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UCC28700DBVR

sujeeva wettasinghe
Intellectual 460 points

HI TI,

I have a question for TI's engineers.  Regarding the attached reference design schematic, why is the safety capacitor C5 between the isolated ground and the positive (hot) side of the transformer?   Why isn't the safety capacitor between the isolated ground and the pseudo line side ground, as per usual?

 

From a PCB layout perspective, it would be easier to connect C5 between the two grounds.  I would like to understand why they did it that way rather than the easier and more normal way.

 

See the attached PDF.

 

Regards,
Sujeeva

tidri42.pdf

  • 0
    TI__Expert 4015 points

    Hi Sujeeva, the hot pin of the transformer is pin 3, the switching pin. Pin 1 is connected to the top of the DC bulk capacitor. Because the bulk capacitor voltage does not change noticeably over one switching period it can be regarded as AC ground. The Y-cap (safety capacitor) is often connected to this node, often as you say because it is easier from a layout perspective.

    In theory this is the same as connecting the cap between primary in secondary ground, but there may be some difference in the EMI scans, particularly at high frequency.

    Thanks

    Billy