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LM5177: High Side Vgs Ringing

Sam Ting
Expert 3600 points
Part Number: LM5177

Hi team,

We've got a Vgs waveform of high side MOSFET in buck mode operation, but found there are some ringings during turning on. Engineers worry that drops may cause high power dissipation because the voltages are below Vgs(th) and Vgp. 

Could you explain the root cause of ringing, comment on if there is any potential risks and provide solutions to resolve this problem? Thanks!

Ch1: Vgs of HS MOSFET (by differential probe)

Ch2: Switching Node

Best regards,

Sam Ting   

  • +1
    TI__Guru 65450 points

    Hello Sam,

    It seems that this ringing is coming from the fast changing switch node and parasitic inductances and capacitances in the layout. In general there is a risk of parasitic turn on due to such a ringing. So you need to either reduce the parasitics in the system (normally this means a layout change) or you need to slow down the change in the switch node. This normally can be done by slowing down the turn on of the low-side switch via a gate resistor for example.

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Expert 3600 points

    Hello Brigitte,

    There are two gate resistors. Can you please guide us when should we increase the high side gate resistor or the low side gate resistor?
    Thanks for your support!

    Best regards,
    Sam Ting

  • 0 in reply to Sam Ting
    TI__Guru 65450 points

    Hello Ting,

    If gate resistors are increased, the on and off time is becoming longer. For ringing, the most important solution is in general a good layout, so if you plan to do a next revision of your board, make sure that the parasitic coupling from the switch node into the gate lines is reduced.

    Afte reviewing the measurements above again, I am a bit confused about the ringing you are seeing. The GS of the high-side MOSFET is already going high to turn on the FET, but then it is suddenly pulled low again at the moment when the switch node is going high. This is somewhat strange to me. I would expect that something gets pulled low when the switch node goes low and pulled high when the switch node is going high. I am confused.

    Could you please check again the signals for me?

    Best regards,
    Brigitte

  • 0 in reply to Brigitte
    TI__Expert 3600 points

    Hi Brigitte,

    I've confirmed that they used the tip and barrel method for the waveforms, and believe that a better layout change can improve the parasitic effect of gate traces to reduce the ringing.
    For this case, we wonder if another cause is Miller Plateau effect from MOSFETs. If so, how do we ensure the switching on and off without potential risks and how to add the snubbers or damping components (the RC snubbers and gate resistors have been added.)? Thanks!

    Best regards,
    Sam Ting

  • 0 in reply to Sam Ting
    TI__Guru 65450 points

    Hello Sam,

    I am sorry, but I am not sure what you are searching for. Would it possibly help to review the schematic of the EVM? LM5177EVM-HP Evaluation board | TI.com

    Best regards,
    Brigitte