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Original question:

LM25118EVAL: Radiated noise of LM25118EVAL

LM25118EVAL: snubber

user6144845
Prodigy 160 points
Part Number: LM25118EVAL
Other Parts Discussed in Thread: LM25118

user6144845
LM25118EVAL: Radiated noise of LM25118EVAL

This is a related question.

Hello.

Although the noise due to the snubber could be reduced, the output voltage fluctuates when the input voltage is set to 30V or higher.
There is a voltage drop of VCC.
As I announced in the previous question, a snubber circuit (10Ω + 2000p) is inserted between GATE and SGND of Q1-Q2 of FET.
Please comment on the insertion location and constants.

  • 0
    TI__Mastermind 46971 points

    Hi Michihiko-san,

    thank you for your question and using E2E.

    You are mentioning that this is a related question, can you please add a reference to the previous question.

    I currently could not find it and without it is hard to understand the context of this question.

    Thanks,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 160 points

    LM25118EVAL: Radiated noise of LM25118EVAL

    user6144845
    Online 
    Prodigy 130 points
    Part Number: LM25118EVAL
    Other Parts Discussed in Thread: LM25118
    Hello.
    We have always been very helpful.
    I am evaluating to get and use LM25118EVAL.
    What are the recommended countermeasures for radiation noise during operation?
    Input voltage is 12V Output voltage is 12V Load current is about 1A for LCD and peripheral circuits.
    The oscillation frequency is 300kHz.
    Radiation noise (30MHz to 1GHz) was measured, but exceeded 30dBuV / m at 47MHz, 90MHz and 207MHz.
    Take measures.
    Thank you.
    • Offline 
      TI__Mastermind 21720 points

      Hello user6144845

      Thank you for using the LM25118 and reaching out with your questions.

      In the frequency range of 30MHz to 1GHz the rising and falling of the switch node and any high frequency ringing can affect the measurements.

      One suggestions is to slow down the switch node by adding an RC snubber to the switch node or adding gate resistance to the MOSFET to slow down the turn on time. A snubbber across the output diode can also help dampen any high frequency ringing.

      Please let me know if you have any questions.

      Thanks,

      Garrett

    • Online  in reply to Garrett Roecker
      Prodigy 130 points

      Hello. Mr. Garrett
      Thank you very much for the answer.

      Ringing was observed when measuring the switching waveform (TP3) of the LM25118EVAL.

      As an improvement method
      Insert CR (5 to 10Ω + 1,000PF to 10,000PF) between the source and GND of FET Q1.
      CR (5-10Ω + 1,000PF-10,000PF) was inserted between the drain and source of FET Q2.
      The ringing waveform does not change much and appears as radiated noise.

      Please let us know if you have any other suggestions for improvement.
      Please show the schematic.

      The ringing frequency is about 50MHz.

      Thank you.

    • Online  in reply to user6144845
      Prodigy 130 points

      Hello. Mr. Garrett

      Attachment location of snubber and measurement result of radiation noise are attached.
      I want to reduce 60MHz noise.

      Thank you.

    • Online  in reply to user6144845
      Prodigy 130 points

      Hello. Mr. Garrett

      Inserting a snubber between the drain and gate of the FET did not improve the ringing of the switching waveform.
      When checking the waveforms of the HO and LO pins of the LM25118, there was ringing. When a snubber circuit was inserted between the gate of the FET and GND, the ringing almost disappeared, and the efficiency decreased by about 1 to 2%. Could be improved.
      Please let us know if you have any comments.
      Thank you very much.

    • Offline  in reply to user6144845
      TI__Mastermind 21720 points

      Hello,

      For noise that is occurring at in this frequency range layout is very important. The snubbers should be placed such that the parasitic inductance is minimized. Typically this is done with surface mount devices. These surface mount device should be place directly across the switching device. RC snubber should  be place arcoss the drain to source of a MOSFET and the anode and cathode of a diode.

      When the snubber is connected to the gated drive this is effectively adding some impedance slowing down the rise and fall time of the switch node reducing high frequency ringing. The same thing can be implemented with  a simple gate resistor. Slowing down the rising and falling of the switch nodes will increase the switching losses, impacting efficiency. There is no way around this.

