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UCC28742: How much voltage take at Vdd pin at first condition?

Masayuki Uchihara
Intellectual 1635 points
Part Number: UCC28742

Hello team,

My customer has a question about the schematic of the UCC28742. How much voltage take at Vdd pin at first condition? In this case, the node is floating in the standpoint of DC voltage. If the Vbulk is equal to the Vdd pin, it's problem for customer design because the VAC is around 200V. That's why I want to make sure it. 

I am awaiting your reply.

Thanks,

Masayuki Uchihara

  • 0
    TI__Genius 9710 points
    Dear Masayuki,

    Thanks for promoting our products.
    You asked " How much voltage take at Vdd pin at first condition? ".
    If my understanding is correct, you want to know what is the turn-on threshold on the Vdd pin.
    The Vdd turn-on threshold should be 21.6V.

    You concern about Vbulk=Vdd, when the VAC is around 200V may damage the IC.
    Don't forget the Rstr is here, so Vbulk won't equal to Vdd.
    The Rstr typical range is between 1M and 10M, depend on power on delay and no load standby power loss.

    If you still have any questions, please feel free to let me know.

    Best Regards
    Kevin
  • 0 in reply to Kuan Wei Chen (Kevin Chen)
    TI__Intellectual 1635 points

    Hello Kevin,

    Thank you for your response. I can't understand first condition Vdd voltage. It looks like open following Rstr (blue circle: Photocoupler and Cvdd) in terms of DC voltage at first condition. How do you divide the Vbulk voltage? Our customer will use Input voltage between 160VAC to 500VAC.

    I am awaiting your reply.

    Best Regards,

    Masayuki

     

  • 0 in reply to Masayuki Uchihara
    TI__Genius 9710 points
    Dear Masayuki,

    The Rstr is not for divide the Vbulk voltage.
    Instead, it provide a small current to charge the Cvdd at first condition.
    The photocoupler is sensing the output voltage for regulation.

    Best Regards
    Kevin