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    convert voltage control to current control

    hi
    in this schematic i want to use uc3846 instead of sg3525 and control the primary current.so i need to add a sense resistor (and rc filter)and use suitable caps and resistors which are in datasheet to run uc3846.
    but is that needed to change other schematic parts?like add or remove other components?
    Click image for larger version. 

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  2. #2
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    Re: convert voltage control to current control

    you can't use current mode with an half bridge - time to do a little research ...


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    Re: convert voltage control to current control

    Quote Originally Posted by Easy peasy View Post
    you can't use current mode with an half bridge - time to do a little research ...
    i do search and realize that current mode can use with an half bridge but it is a little complex.
    i use the same sg3525 and add an external current limiter and it worked.
    but i have a problem:
    output voltage isn't double and is single 32 volt 20 amper.core is etd44 and pc40 material.primary winding is 30 turn litz wire and secondary is 7 turn litz wire(include 21 number 0.55 wire)but at 20 amper after 1 min secondary winding become very hot and i can't use it more than 2 minute.
    what is problem you think?



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    Re: convert voltage control to current control

    you haven't - in the schematic, connected the the CT of the Tx to the zero volt of the output caps...! this will cause problems ...

    you need more copper on the sec side by the sound of it - can you post pictures ...?

    30T : 7+7 from 230Vac rectified mains is 280VDC : 65.3 + 65.3 -> too high ...? ...



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    Re: convert voltage control to current control

    Quote Originally Posted by Easy peasy View Post
    you haven't - in the schematic, connected the the CT of the Tx to the zero volt of the output caps...! this will cause problems ...

    you need more copper on the sec side by the sound of it - can you post pictures ...?

    30T : 7+7 from 230Vac rectified mains is 280VDC : 65.3 + 65.3 -> too high ...? ...
    output isn't double and is single 32 volt.
    in half bridge primary voltage is 155 volt.transformer is 30:7 so secondary voltage is 36 and set to 32 volt by feedback.
    secondary wire is 5mm2 and current density is 4 A/mm2 so i think its enough and that extereme hot coming from different reason that i don't know and need help



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    Re: convert voltage control to current control

    you show a centre tapped output in the schematic above,

    are you saying 21 x 0.55 dia = 5mm^2, OK, but if you have a simple sec over pri wdg construction, then you will get losses due to the 50kHz,

    for 600-700 watts you need to interleave the Tx wdgs to reduce these HF AC losses ....


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    Re: convert voltage control to current control

    i do search and realize that current mode can use with an half bridge but it is a little complex.
    Please read the attached



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    Re: convert voltage control to current control

    I have done for you here a Half Bridge SMPS with vin =310vdc, and vout = 32v and iout = 20A.
    I did it in current mode, as current mode with slope compensation does not suffer any instability for a half bridge on the LTspice simulator. Just convert it to voltage mode for use with a voltage mode controller…some of which I sent you earlier.
    For the transformer, I have used your ETD44. A Ferroxcube one.
    I wound the primary with 24 turns of 2 pieces of 7/0.3 TEX-ELZ triple insulated wire. (2 strands in parallel)
    There is a split secondary, and each secondary comprises 10 turns of [5 x 0.5mm] Enammelled copper wire. (5 strands in parallel)
    I would sandwich the primary between the two secondaries.
    I gave it a Sync FET output, as in the sim and schem.

    (attached is the ltspice simulation, schematic, and brief calculation document)

    (By the way if you dont have enough input capacitance to keep the vin at 310VDC , and it drops significantly below 310v every 10ms, then it will need to be re-done with the lowest input voltage used as the design input voltage parameter)

    for 600-700 watts you need to interleave the Tx wdgs to reduce these HF AC losses ....
    Thanks, ...May i ask do you mean the primary interleaved inbetween the two secondaries, as i have done here?

    - - - Updated - - -

    BTW, the reason you cant do a half bridge in current mode is because the rail splitting input capacitors wont actually be of exactly the same value due to tolerance.....so you will get one charging up more than the other and unbalancing it.
    Last edited by treez; 13th December 2019 at 18:22.



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    Re: convert voltage control to current control

    We have discussed here how Current mode control can’t be used in a half bridge SMPS.
    However, the following document states that a “Current Doubler rectifier “ can be used with a Half Bridge SMPS…………….
    https://www.ti.com/lit/an/slua121/slua121.pdf

    ..this same document also states that “Current Doubler rectifiers “ must be used with Current Mode control, otherwise the currents in the output inductors will get un-balanced.

    It looks like there is a contradiction in this document?



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    Re: convert voltage control to current control

    There is - or should be - no contradiction - you can use a very small amount of current mode control in an half bridge ( but not too much ..! - the rest of the ramp comes from Vin - suitably clamped) - you can use current doubler output with half bridge and average current control ( or volt mode ) - we have a product that does this ...



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