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UCC28740-Q1: Why there is ringing at Switch node when no current flow for flyback topology?

Ted Zhang
Intellectual 2280 points
Part Number: UCC28740-Q1

HI Team,

One basic question: 

Why there is ringing at Vswitch when there is no current flow(circled in red)?

Thanks

  • 0
    TI__Guru* 85616 points

    Hello,

    When the output is not taking any more current there is still energy stored in the transformer's primary magnetizing inductance (Lpm).  That energy will be transferred and ring with the parasitic switch node capacitance  (Csw).   There is current flowing back and forth from Lpm and Csw during this time. 

    The ring frequency (fr) can be determined with the following equation.

    fr = 1/(2*3.14*(Csw*Lpm)^0.5)

    Regards,