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LM25118EVAL: Radiated noise of LM25118EVAL

user6144845
Prodigy 160 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.
  • 0
    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

  • 0 in reply to Garrett Roecker
    Prodigy 160 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.

  • 0 in reply to user6144845
    Prodigy 160 points

    Hello. Mr. Garrett

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

    Thank you.

  • 0 in reply to user6144845
    Prodigy 160 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.

  • 0 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