      Please let me know if you have any questions.

      Thanks,

      Garrett

  • 0 in reply to Stefan Schauer
    Prodigy 160 points

    LM25118EVAL: Radiated noise of LM25118EVAL

    user6144845
    Online 
    Prodigy 130 points
    Part Number: LM25118EVAL
    Other Parts Discussed in Thread: LM25118
    Hello.
    We have always been very helpful.
    I am evaluating to get and use LM25118EVAL.
    What are the recommended countermeasures for radiation noise during operation?
    Input voltage is 12V Output voltage is 12V Load current is about 1A for LCD and peripheral circuits.
    The oscillation frequency is 300kHz.
    Radiation noise (30MHz to 1GHz) was measured, but exceeded 30dBuV / m at 47MHz, 90MHz and 207MHz.
    Take measures.
    Thank you.
    • Offline 
      TI__Mastermind 21720 points

      Hello user6144845

      Thank you for using the LM25118 and reaching out with your questions.

      In the frequency range of 30MHz to 1GHz the rising and falling of the switch node and any high frequency ringing can affect the measurements.

      One suggestions is to slow down the switch node by adding an RC snubber to the switch node or adding gate resistance to the MOSFET to slow down the turn on time. A snubbber across the output diode can also help dampen any high frequency ringing.

      Please let me know if you have any questions.

      Thanks,

      Garrett

    • Online  in reply to Garrett Roecker
      Prodigy 130 points

      Hello. Mr. Garrett
      Thank you very much for the answer.

      Ringing was observed when measuring the switching waveform (TP3) of the LM25118EVAL.

      As an improvement method
      Insert CR (5 to 10Ω + 1,000PF to 10,000PF) between the source and GND of FET Q1.
      CR (5-10Ω + 1,000PF-10,000PF) was inserted between the drain and source of FET Q2.
      The ringing waveform does not change much and appears as radiated noise.

      Please let us know if you have any other suggestions for improvement.
      Please show the schematic.

      The ringing frequency is about 50MHz.

      Thank you.

    • Online  in reply to user6144845
      Prodigy 130 points

      Hello. Mr. Garrett

      Attachment location of snubber and measurement result of radiation noise are attached.
      I want to reduce 60MHz noise.

      Thank you.

    • Online  in reply to user6144845
      Prodigy 130 points

      Hello. Mr. Garrett

      Inserting a snubber between the drain and gate of the FET did not improve the ringing of the switching waveform.
      When checking the waveforms of the HO and LO pins of the LM25118, there was ringing. When a snubber circuit was inserted between the gate of the FET and GND, the ringing almost disappeared, and the efficiency decreased by about 1 to 2%. Could be improved.
      Please let us know if you have any comments.
      Thank you very much.

    • Offline  in reply to user6144845
      TI__Mastermind 21720 points

      Hello,

      For noise that is occurring at in this frequency range layout is very important. The snubbers should be placed such that the parasitic inductance is minimized. Typically this is done with surface mount devices. These surface mount device should be place directly across the switching device. RC snubber should  be place arcoss the drain to source of a MOSFET and the anode and cathode of a diode.

      When the snubber is connected to the gated drive this is effectively adding some impedance slowing down the rise and fall time of the switch node reducing high frequency ringing. The same thing can be implemented with  a simple gate resistor. Slowing down the rising and falling of the switch nodes will increase the switching losses, impacting efficiency. There is no way around this.

      Please let me know if you have any questions.

      Thanks,

      Garrett

  • 0 in reply to Stefan Schauer
    TI__Mastermind 46971 points

    Hi,

    sorry for all the questions but I am trying to get the full picture and to understand the issue.

    So you are using the LM25118EVAL and have modified it with a snubber between the FET (Q1 and Q2) GATE and GND.

    Then you are measuring the scope plot above.

    Is it correct the the waveforms from top to down can be mapped to CH1 , CH2,  CH3 in this order.

    What is the operation condition for this scope shot (input voltage and output load) ?

    Thanks,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Mastermind 46971 points

    Hi ,

    I have not seen an update on this thread for the last 8 days.
    If the issue is solved it would be great to press the is solved button.

    Best regards,

    Stefan