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  1. #1
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    Which common mode choke has the lowest leakage inductance?

    Hello,
    Please assist on the best way to make a common mode choke for our SMPS output?

    We are doing a common mode choke for the output of our 48V 6.7A output SMPS.
    We want as little differential mode inductance as possible (as little leakage inductance as possible).
    This is because output inductance will not help the stability situation.

    Which Common mode choke in the attached will have the lowest leakage inductance? Can you estimate it in each case?
    (a single three turn coil has a 10.75uH inductance)

    The common mode choke is well needed because we have several SMPS’s in parallel, and without the chokes (at the output of each one) , there will be large ground loops in which noise currents can flow.

    Torroid datasheet:
    http://www.farnell.com/datasheets/18...SAAEgIWQfD_BwE

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    Re: Which common mode choke has the lowest leakage inductance?

    Wind the two wires bi-filar - and make sure they have insulation - e.g. double insulated wire ( teflon ) or for LV, e.g. 48V: PEI 180 deg C

    this is the lowest DM inductance - or leakage on the CM choke.


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    Re: Which common mode choke has the lowest leakage inductance?

    Thanks,
    The only wire available to buy , in our small quantities , is enamelled copper wire…
    https://uk.farnell.com/pro-power/ecw...0copper%20wire

    Since its too dangerous to wind this bi-filar (?), we will have to go with the “separated coil” common mode choke method. Hopefully the leakage inductance is not too high? Hopefully it will be around 300nH?..and no more.
    We may just wind two turns on each half of the coil, in order to reduce the leakage inductance value.

    The actual inductance of two turns is just 4.68uH.....so i would think leakage would be a maximum of 3% of that(?)...thats 140nH



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    Re: Which common mode choke has the lowest leakage inductance?

    for 48VDC - just sleeve one or both of the wires, heatshrink - any thing ...


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    Re: Which common mode choke has the lowest leakage inductance?

    I'd expect that enamelled wire of any category is safe for 48 VDC, without supplementary insulation. If you are not sure about appropriate manufacturing, you may perform a Hi-Pot test, test voltage 250V according to IEC 61558.



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    Re: Which common mode choke has the lowest leakage inductance?

    Thanks, will do...we need to purchase a low quantity of 0.25mm Enamelled copper wire....say just 10 metres of it as we only want to wind 8 common mode chokes.
    Unfortunately, it doesnt seem to sell in such low lengths......maplins used to sell it such but they are now gone. Sorry to ask, but Do you know of any low-length vendor for enamelled copper wire?



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    Re: Which common mode choke has the lowest leakage inductance?

    Bifilar is fine for 48V.

    Wire is so cheap it doesn't make sense to sell it in such small quanities. 100m should be less than 10 Euro.

    0.25mm is used in many ribbon cables, I think. I've used it for prototype multifilar windings in the past with good results.


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    Re: Which common mode choke has the lowest leakage inductance?

    Thanks, the cheapest 0.25mm ECW is well over 10 Euros unfortunately...maybe Sterling has droped some more.
    https://uk.farnell.com/pro-power/ecw...0copper%20wire



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    Re: Which common mode choke has the lowest leakage inductance?

    Thanks, we’d like to heatshrink one of the coils of enamelled copper wire in case the ECW gets nicked.

    Interestingly, if we only heatshrink one of the coils, then when I twist them together, all that happens is that I end up twisting the non-heatshrinked coil round the heatshrinked one….this means that the non-heatshrinked coil ends up obviously being a longer length than the heatshrinked coil (the heatshrinked coil just remains straight). As such, will this have any effect on the coupling between the coils?...ie, will it mean there is more leakage inductance? (ie diff mode inductance).



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    Re: Which common mode choke has the lowest leakage inductance?

    1) any asymmetric solution is a compromise

    2) the further the wires are apart the higher the leakage

    3) oftentimes a practical compromise is the best engineering solution


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