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  1. #1
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    Which transformers can pass approvals testing rergarding isolation voltage testing?

    Hello,
    Our contractor designed a 10W, 24Vout, offline Flyback SMPS for 240VAC input. He had a split primary and a bias coil on the primary side, both wound with enamelled copper wire. He also had a secondary of Triple Insulated wire.
    He used an SMD transformer which was EFD25 (Page 423 of the below Ferroxcube datasheet).
    This failed approvals testing involving testing at 3.5kV for 1 minute.

    I am surprised about this, since the secondary was wound with triple insulated wire.
    Is it not possible to wind a transformer to pass “3.5Kv for one minute” with the EFD25 transformer?

    The approvals house also told us that the PTH version of this transformer would also result in failure. (PTH version is on page 422 of the below datasheet)

    The strange thing is that an offline flyback wound on the PQ26/25 core, can pass the “3.5Kv for one minute” approvals test.

    Do you know how we can find out which of the transformer bobbins, shown in the ferroxcube datasheet below , can be wound to pass the “3.5Kv for one minute” regulation?
    I thought that having a secondary made of triple insulated wire was enough? (obviously including the 8mm primary to secondary gap)


    Ferroxcube ferrite datasheet:
    EFD25 former is on page 422 and 423....
    PQ2625 bobbin is on page 732....
    https://www.ferroxcube.com/en-global...ad/download/11

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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    Quote Originally Posted by treez View Post
    Do you know how we can find out which of the transformer bobbins, shown in the ferroxcube datasheet below , can be wound to pass the “3.5Kv for one minute” regulation?
    I thought that having a secondary made of triple insulated wire was enough? (obviously including the 8mm primary to secondary gap)
    i do not think the issue is the bobbin
    i think the issue is spacing

    the only 8mm spacing i see on the bobbin you called out on page 732 is between the left side and the right side of the top view (drawing on the left)
    if left to right pins is the separation between primary and secondary, i suggest you do a top and bottom separation - 25.4 mm

    as for the triple insulated wire, what is its rating?
    maybe double insulated on primary and on secondary - net 4 layers, assuming it will fit.



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    TEX-ELZ Triple insulated wire by Furukawa is surely rated to 3.5kv for one minute?

    https://www.furukawa.co.jp/tex-e/en/...e_feature.html
    There is easily >8mm of spacing between primary and secondary pins of the EF25 former, so i cannot understand why this has failed approvals?

    I suspect that the problem is something to do with the creepage distance from the point where the triple insulated wire is terminated (on the former pin), to the place in the bobbin, where the coils of primary connected enamelled copper wire are situated. I am not sure, with any bobbin, how one is supposed to be able to measure what this would be (ie, whether it could be made more than the required 8mm).

    Quite frankly, I would be amazed if the EFD25 wound transformer flashed over during “3.5kv for one minute” approvals testing, so I am not quite sure how the approvals body have managed to assure that we have failed?
    Last edited by treez; 13th October 2019 at 18:17.



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    Did you design the transformer yourself? What's the clearance between primary windings and secondary terminals? If it's actually an isolation failure of TIW, I suspect a handling problem.



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    Did you design the transformer yourself?
    Yes, and as discussed, its primary, bias and secondary with TIW.
    The primary bias coil is the last to be wound on it, (so its the top layer)

    What's the clearance between primary windings and secondary terminals?
    The creepage or clearance from secondary termination terminals, to the primary coils on the bobbin is hard to measure.



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    The creepage or clearance from secondary termination terminals, to the primary coils on the bobbin is hard to measure.
    It's necessary to design the transformer with sufficient clearance in this place. By the way, what's the safety specification (overvoltage category, basic or reinforced isolation?


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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    Thanks, its re-inforced insulation.



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    no disrespect intended
    copied and pasted from the Furukawa Electric Company website you quoted:

    Certified as reinforced insulation; three layers can resist up to 3,000 ACV for one minute.


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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    I'm assuming the TIW insulation didn't fail, unless it was damaged during handling (as FvM suggests). Was insulating tubing used to protect the windings as they terminate to the pins?

    What was the pinout? Did they add any slots to the PCB? Could have been insufficient creepage between pins. Though if it didn't meet 8mm creepage than it should have failed before even bothering with a hipot test.


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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    Was insulating tubing used to protect the windings as they terminate to the pins?

    What was the pinout?
    Thanks, the pinout is as attached. There was tubing used yes.
    As you know, There is easily 8mm betweeen primary pins and secondary pins.

    Did they add any slots to the PCB?
    Thanks, as you know, there is no need for slotting as there is more than 8mm from primary pins to secondary pins.
    Its our PCB and we didnt slot it.

    - - - Updated - - -

    Repeating myself from above...
    I suspect that the problem is something to do with the creepage distance from the point where the triple insulated wire is terminated (on the former pin), to the place in the bobbin, where the coils of primary connected enamelled copper wire are situated. I am not sure, with any bobbin, how one is supposed to be able to measure what this would be (ie, whether it could be made more than the required 8mm).

    Quite frankly, I would be amazed if the EFD25 wound transformer flashed over during “3.5kv for one minute” approvals testing, so I am not quite sure how the approvals body have managed to assure that we have failed?
    I am thinking there must be a list of common off-the-shelf formers which cannot pass "1 minute at 3.5kv " regulations?

    Its certainly needed, we are having to rip up our whole PCB because of this, because the alternative , conforming EFD25 derived former is bigger than our ferroxcube EFD25 one.

    I am quite surprised that there are offtheshelf bobbins which are not possible to conform to "One minute at 3.5kv" regulations.



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    Unless you have additional spacers, the clearance between winding and SMD pin isn't larger than about 5 mm.


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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    as i read the ferroxcube data sheet, page 423 (acrobat reader page 424)
    the surface mount pad layout has 2 mm wide pads on 3.75 mm centers
    therefore the separation between pads is 1.75 mm

    the separation between the primary (pins 1,2,3 in schematic) and the secondary (pins 9 and 10)
    is 28 mm and should not be an issue

    the separation between the primary (pins 1,2,3 in schematic) and the aux (pins 5 and 6)
    depends on pin 4
    if pin 4 was not removed carefully, and the pad is on the PWB for pin 4, then the spacing is
    1.75 mm from pin 3 to pin 4 and 1.75 mm from pin 4 to pin 5, or 3.5 mm

    if pin 4 was removed carefully, and the pad is not the PWB for pin 4, then the spacing is
    1.75 mm from pin 3 to pin 4 and 1.75 mm from pin 4 to pin 5, plus 2 mm for pin4, or 5.5 mm

    usual test like this ties pins 1,2,3 together, and 9 and 10 together and apply the 3.5 kV
    between 1,2,3 and 9,10
    how were 5 and 6 treated? primary or secondary?

    are there and scorch/burn marks to indicate where the breakdown occurred?


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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    how were 5 and 6 treated? primary or secondary?
    5 and 6 are primary bias.
    I am not sure if their were scorch marks, they just told us we failed.
    If it goes over 3mA when 3.5kv is applied for one minute then its a fail.....

    - - - Updated - - -

    ..presumably if we had wound all primary and secondary turns with TIW then we would have been OK?



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    Re: Which transformers can pass approvals testing rergarding isolation voltage testin

    With TIW used for secondary winding, the critical clearance is pin 9/10 to primary windings, including aux. Changing the secondary pins to 7/12 slightly increases the clearance. Or use additional tape.

    ..presumably if we had wound all primary and secondary turns with TIW then we would have been OK?
    Yes, but does it fit?


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