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UCC28780: Understanding Auxiliary winding in a transformer

Mo Mo
Prodigy 130 points
Part Number: UCC28780

Hello to all,

I'm designing an AC-DC based on the UCC28780 active clamp flyback controller.

I have a question about the primary auxiliary winding that is supposed to power the UCC28780 and the gate driver. My question is, if at instant t0, my controller and my driver are not powered, that means that i have no current flowing across the primary winding. The primary auxiliary winding is nothing but a secondary winding without isolation. So how is my this auxiliary winding is supposed to power my controllers if there is no power flowing through the primary ( because the IC is off, hence the flyback main switch is off).

Thank you in advance for you answers.

  • 0
    TI__Guru* 81346 points

    Hello,

    The UCC28780 have a VDD(on) turn on of 17.5 V and a turn off of 9.8 typical.

    1. This was done so you could use a resistor/resistors to trickle charge the VDD capacitor.
    2. In the design you would normally set Na/Ns = VDD/Vout = 12V/Vout.
    3. At power up VDD will charge to 17.5 and the VDD capacitor needs to be sized for holdup based on IDD loading and time required for the aux winding to supply power to the UCC28780.
      1. Idd = Cvdd*dv/dt
      2. Cvdd = Idd*dt/dv

    Regards,