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  1. #1
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    Problem with flyback SMPS

    i trying to take 1 A current from my smps but it gives up to 200 mA.

    After looking a lot of source, i made a transformer : its tronsformers primer 132 turn and approximately 0.3 mm 1 strand, seconder is 11 turn and approximately 0.4 mm 7 starnds !
    Our country line voltaj is 220 Vac.
    The SMPS gives 24 Volt and higher voltaj but the current not high. I tryed to take 12v only and hope current may be high but it didnt i hope.

    Anyone give me any idea or comment what is wrong ?

    is the problem transformer, controller frequency, power switch etc ?

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    flyback smps simple

    Could you post also the calculation that have you did to size the coupled inductors?

    Also I can't see from your schematic the supply line for the controller when the converter have run the startup, usually this will be done by using a auxiliary windings.

    Bye
    Powermos



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    ferroxcube flyback transformer design

    Only value about transformer is primary inductor and it is aproximatly 1mH. The turn numbers is actualy aproximatly from many calculation and some sources, because the calculations for transformar are too many and complicated.

    Uc3842 power pin 7 and during the operation and start up it is run the over the 75k resistor and line voltage, it works.



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    how the smps really works

    Dear mesutyeni

    Please click on the LINK . Here you will a practical circuit. (you forgot to write the core material). Anyway Most of the ferrite can do the job, but Ferroxcube 3C8 core will be good. But do remember to have a winding for converter power supply. I used 220K instaed of 75K. My output from the approx same circuit as in the link above was 16V 4A. It is working fine



  5. #5
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    ferroxcube india

    Here is one example of an SG3524-based supply:
    http://hmin.tripod.com/als/andysm/pa...supp8.html#sg3

    I have more somewhere :), will add them as soon as I find them.



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    secondary turns calculation for care smps

    Hi,
    One more important think ,you have to take care of winding polarity.

    and also current sense resistor 1.5 is more than for expected output max current



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    uc3842 high current smps

    Hi,
    grittinjames has a VERY important point for a flyback (or for any SWMPS topology too), the start of your secondary winding should be opposite to your primary winding, if the start of the primary is connected to the MOSFET make sure that the start of the secondary is at the rectifier diode end. If it is wrong it will not work correctly at all, try swapping the connections of ONE of the windings if you are not sure. A flyback design uses the energy storeded in the core inductance to dump into the secondary winding when the primary input is NOT being energised.
    Regards,
    Bob.



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  8. #8
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    al value smps transformer

    can you give the full spec like

    min i/p volatge max i/p voltage and out voltage, current i ll verify your design.....

    darla



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    inductor current sensing in flyback

    give the full specification

    or

    try this tips

    1.decrease the trans former primary inductance.

    2.decrease the current sense (mosfet ) resister 1.5Ω value ( Note: decrease this resister value some times output short circuit condition mosfet was damage)

    give the your requirement ( like input voltage range, load max. current etc...)



    :D



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    flyback core to big

    Check your diodes. I think they may be inadequate in terms of reverse voltage.

    I would expect the reverse voltage on the secondary diode to be well in excess of 40V (spec of 1N5822).

    Also the 1N4007 should probably be replaced by a fast rectifier.

    oddbudman



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    use flyback transformer core smps

    thanks to everyone.

    i test all of your suggestion. it work better than before.

    its current now 500-600mA. But heat of transformer and mosfet i think too much.

    specifications :

    Vin : 90-220 Vac ( i think smaller Vin cause smaller inductance and smaller transformer )

    Vout : 15 V and 0.7-1 A
    swiching frequency : 100 Khz ( because smaller transformer )

    Added after 11 minutes:

    --Transformer polariyties are true i think, i tested and if reverse it the voltaj out smaller.

    --I add auxiliary windings for supply 13v to controller. ( add 220uF big cap)
    And it is really works , out current was larger 2-3 times before.

    --I decreased the current sense (mosfet ) resister 0.5Ω value ( two parallel 1 Ω 5w)

    -- i decrease primary inductance by turn number 80. New transformer calculations based on ST flyback transformer design paper ; for Ef20 Ae:0.315 cm2 and for EF25 Ae:.395cm2, so primary inductance 650 uH. From different source calculations inductance was 1mH.

    - i replaced the seconder diode 3A schotky after a couple of break down.



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    how to find flyback primary winding

    give me the transformer core details (like EC,EI,ETD series)

    decrease the primary turns by adjusting the air gap



  13. #13
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    offline flyback smps

    --I buy a PQ core , i dont know its manufacturer, but dimensions as remember A1 26 , A2 20. i read their Ae value grater ( 1 cm2 ) so made a transformer ; 35 turn primary and 8 turn secondery so its work too. And i think its heat smaller than EF core but still hot.

    i dont know its true ; PQ used for flyback smps , their efficiency, stability and long usage warranty.

    --Most important problem is Mosfet, i saw a lot of blow up and their cost. i dont know the reason heat, oscilation , over current or voltage.

    Added after 7 minutes:

    acctualy i dont know its parameter because the store i buy them also dont know their manufecturer.
    i uses aproximatly value only Ae 0.315 because its dimensions approximatly same EF20 and Ae:0.395 for EF25 core.

    Added after 55 seconds:

    I have with gap and no gap versions



  14. #14
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    flyback smps + high efficiency

    May be nice if you can publish all the schematics and the calculation for the power supply elements (also the magnetic one) or better write a simple tutorial for other users.

    Bye
    Powermos



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    calculating power loss startup resistor smps

    Hi,
    The destruction of the MOSFET could be due to the fact that your core is not gapped, if the core saturates this can happen. An offline flyback transformer core should really be gapped, it is not easy to calculate the optimum gap without the core parameters, which you do not have but you can easily put a gap into it by inserting a thin piece of high melting point plastic sheet between the two halves of the 'E' core at each contacting face, this will put 0.6mm of gap into the core (2 * 0.3mm as it is in two places), normally the gap calculates somewhere between 0.2 and 1mm. Try about 0.6mm as a start, this will help to ensure that there is no core saturation on voltage peaks. (you may need a few more primary turns with a gapped core incidentally as this does lower the efffective Al value of the core). What switching frequency are you using (I calculate 150kHz from your circuit), if this is too high for the core this will have a high core loss and hence get hot, your MOSFET should not really get too hot with the loads that you have on your circuit.
    Regards
    Bob.



  16. #16
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    Flyback SMPS help

    Just rewind the transformer, Im sure overheating will reduce efficiency.
    1. Change 4007 to high speed diode in primary clamping circuit. Select PIV at least 4 times (1KV).
    2. Rewind transformer to avoid insulation failure. Short circuited loop may cause induction heating and produce excessive heat.
    3. Check one of your output diode, replace it if require. you can use SR360 also.
    4. Adjust air gap. Too much air gap will resulta energy loss in heat. too small will reduce frequency. Dont use metal foil in air gap. it will work as induction heater.

    Till you reduce heat, your SMPS is not currect
    Thanks



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