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LM5180: RCD snubber

K.Hamamoto
Guru 12155 points
Part Number: LM5180

Hi

I would like to ask about RCD snubber at primary side. Regarding the snbber, it's describing at page 20 on datasheet.


Can we use RCD snubber instead?

At first , they added RC snubber as it's recommended on datasheet. But this configuration had not effected enough to the surge of FET.

(As the another background, the components they can use at their company is limited.)

So that , they tried to use RDC snubber but the switching waveform become unstable frequency at light load condition.

RCD Snubber: R=10kohm , C=4700pF , D= FRD

(in addition , they added RC snubber in parallel. R=18ohm, 470pF without DZ clamped. But it does not effect.)

Is not RCD snubber acceptable for LM5180 ?

Best Regards,

Koji Hamamoto

  • 0
    TI__Expert 8245 points

    Hi,

    It's OK to connect a RC snubber circuit across the flyback diode, and please just use the snubber value as datasheet recommends as a start point, which are 100 Ω and 22 pF.

  • 0 in reply to Iven Su
    Guru 12155 points

    Hi Iven,

    Thank you for your prompt reply.

    I understood about RC snubber. My question was about RCD snubber as bellow.

    Is the RCD snubber possible stead of RC snubber?

    Because if our customer was added RCD snubber at primary side then the switching become abnormal.

    Best Regard,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    Guru 12155 points

    Hi,

    Do you have any idea about RCD snubber for LM5180?

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Expert 8245 points

    Hi,

    Could you please help probe the abnormal SW node voltage, thanks.

  • 0 in reply to Iven Su
    Guru 12155 points

    Hi Ivan,

    Here is SW pin waveform.

    This look shows SW node does not stable at light load condition with RCD snubber.

    (At heavy load condition , the waveform will be same between without snubber and with RCD snubber. )

    So, that's why I am asking whether LM5180 accept SCD snubber instead of RC snubber.

    I suspect RCD snubber causes the unstable of SW node detection.  What do you thnk?

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    Guru 12155 points

    Hi Ivan,

    In additional question.

    I think the snubber may affect something to specification of Flyback operation.
    （efficiency or rise/fall time etc)

    If LM5180 detects SW node voltage in few second after FET turn off and ON time is defined by that voltage, RCD snubber may causes by affecting that detected voltage then the switching jitters in that case.

    How does LM5180 define ON time?

    If LM5180 monitors SW node voltage to define ON time, does LM5180 has blanking time?

    If so, that time changes at light load and heavy load condition?

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Expert 8245 points

    Hi,

    Do you have a  dummy load in the output? What the no-load output voltages with/without RCD snubber? Thanks.

  • 0 in reply to Iven Su
    Guru 12155 points

    Hi 

    There is no dummy load in the output.

    And the output voltage at no-load are;

    without RCD : 15V

    with RCD : 16V (the output is a little higher than without RCD.)

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Expert 8245 points

    Hi,

    Then what's your target output voltage? Please apply one 1kOhm dummy load in output to see if the converter with RCD works well.

  • 0 in reply to Iven Su
    Guru 12155 points

    Hi

    >Then what's your target output voltage? 

        The target output voltage is 15V.

    >Please apply one 1kOhm dummy load in output to see if the converter with RCD works well.

    We already added dummy load what the load current flow 100mA and measured the switching.

    The switching waveform become same both with RCD and without RCD at this condition.(It means RCD works as we expected. Only surge peak voltage reduced.)

    (On the other hand, at Io=30mA, the switching waveform(with RCD and without RCD) is different as I asked above.)

    That is why I asked above question whether the detecting time at light load condition is shorter than heavy load condition.

    Best Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    Guru 12155 points

    Hi Ivan,

    Do you have any update for this?

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    TI__Expert 8245 points

    Hi,

    OK, it looks you are not recommended use RCD snubber in your light load application because the SW voltage is important in feedback loop but RCD circuit will change its behavior when light load.