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UCC28704EVM-724: Adaptation for Non-Isolated Design

Garret Holt
Intellectual 1050 points
Part Number: UCC28704EVM-724
Other Parts Discussed in Thread: UCC28704

Hi Team,


We are working with the UCC28704 EVM and are adapting it for a non isolated solution, operating off of line voltage input. We connected the secondary negative/ground to the neutral on the EVM with a short (~ 2”). This led to increased conducted emissions in frequency range of about 500KHz to 30MHz (by more than 10 dB). What is causing the emissions? Do you have suggestions for board level changes to the EVM that can reduce the emissions? The layout of the EVM is designed for an isolated solution, but I suspect that if this was redone with non-isolated application in mind we would see improvement. Let me know if you have any suggestions for implementing this.

Regards,

Garret

  • 0
    TI__Guru 65935 points

    Hello,

    When you short the output to ground to primary ground, you are shorting out C10.  This capacitor works to help with transformer T1 to form a common mode filter.  By shorting this capacitor out EMI signature for common mode nose will increase.  You could try getting a high voltage XCap with the same value and placing it across the bulk and output to see if that helps reduce EMI. 

    Meeting EMI is not the easiest of things to do.  The UCC28704 does not have a common mode choke on the input.  Adding one could help reduce EMI with X and Y capacitors could also help.

    The following link will bring you to a TI seminar paper on designing flyback transformers to meet EMI that you may find helpful as well.

    Regards,