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LM5022: LM5022 flyback over current protection

Neo.Wang
Intellectual 950 points
Part Number: LM5022
Other Parts Discussed in Thread: LM51551, LM5155

Hi there,

In my design, can you help calculate the OCP threshold per below Rsense used? Here Vin range is 9-60V, Vout is 12.8V/2A, transformer is 1:1 ratio and primary inductance is 8uH.

In case the flyback Vout is shorted to GND, then we power on the flyback circuit, how LM5022 protect the circuit?

If a OC happens like 0A->10A current step, how does LM5022 protect itself?

Thanks.

  • 0
    Intellectual 950 points

    Kindly provide the equations to calculate LM5022 flyback OCP threshold accordingly. Thanks.

  • 0 in reply to Neo.Wang
    TI__Mastermind 22720 points

    Hi Neo,

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

    Best Regards,

    Feng

  • 0 in reply to Feng Ji
    TI__Mastermind 19595 points

    Hello Neo,

    Thanks for reaching out to us via e2e.

    Here is a rough calculation:
    The OCP voltage threshold is 0.5V.
    With 0.02 Ohm this results in 25A peak on the primary side (throug the FET).
    When your input is 60V and your output is 12.8V, this converts to about 117A peak on the secondary side.
    On the other hand, when your input voltage is 9V, you will get 17.5A on the output.
    This would be 224W which is far too much for a Flyback.

    With a wide range of the input voltage, you have to select the current sense resistor in a way that the required output power can still be achieved at the lowest input voltage.
    Then select the rating of the other omponents according to the calculations for the high input voltage.

    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

  • 0 in reply to Harry Hofner
    Intellectual 950 points

    Hi Harry,

    Thanks. Incase primary OCP is triggered, will LM5022 latch off (Primary FET held off) or keep retrying to re-enable?

    If higher Rsense used to lower the OC threshold (e.g. 0.05ohm to give ~10A primary peak current limitation), how does the Rsen change impact the loop? 

    Thanks

    Neo.

  • 0 in reply to Neo.Wang
    TI__Mastermind 19595 points

    Hello Neo,

    There is no complex (long term) overcurrent handling inside the LM5022.
    It will only react on a cycle-by-cycle basis.

    So, it will not latch off and try again at every beginning of a new cycle.

    Rsense has to be adjusted, so that the maximum current can be handled by all components (inductor, FET, diode, but also the supply (battery).
    It will limit the amount of energy that can be transferred within one cycle.
    If there is a sudden increase of current on the output, it mya take more cycles to reach a stable output condition again.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 950 points

    Hello Harry,

    How the higher Rsense impact the loop compensation? Will it lower down the bandwidth and phase margin?

    As for isolated flyback with LM5022, is there any other controller can be used to replace LM5022 with latch off after OCP? Prefer lower OCP threshold to reduce the Ploss on Rsense and improve the efficiency.

    If yes, can we maintain the power components (Mosfet/transformer/diode/) while only do minor changes to external circuit for the new controller?

    Thanks

    Neo

  • 0 in reply to Neo.Wang
    TI__Mastermind 19595 points

    Hello Neo,

    Changing Rsense will not have an impact on the voltage loop and its compensation.

    The LM51551 does provide a hiccup mode.

    Please use the Quick-Start Calculator of the LM5155 to verify, if your existing power components will match.

    Best regards
    Harry

  • 0 in reply to Harry Hofner
    Intellectual 950 points

    Thank you Harry. Let me check on this.

    Neo