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LM74670-Q1: Vgs signal abnormal

Betty Guo
Expert 7300 points
Part Number: LM74670-Q1

Hi,

customer use LM74670-Q1 as full bridge rectifier, but they found the Vgs is abnormal and has noise on Vin. 

customer use 1uF as charge pump capacitor and use CSD18563Q5 as transistors. the waveform as below show.

could you help on this?

Vin is 24VAC, below is Vin wavform:

 

the Vgs waveform as below:

  • 0
    TI__Genius 16305 points
    Hi Betty,

    I am looking into your question and will get back on this next week.
    Please let us know if you need an urgent response.


    Regards,
    kari.
  • 0 in reply to Karikalan Selvaraj
    TI__Expert 7300 points

    Hi Kari,

    could you give reply by Wednesday?

    thank you

  • 0 in reply to Betty Guo
    TI__Genius 16305 points
    Hi Betty,

    For LM74670-Q1 to act as part in the full bridge rectifier, it needs to block any reverse current flowing back across the MOSFET it controls.
    In order to control this reverse current, LM74670-Q1 turns off the MOSFET by pulling gate down.

    In the waveforms, gate is being pulled down (Vgs = 0V) for every cycle. Because this pulldown current is fast, there is a bit of inductive ringing coupled onto VIN that you see when Gate is turned Off.

    I dont see a concern here. is the rectifier not functioning properly?

    Regards,
    Kari.