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Original question:

INT- LM5118 EMI issue

LM5118: LM5118

Pruthvik GS
Prodigy 60 points
Part Number: LM5118

Hello,

We are using the LM5118 Buck boost IC in one of our projects. The switching frequency for the same has been set to 420kHz. We are observing lot of emissions from the particular IC which is resulting in the failure of RE102 test. The emission is observed in 100MHz to 200MHz band and 200MHz to 1GHz frequency bands. The emission level around 130MHz to 180MHz is crossing 50dBuV/m.

We also have a Common mode filter and EMI filter prior to Buck Boost IC.

In one of the article by TI, I read that the Emissions could be reduced by adding a resistance to the High side Mosfet but no where the value of resisitor to be used has been specified. 

 
could anybody please provide an appropriate solution for the same.

  • 0
    TI__Mastermind 21720 points
    Hi Pruthvik,

    Thanks for reaching out with your questions.

    Emissions in the frequency range mentioned above are caused by high frequency ringing, usually on the switch node. By adding the resistor to the gate resistor the rise time of the switch node is increase reducing the amount of ringing. The resistor is typically in the range of 1Ihm to 15Ohm. The best way to find the correct value is simply testing it on the board.

    The other option to help reduce the ringing to to add a snubber to the switch node.

    Thanks,

    Garrett
  • 0 in reply to Garrett Roecker
    Prodigy 60 points
    Hi Garrett Roecker,

    We tried adding 10ohm gate resistor to the gate of high side Mosfet. This did reduce the amount of ringing but the emissions were still high.
    I shall try using other resistor values and let you know the results. Please also suggest any alternative which could reduce the emissions.

    Thanks,
    Pruthvik
  • 0 in reply to Pruthvik GS
    TI__Mastermind 21720 points
    Hi Pruthvik,

    The other item to look at is the layout of the circuit. Large loops that involve the switching current can cause issues that the mentioned frequency band.

    Is the ringing on the switch node the same as what is being measured during the EMI test?

    Thanks

    Garrett