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LM5020: TL431L

pooja kattimani
Prodigy 80 points
Part Number: LM5020


Tool/software:

Hello , I have designed duel output Isolated DC-DC Flyback converter using LM5020_1  . when load connected the unregulated output (Vout1)voltage is drooping.

can suggest how to overcome this problem.

Vin=36-75V

Vout1=12V

Iout1=1A

Vout2=12V

Iout2=3A

Fsw=100KHz

  • 0
    TI__Mastermind 19715 points

    Hello Pooja,

    Thanks for reaching out to us via e2e.

    In general, the cross-regulation between two secondary windings will always have limitations.

    Can you please specify if you have also connected a load on the regulated output?


    If this is not the case, the output capacitors of the regulated output are always full.
    So, the controller will always see that the desired output voltage has been reached.
    and it cannot react on a load change on the unregulated output.

    When there is some reasonable power consumption on the regulated output, the unregulated output will "steal" some of that energy.
    In that case the controller will need to send more energy to compensate for that.


    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Prodigy 80 points

    Thanks for your response,

    both regulated and unregulated outputs are drooping when it is loaded individually.

    Should I consider redesigning the feedback and compensation circuit? if in that case which value i need to tune .

    feed back section circuit  image attached below .

  • 0 in reply to pooja kattimani
    TI__Mastermind 19715 points

    Hello Pooja,

    I am sorry that I need to get back to you on this again, but we need a more precise description.

    From your first / original case:

    You start with no load on VOUT1 (unregulated) and no load on VOUT2 (regulated). In this case both output voltages are good.
    Is that correct?
    Then you apply some load (1A) on VOUT1 and still no load on VOUT2. Is that correct?

    In this case, VOUT1 is dropping. Is that correct?

    What happens to VOUT2 in that case?


    From your second description:

    >> both regulated and unregulated outputs are drooping when it is loaded individually

    What exactly does that mean?

    When you apply a load to VOUT2, it is dropping and the controller cannot compensate for it?

    Is VOUT2 then permanently too low and will never reach the target of 12V again?

    What happens to VOUT1 in that case?
    Will it also drop?
    Will the dropped voltage be about equal to VOUT2?


    >> both regulated and unregulated outputs are drooping when it is loaded individually
    Does this mean:
    - when you apply a load to VOUT1, BOTH output voltages are dropping?
    - when you apply a load to VOUT2, BOTH output voltages are dropping?

    Please describe in a detailed way.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Prodigy 80 points

    hello, thank you for your reply, kindly find answers to the questions.

     You start with no load on VOUT1 (unregulated) and no load on VOUT2 (regulated). In this case both output voltages are good.
    Is that correct?

    >> NO, VOUT1 (unregulated)=10V & VOUT2 (regulated)=12V


    Then you apply some load (1A) on VOUT1 and still no load on VOUT2. Is that correct?

    In this case, VOUT1 is dropping. Is that correct?

    What happens to VOUT2 in that case?

    >> I applied some load (500mA) on VOUT1. VOUT1 is dropping and no drop in VOUT 2 it is constant 12V


    From your second description:

    >> both regulated and unregulated outputs are drooping when it is loaded individually

    What exactly does that mean?

    When you apply a load to VOUT2, it is dropping, and the controller cannot compensate for it?

    Is VOUT2 then permanently too low and will never reach the target of 12V again?

    What happens to VOUT1 in that case?
    Will it also drop?
    Will the dropped voltage be about equal to VOUT2?

    >> when VOUT2 Loaded, no drop in VOUT1 and VOUT 2


    >> both regulated and unregulated outputs are drooping when it is loaded individually
    Does this mean:
    - when you apply a load to VOUT1, BOTH output voltages are dropping?

    >> when VOUT1 Loaded, VOUT1 is dropping & no drop in VOUT2 =12V is Constanta


    - when you apply a load to VOUT2, BOTH output voltages are dropping?

    >> when VOUT 2 Loaded, VOUT1 =12V & VOUT2 =12V both Constanta

    Thanks and Regards,

    Pooja

  • 0 in reply to pooja kattimani
    TI__Mastermind 19715 points

    Hello Pooja,

    Thanks for your answers.


    The fact that VOUT2 is always stable at 12V shows us that the feedback loop is working fine and no matter which load you apply on which output.
    The regulated output VOUT2 is properly handled by the controller.

    Output VOUT1 can only be handled indirectly.
    As described earlier on, the unregulated output will "steal" some of the energy from VOUT2.
    In that case the controller will need to send more energy to compensate for that.

    This concept can only work when there is a proper coupling between the two secondary windings of the transformer.

    So, it appears that there is only a loose coupling between the two secondary windings in the transformer that you have chosen.
    VOUT2 does not see too much of an impact when a load is applied to VOUT1, and so there is no need for the controller to compensate for anything.

    There is nothing that you can do against it, except for choosing a different transformer with a better coupling.


    All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults” and is subject to TI’s Important Notice (www.ti.com/.../important-notice.shtml).

    Best regards
    Harry