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PMP8740: EMI Filter Design

Abhishek Dalal
Prodigy 80 points
Part Number: PMP8740

Tool/software:

In the EMI filter design section of SLUP349 document, the EMI filter is designed to attenuate 3rd harmonic and higher switching frequency component. Why fundamental switching frequency component is not targeted?

Please refer to page-11 of the document.

  • 0
    TI__Expert 5805 points

    Hi Abhishek,

    Please consider that the PFC stage works in CCM mode at fixed frequency (130 kHz). This means the fundamental of the triangular ripple current falls outside the EMI measurement range, which is 150 kHz - 30 MHz.

    The second harmonic of the triangular waveform is typically very small so the most important harmonic will be the third (3 x 130 kHz = 390 kHz, which falls in the measured range).

    Regards,

    Roberto

  • 0 in reply to Roberto S.
    Prodigy 80 points

    Thanks Roberto,

    I've oner more question... What is the role of L3 in the differential and common mode filter and why the leakage inductance of L3 is not added with L4 leakage in the calculation of filter cut off frequency.

  • 0 in reply to Abhishek Dalal
    TI__Expert 5805 points

    Hi Abhishek,

    As described in the white paper of this reference design ("Design review of a 2-kW parallelable power-supply module"), L3 provides some differential-mode filtering but should take care of the high-frequency range; if the converter fulfills EMI limits with L4 alone, it will also fulfill EMI limits with L3. This is why I didn't add the leakage inductance of L3 in the differential mode filter calculations, since it provides extra filtering. At the end of the day, since I didn't have time to fine-tune the EMI filter, I decided to use the same common mode choke for both L3 and L4. I suggest anyway to select for L3 a lower inductance common mode choke specialized for high frequency range. I hope this clarifies the topic...

    Regards,

    Roberto