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SMPS transformer core getting hot advice

  1. Jun 18, 2016 #1
    I had a bit of rattle about this here before but I hope you folks ain't fed up with me yet.:)
    So after some trial and error i've built a working smps for my amplifier , it's pretty high power ,when testing dummy loads i got up to 2KW , since I don't need more for my project I never went further as I had no need.
    It's not regulated but it dropped some 15 volts at max dummy load so works for me.

    The problem is this , after trying out some leftover ferrite cores simply to understand that there's no luck with them I bought a new EPCOS made core.It's the ETD59 , the material is N87 with permeability of 5300nH.
    the size of the core is 59.8 x 31.2 x 22.1mm

    Based on other smps using the same materials I made 12 turns in the primary (maybe 11 can't remember)
    And the secondary even though doesn't play a role in this problem is some 2x7 turns.the primary is wound with some thick litz wire salvaged from some old smps.Haven't exactly measured but I would say some 3 mm2.
    the transformer frequency is +- 50 Khz, driven by a pair of half bridge IGBT'S.


    The problem is this , under load the core heats up , I kind of predicted this so I made my heatsink such that it also touches the core , in other words I have an aluminum box with large surface areas and everything is thermocoupled to the box.
    But still I think the core gets too hot even with lighter loads , something probably isn't right.The windings themselves stay cool , only slightly warm but then it could also be the effect from the heat given off by the core.

    When I was younger i made a few mains transformers with hand and I once got my turns calculations for the primary wrong and ended up winding fewer than necessary turns , but back then the windings got warm not the core , since there wasn't enough inductance and the wires went through some resistive/ohmic heating on each mains cycle.

    So I wonder what could be the problem , my own guess is too few or too many turns in the primary, rather too few but I'm not sure.
     
  2. jcsd
  3. Jun 18, 2016 #2

    Baluncore

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    A heatsink on a ceramic core will likely fracture the core due to differential thermal expansion between the hot and cooled regions. Cracks in the core will reduce both thermal and magnetic conductivity. You are better to cool the entire transformer in free or forced air.

    Using thinner wire will permit a smaller window and so a shorter magnetic circuit on a different core.

    An optimised design will have a core temperature the same as the windings.
    If the core is very hot and the windings cool then either;
    1. The core material is not optimised for the selected frequency of operation.
    2. The flux in the core is too great. Increase the section area of the core.

    Have you investigated adjustment of the air gap in the core assembly ?
     
    Last edited: Jun 19, 2016
  4. Jun 18, 2016 #3
    Well my core has no air gap, the two E sides match together with no gaps and I did not make one since according to what I know it's not needed.I just put them together and the whole core is then kept together by a adjustable metal squeezer so the core is pretty tight held to the side of the heatsink which also works as its support.

    the frequency range is noted as 25 to 500khz if i'm correct so 50khz should be good enough.
    Not sure what to do , the thing is the very transformation is working good enough, the IGBT's run barely warm , the heatsink for the smps in general is barely warm , everything is barely warm except the ferrite core which after some 20 mins of music through a light load speakers got pretty hot to touch.

    I think I have 11 turns of primary now , should I try to take one off? Would not be that complicated since I made everything such that I can access the critical parts.
     
  5. Jun 18, 2016 #4

    Baluncore

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    EPCOS ETD59 in N87 is available with a gap of 0, 1.0, 1.5 or 2.0 mm. One or both of the E core halves provided may have the gap as a shorter centre pole. You will probably not notice it without a straight edge across the faces of the E core poles.

    The inner pole is shortened so as to avoid flux leakage from an external gap. You can experimentally adjust the gap by using a shim of non-magnetic material that will survive the core temperature.
     
  6. Jun 19, 2016 #5
    Oh now I remember , the cores did come in air gap sizes and I bought the one that has no airgap.
    prior to assembly which is rather straight forward , I looked at the two half sides put together and there wasn't light coming through the middle pole on which the bobbin sits so for all practical considerations there is no air gap in this core.
     
  7. Jun 19, 2016 #6

    anorlunda

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  8. Jun 19, 2016 #7

    Nidum

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    Have you considered the possibility of there being a shorted turn somewhere in your metalwork ?
     
  9. Jun 19, 2016 #8
    What do you mean by shorted turn in my metalwork? If you think a shorted turn in either the primary or secondary winding , then no because given such few turns I would immediately notice the difference in output voltage.
    Output voltage hasn't changed since the first day I wound this particular transformer.

    thanks for the link @anorlunda , I will check it out.
     
  10. Jun 19, 2016 #9

    Nidum

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    A shorted turn results when any combination of metal parts in the transformer assembly forms a complete loop around the transformer core . Most commonly loop consists of the transformer centre bolt and external connections via mounting brackets , chassis , earthing straps and heat sinks .
     
    Last edited: Jun 19, 2016
  11. Jun 22, 2016 #10
    Today I plugged it it once more and listened to some music , wasn't long some 15 mins. maybe bit less and the core got to the boundary between very warm and hot.
    So before I take the transformer out and start solving the problem would your 'consensus" be that I need to add an extra or two turns to the primary winding?
    I now have 11 turns in the primary.
     
