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  1. #21
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    Re: Off-the shelf transformer for push pull converter

    Yes..the 1:1 core you showed me in post #18 above cannot do 5vout from 5vin in pushpull connection...neither with currnt doubler connection, nor split coil output.
    The attached ltspice , which is pranged to be on abs max duty cycle, shows it cannot do 5vout from 5vin

    Do you still want to do the pushpull now?

    The chances of finding one with the right turns ratio, i woudl have thought, is very slim...

    flyback instead?

    - - - Updated - - -

    anyway, i think you get the general picture, you do di = V.dt/L to get the magnetising current rise...then check its not greater than 300mT.


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  2. #22
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    Re: Off-the shelf transformer for push pull converter

    @ c_mitra:
    1.) That can only be checked by means of a prototype, as only the off-the shelf transformer is usually tested at a specific frequency, see [1]. E.g. [2] does not state any frequency at all.

    2.) How should I estimate the power handling capability of the core? An off-the shelf transformer doesn't give information of the used core material, see [1], [2].

    3.) As the title of this thread indicates, an off-the shelf transformer should be used. As the maximum flux is determined by Φ_max = V_RMS • sqrt(2) / N, it is hard do calculate the maximum flux as the number of windings are usually not stated, see [1], [2]

    @ treez:
    I am awer of the lower secondary side voltage, which is caused by the non-idealities of the transformer. Further, diode voltage drop and switching MOST voltage drop has to be taken into account. Thus a slightly higher primary voltage should be used, or the secondary electronics should be operated at a lower voltage level e.g. 3.3V. That has not be defined until now.

    Thank you for the Spice file, I will try it in more detail and I will come back to you. THX!

    [1] https://www.we-online.de/katalog/dat.../750315090.pdf
    [2] https://products.pulseelex.com/files...files/P730.pdf



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  3. #23
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    Re: Off-the shelf transformer for push pull converter

    Both of them DO mention the frequency in passing somewhere. For example, ref [2] does mention 200 kHz in the diagram (connection; last col)

    In the same way, you can roughly estimate the power from the electrical specs given: for the first one (ibid) it says 12V at 600 mA and we can estimate that the max power will be around 7-8W.

    I do not know the intended application, but making a prototype does make very good sense.


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  4. #24
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    Re: Off-the shelf transformer for push pull converter

    Stenzer you are rightly being conscientious. Many people would simply make a prototype, paint it with black matt paint, and then have a “Look” at it through the thermal camera to see how hot it was….in fact, many would simply touch it on full power and if it wasn’t too warm they would pass it.
    In other words, people would not be as conscientious as you.

    They might even make up the smps with the offtheshelf sample...and then find the efficiency...and they would then guesstimate how much of the loss was in the transformer...and then they would kind of make an experienced judgement call over whether it was ok.

    Perhaps they would also make up a prototype......put it in a thermal chamber at max ambient, and at min vin and max looad...and see if the power supply survived 24 hrs in there and scuesfully produced correct vout and correct power...if it survivied, they would give it the thumbs up.


    What you really need is a ferrite datasheet like the following DMR40 one…then you can look at the graphs on the second page…the top four ones. You can compare these with known ferrites that you have used before.
    You really also should have a full transformer datasheet as in the attached. It tells everything from the wire used, to the exact ferrite material used etc etc

    …but if you are going offtheshelf…you will never get this information…because if they gave it to you then you would go to a transformer manufacturer and get the transformers made for you and probably/possibly find out its cheaper than buying off the shelf….
    So good luck…you can ask the manufacturer for the type of datasheets that I have given you here…but they will not give them to you , I am sure….sorry….as you know…its called Business unfortunately
    Last edited by treez; 10th April 2020 at 20:18.



  5. #25
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    Re: Off-the shelf transformer for push pull converter

    Also, your SMPS is just 5W....even if only 85% efficient...thats just 750mW of total loss....even if half of that was in the transformer.......its only 375mW.....i reckon most of the ones youve shown will be OK...unless your ambient is really hot.
    Two 1206 resistors could handle 375mW and some...so i reckon those transformers you've shown will be OK.

    You can see if it saturates by looking at the current ramp in the sense resistor

    - - - Updated - - -

    Sorry not to be able to offer great help here….but such does not exist…the info you want has deliberately not been supplied…because now of course you need to contact the apps department of the OTS txfmr company…..and then they can cultivate you and manipulate you into some ridiculously expensive deal for supply of their OTS transformers….they will take all your spec off you and supply you with an “OTS” txfmr (which they got custom made to your spec)…and charge you a fortune for it…when all along, it would have been cheaper for you to do a custom wind…because then you control the spec…you own the spec, and you can take it to any winder you like…the cheapest one.



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  6. #26
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    Re: Off-the shelf transformer for push pull converter

    They might even make up the smps with the offtheshelf sample...and then find the efficiency...and they would then guesstimate how much of the loss was in the transformer...and then they would kind of make an experienced judgement call over whether it was ok.
    Best suggestion that always works when the information is in short supply. Be slightly conservative in your design and you should be ok.



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