causes hum in laptop external mics with charger on. in addition to other ingress/egress issues.

Primary secondary separation and dielectric constant are key.

Although it is this CM high impedance ratio in each direction that shunts the CM noise with a shunt cap limited by the IEC safety current limit.. What is it now 0.25mA per device?
 
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Thanks, obviously common mode chokes don't saturate with the differential mode current, (since the fields of each coil cancel each other out) ..however, they will saturate on the magnetising current of this....eventually. When saturated, the leakage inductance will also drop to around zero.......what, do you believe is the differential current that will saturate this common mode choke?............
Which current is meaned with "of this"?

Even if the common mode choke saturates due to high common mode current, the leakage inductance won't drop much because it's almost equal to the inductance of the respective air-core inductor.
 
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Vbase

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Theoretically the currents in both coils are always equal and in opposite direction so the core never saturates. In reality it can happen for short periods of time. The EMC directive allows to generate interference for short periods.
Also theoretically if you short one coil the inductance of the other coil is 0.
 
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treez

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the leakage inductance won't drop much because it's almost equal to the inductance of the respective air-core inductor.
The inductance of the equivalent air core inductor couldn't possibly be up to 25% of the coil-on-core inductance..

By "of this", I mean there is a magnetising current, and this will saturate the core if it gets high enough...I am saying that the mains input current, with its 5 amp peaks, results in this magnetising current being so high that the core saturates.

Theoretically the currents in both coils are always equal and in opposite direction so the core never saturates.
...yes, theoretically ideal...but in reality , as you know, there is a magnetising current, this will saturate the core....I am not talking about common mode current, I am talking about "Magnetising current"
 
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FvM

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I reviewed the Wuerth catalog, they specify a leakage inductance of only 1% of common mode impedance for similar chokes, so my previous estimation is obviously much too high.

It's true that part of the core can be saturated by differential mode due to the field asymmetry, but at only a higher currents. If so, this would primarly affect the common mode inductance.
 
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treez

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thanks, but I am not talking about such asymmetry, I am talking of magnetising current, and how this can saturate a common mode choke......specially one like the one in the top post, which has a high inductance (2mH) for such a small size, so even small magnetising current will saturate it.
When saturated, it is just an air cored coil.......and it will have little common mode impedance......and the differential mode choke that everybody (everyone in industry, not people in this discussion) always says is in a common mode choke by dint of the leakage, will also be very small.
 

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We neglect the effects of stray impedance (DCR, Cww, Cp, RL etc.) that are always present to a greater or lesser extent in a real choke. This assumption is reasonable, because in a well- designed choke, the stray impedance will always be negligible compared to the circuit source and load impedance.

I think the biggest risk is remenance for excitation or saturation in DM mode but any imbalance in the CM core current will slowly self centre. So start stop stop may be a stress factor. It certainly is in big iron core transformers with >10x on small and 8x on large, but much smaller on ferrite.

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Imbalance in the primary bridge ESR will always create a CM imbalance, so centre tapped half bridge has fewer variables and lesser ESR as an excitation load to the CM effects. Also for better load and cross regulation, feedforward multiple outputs use this, but on primary side quasi resonant mode with valley switching is more balanced and efficient.
 
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treez

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thanks, and we are interested in whether or not the above choke (#1) saturates at the 5 amp peaks of the mains input current? (saturation due to magnetising current). We all accept that such a choke has a magnetising current.
 

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When saturated, the leakage inductance will also drop to around zero
as FvM says, this is not correct, the leakage will drop very little if the main core is saturated. The core cannot saturate if the currents are exactly equal and opposite - i.e. there is no net magnetising current in this case, for very hi mu cores it takes only a very little difference in the currents to saturate the cores, and care must be taken in designs to avoid this!
 

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thanks, and we are interested in whether or not the above choke (#1) saturates at the 5 amp peaks of the mains input current? (saturation due to magnetising current). We all accept that such a choke has a magnetising current.
HMm that contradicts your title and initial comments.

The Rated current is in the CM part number
SS11(1)-R22020-CH 2.2 is rated for 2.2A so anything over this will be called saturation.