  12. Jun 22, 2016 #11

    Baluncore

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    I would first put an 0.2mm insulated spacer between the outside limbs of the E cores and test it again. That will identify if the gap is important in the design.
     
  13. Jun 22, 2016 #12

    jim hardy

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    Can we put a number on your flux density ?
    On back of an envelope
    how many volts per turn from your transformer? 200/11 = somewhere around 20 ?

    Now to get 20 volts in one turn requires by e=ndΦ/dt
    around 20 Webers per second ? Twenty microWebers per microsecond ?
    At 50 khz, a half cycle is ten microseconds ?
    So your rate of change of flux is 200 microwebers per half cycle? Meaning flux likely swings from -100 to +100 microWebers?
    What is area of your core ?
    Flux density is ~ 100 microWebers divided by that area....

    Repeat with your real numbers......

    How does your result compare to this rule-of-thumb 500 to 2500 Gauss from Magnetics ?
    http://wcmagnetics.com/wp-content/uploads/2015/02/appnote11.28.10.pdf
    That's 0.05 to 0.25 Teslas if i remember my SI ....

    Better calculation procedure here but it'd take me a week to learn.
    http://ecee.colorado.edu/copec/book/slides/Ch15slides.pdf

    old jim
     
  14. Jun 22, 2016 #13
    Since it's a half bridge and my mains is 230 AC , the rectified DC is 325 volts of which the primary sees only half so about 160/11 would be closer.
    I will try to do the maths after I come home after two days.

    Well I like your suggestion Baluncore , mostly because it's the first easiest things to do.I wonder what kind of material I could use for the spacers to separate the side legs to get the air gap in the middle one ?
    Ordinary cardboard? thick paper?

    Also can you briefly explain what does the air gap in a transformer actually does? All I know in low frequency transformers or audio output transformers it only decreases performance.I assume since air is much much worse at conducting flux or permeability the air gap would kind of "brake" the magnetic field a little?
    Isn't the magnetic field also stretched in the place of air gap due to the field lines being concentrated in the core but as they leave they spread?
     
  15. Jun 22, 2016 #14

    jim hardy

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    160 / 11 = 14.5 volts/turn
    14.5 microwebers per microsecond
    145 microwebers per half cycle is Δflux
    half that is 72.5 microwebers

    22.1 X 31.2 mm is 0.000689 m2
    72.5 microwebers divided by 0.00068952 m2 = 0.105 Tesla ? 1050 Gauss ? Check my arithmetic.?.....

    Does your core datasheet suggest an operating flux ? I couldn't find N87 curves.

    old jim

    EDIT found one
    http://en.tdk.eu/blob/528882/download/4/pdf-n87.pdf

    FluxN87core.jpg
     
    Last edited: Jun 22, 2016
  16. Jun 22, 2016 #15
    I will continue this after I get back as I have to go on a small trip today.
    My parameters in short are 50khz , 160v primary with 11 turns , the duty cycle is rather large , almost to it's fullest which would be 50% if i'm correct , well that's about it , yes it's an N87 core made by EPCOS.
     
  17. Jun 22, 2016 #16
    Unless I messed up somewhere, I get 40W to 50W in core losses, which would explain the temperature rise. This is based on Jims calculation of core area, the core diameter in the OP, and the graph of losses Jim posted. Please double check this, it's late & I'm tired.
     
  18. Jun 25, 2016 #17
    thanks Tom for contributing, About 40/50watts of core loss is overly high for such a small ferrite core which cannot dissipate heat not nearly as good as larger iron cores and even then the losses would be bit high.

    I wonder Tom can you write down the math that you used to come up with these losses ?I want to understand which variable has to be changed to get smaller losses as I am sore these cores are capable of some 1KW in power through without getting so hot.
     
  19. Jun 25, 2016 #18

    jim hardy

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    I had better luck with searches this morning

    ferrite_epcosTD59.jpg

    Well now

    72.5 microwebers divided by 368 mm2 X 106 mm2 / m3 = 0.197Tesla ? 1970Gauss ?

    Which puts us on the ~200 milliTesla curve instead of where i drew the oval in earlier post

    in fact let's use the TDK N87 curve i stumbled across just a couple minutes ago
    it's from their 625 page catalog at http://en.tdk.eu/blob/519704/download/2/ferrites-and-accessories-data-book-130501.pdf , page 84
    and there's tons of info there, download a copy for your e-library....

    FluxN87core2.jpg

    maybe 275kw/m3 ?

    275 kw/m3 X 51200 X10-9 m3 = 14 watts ?
    Still seems a lot for such a small core.

    You'd better check my arithmetic.
    They say those curves are for sinewave and i dont know how to adjust them for square wave.
    But this seems a reasonable approach for making an estimate. It's not like we're building a Steinway here .

    I learned a lot today, thank you Tom.G and Salvador !

    old jim
     
    Last edited: Jun 25, 2016
  20. Jun 25, 2016 #19
    Well you don't have to say thank me, Jim I'd rather have to say thank you as I am the one puzzled by this.
     
  21. Jun 25, 2016 #20

    jim hardy

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    @Tom.G Does this look right ?
    FluxN87core3.jpg


    @Salvador I dont know how difficult it is to add primary turns.
    You've got me curious so i hope you try it.. Is this heat just from flux amplitude or something else ?

    old jim
     
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