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LM5022: LM5022 flyback operation mode transition

Neo.Wang
Intellectual 840 points
Part Number: LM5022

Hi Experts,

When a very short and high current pulse occurs on flyback output, noticed the 12Vout has a dip and then the operation mode change from DCM to CCM for a few cycles to recover.

Is the operation mode change expected? 

During this very short CCM duration, the SW stress on pri MOSFET and sec Diode is quite high, which is exceeding the rated voltage of FET and Diode. How can this be mitigated?

Here are the waveforms.

Vin 60V, Vout 12.8V, turn ratio 1:1, primary inductance is 8uH.

Yellow- voltage across the secondary diode (Postive tied to Cathode).

Green- flyback output voltage

Red- flyback output current spike

zoomin.

further zoom in.

Here is the schematic.

Thanks.

  • 0
    TI__Genius 13655 points

    Hi Neo,

    Due to bank holiday, please expect a reply by Friday.

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    TI__Genius 16245 points

    Hello Neo,

    Thanks for reaching out to us via e2e.

    DCM and CCM are not different "modes" of our controller.
    They are just the consequence of different timing.
    When more energy is needed at the output (e.g. due to a sudden increase of the load current). the low side FET will be turned on for longer to store more energy in the inductor.
    So it will also take longer to consume this energy and it will not go down to zero before the end of the cycle.
    This is the expected behavior / response of the regulation.

    To limit the current, you should change the current sense resistor, so that the maximum is below the max rating of the FET and diode.

    In addition, you can try to change the voltage feedback loop so that it will react slower on sudden changes.


    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 840 points

    Hi Harry,

    When looking at the 2nd waveforms above, actually the duty cycle increased only when the sudden load current drops to close 0A, but not at the beginning of load increase. Is this behavior expected as well?

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Genius 16245 points

    Hello Neo,

    Yes, this is expected.
    The "inner" peak current control loop is fast and can react cycle-by-cycle.
    The "outer" voltage control loop is slower and will only react on voltage changes after a w cycles delay.

    The voltage control loop compensation should be trimmed to avid overshoots and to show a smooth response on sudden changes.
    This alone tells you that it cannot react as fast as the current control loop.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 840 points

    Hello Harry,

    For below statement, do you mean the COMP voltage change is a few cycles delay than Vout drops, thus the initial few pulses keep the duty (MOSFET on time) until COMP voltage increases?

    The "outer" voltage control loop is slower and will only react on voltage changes after a w cycles delay.

    As for the loop compensation, can you help check below values?

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Genius 16245 points

    Hello Neo,

    > do you mean the COMP voltage change is a few cycles delay than Vout drops, thus the initial few pulses keep the duty (MOSFET on time) until COMP voltage increases?
    Correct. The voltage feedback loop will react much slower than the cycle-by-cycle peak current control loop.

    R933 and C1100 should be calculated as for a regular booster (as if they were connected to the FB pin).
    R570, C401 and C1153 may not even be necessary.

    But anyway, the calculated values can only be a starting point.
    There are so many parasitics on a real board.
    Therefore, the ideal values for the compensation can only be determined by experiments and measurements on a real, physical board in the lab.

    Whatever you do, optimizing the feedback compensation will never give you a rectangular response on a change of the output load.
    And you have to avoid over-/undershoots and ringing of the switch node, as this would overstress and maybe even break your system.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 840 points

    Thank you Harry for the comments.

    Neo