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UC3845: uc3845 Rvcc component calculation

mst er
Prodigy 110 points
Part Number: UC3845

Hi Ti support team . I have a simple problem. I am designing flyback converter with use UC3845 IC. I don't understand  Rvcc resistor explanation from datasheet. How is Rvcc resistor affect this UC3845 circuit?

Below is the circuit diagram that I use as a reference. Thank you

  • 0
    TI__Mastermind 34465 points

    Hello ,

    The resistor Rvcc is used to filter out any noise spike on Na winding.
    The spike is due to leakage inductance of the Na winding and it needs to be filter out because it can result in excessive voltage on VCC bias 
    supply to the pwm controller.
    The resistor is chosen by test and it is normally 22 Ohms or less.

    Regards

    John

  • 0
    TI__Mastermind 31580 points

    Hi

    Rvcc is used to limit the current into the internal Vcc clamp circuit when it sinking current to keep the Vcc voltage below the maximum rated voltage of 34V. The Icc must be limited to 25mA, this includes the current consumption of the IC and the additional current taken to drive the external mosfet, which is given by the gate charge multiplied by the switching frequency, Qg X Fsw.

    Regards

    Peter

  • 0 in reply to Peter Meaney
    Prodigy 110 points

    Hımm. I understand now. Thank you  for answer and help

    Regards

    Mesut ERDEN