This is of course differential mode current as CM currents cancel out for the most part.

So 5A is certainly a big problem.

Maybe the questions should have been ;
    1. Does excitation current get included in rated current ? Yes
    2. Does CM current get rated for saturation No, they cancel in this AC application
    3. Does CM excitation cause the saturation of 5A? No it is the design that causes saturation if used when exceeding ratings of 2.,2A
    4. Is the excitation current Common Mode?
      No
    5. is 5A saturation on a 2.2A CM choke a leakage inductance problem NO
 
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is rated for 2.2A so anything over this will be called saturation
nope, for a differential current only a few tens of mA is needed to saturate a high mu core.

2.2 amps is the thermal rating of the wire (usually for 50C temp rise)
 

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thanks, but I am not talking about such asymmetry, I am talking of magnetising current, and how this can saturate a common mode choke......specially one like the one in the top post, which has a high inductance (2mH) for such a small size, so even small magnetising current will saturate it.
Your post shows little understanding of magnetic circuits. Only asymmetries can cause core magnetization for purely differential currents. The respective magnetical path goes through the air where most of it's reluctance is located. So the saturation ampere turns number in differential mode is quite large.

 
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mtwieg

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The Rated current is in the CM part number
SS11(1)-R22020-CH 2.2 is rated for 2.2A so anything over this will be called saturation.
It's almost certain that those current numbers don't refer to a saturation point, but rather a 45C temperature rise due to conduction losses. That means that it's a limitation on RMS current applied over long periods of time (minutes, probably).

The short answer to the original question is that there's no way to tell what the saturation characteristics are from the documentation, for either differential or CM current. Either contact the manufacturer, or try and measure it yourself.

I highly doubt you'll see any saturation behavior due to differential currents, at the very least it would be a very soft saturation since the leakage flux should not penetrate the core very much.
 
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It's almost certain that those current numbers don't refer to a saturation point, but rather a 45C temperature rise due to conduction losses. That means that it's a limitation on RMS current applied over long periods of time (minutes, probably).

The short answer to the original question is that there's no way to tell what the saturation characteristics are from the documentation, for either differential or CM current. Either contact the manufacturer, or try and measure it yourself.

I highly doubt you'll see any saturation behavior due to differential currents, at the very least it would be a very soft saturation since the leakage flux should not penetrate the core very much.
This may be your experience but not my experience with Tokin ferrite which is now partnered with Kemet which is the part in question. Cheaper quality cores have more or equal eddy current losses than saturation induced conduction losses, true , but not these. The ferrite quality is so high that they can use thin cores with fatter conductors and thus conduction losses dominate when saturation occurs, not eddy current.

"The core is thinner to make room for thicker copper wiring. Electrical resistance is suppressed and loss is dramatically reduced, allowing NEC TOKIN’s inductors to produce minimal heat. In addition, compared with inductors assembled with ferrite bulk cores which structurally tend to produce acoustic noise, metal composite inductors are composed of single-piece molded metal powder material, and does not tend to produce no noise."

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To add more clarity, the Tesla rating for Effective Saturation Magnetic Flux Density is defined by this below , which is well below the effective saturation point and the peak saturation point which is BS, which is not the point I was referring to.

Bms is where saturation threshold s defined
mt I believe you were referring to BS.
 
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Cheaper quality cores have more or equal eddy current losses than saturation induced conduction losses, true , but not these.The ferrite quality is so high that they can use thin cores with fatter conductors and thus conduction losses dominate when saturation occurs, not eddy current.
For the 50/60Hz component of the differential current, resistive losses are always going to dominate for any type of ferrite or powdered iron. Regardless of whether saturation is occurring. If the max current number had anything to do with core losses then it would have to be specified along with a frequency (probably ~100kHz), without that it would be meaningless.

To add more clarity, the Tesla rating for Effective Saturation Magnetic Flux Density is defined by this below , which is well below the effective saturation point and the peak saturation point which is BS, which is not the point I was referring to.

Bms is where saturation threshold s defined
mt I believe you were referring to BS.
Looking at their documentation, their Bms values are around 400-600mT, which is pretty typical for soft ferrites, a little high if anything.
 
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