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LM5181: The snubber circuit doesn't suppress DCM ringing

Alessandro Taddei
Prodigy 85 points
Part Number: LM5181

Tool/software:

Hi Ti Team,

we are using LM5181 to create a 13V DC isolated output from a 24V AC input.

From the first test on the board I see that the circuit works quiet well (I can measure 13V output) but capturing the waveform on SW pin 1 of LM5181 It seems that the snubber circuit  (R39-C15, suggested by WeBench) doesn't  suppress the ringing of this pin.

I attached the waveform measured on SW pin1, the 13V output and the circuit schematic.

Can you help me on improving the snubber performance?

 Thank you 

Alessandro

 

  • 0
    TI__Guru* 78270 points

    Hi Alessandro,

    Thank you for designing with the LM5181.  Actually your waveform look pretty clean and the snubber works well. the SW "ringing tail" that you marked inside the 1.169us interval has nothing to do with the snubber.  This is the DCM mode SW resonance in dead time, which is after the secondary current falls to zero and before the next cycle starts. The transformer inductance resonates naturally with the capacitance seen at the SW node during the dead time.  It is normal.

    When you increase your load, this resonant tail will become shorter, until it disappears after running into BCM mode.

    The output ripples are mainly related to the measurement method, mostly unrealistic due to the long ground lead of the probe that picks up noise owing to antenna effects.  Please use a short ground lead probe as shown below to measure the actual output ripples. 

    In my opinion, your circuit works well.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 85 points

    Hi Youhao,

    thank you for your clarifications.

    Actually I know that the part marked inside the waveform is due to the ringing between the drain capacitance of the mosfet switch and the primary inductance of the transformer in DCM mode when the current on the secondary inductance goes to zero .

    Basing on my experience, the snubber role is to gradually dump the oscillations but in the captured waveform I don't see this effect (the amplitude of the two peaks are roughly the same).

    I attach a waveform of a very similar flyback(same output voltage, same input) from another supplier where you can see the snubber effect clearly (the amplitude during oscillations gradually decreases).

    My concern is about the radiated emissions that could be originated by the undumped ringing captured on SW pin 1 (I'm already struggling with EMI problems icreated by these oscillations on mosfet switch drain in DCM on other products ).

    Assuming that I follow all TI layout guidelines, how can I reduce these oscillations?

    Maybe I can increase the load (I have no power limit from the input) and enter BCM mode? Which is the current threshold for this?

    Thank you for your help

    Alessandro

  • +1 in reply to Alessandro Taddei
    TI__Guru* 78270 points

    Hi Alessandro,

    You snubber works well.  If it does not, you would see a lot of ringing in the circled duration, following the FET is switched off.  The below waveform is very good.

    Regarding the DCM resonant tail it is natural resonance in DCM flyback, and it cannot be removed unless you keep you circuit from running into deep DCM.  It is not because LM5181.  It is physics, and all flyback circuits in DCM behave like this.

    I understood you concern but I am afraid you may be over concerned. Could you run a quick EMI test and let me see if you need to take extra circuit or method to pass?

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Guru* 78270 points

    Hi Alessandra,

    Please allow me to close this thread due to inactivity.  If you still need support please feel free to reopen by adding a new post, or start a new thread.

    Regards,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 85 points

    Hi Youhao,

    for the moment we can consider the topic closed because we are far from the  EMC certification phase of the board.

    I'll open another thread in the future focalized on EMC issues if necessary.

    Thank you

    Alessandro