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PMP8740: Auxiliary Power supply

Mehdi
Prodigy 160 points
Part Number: PMP8740
Other Parts Discussed in Thread: UCC28070

Hello,

In PMP8740 design, the rectified AC input voltage is connected to the aux. power supply with a 22uF capacitor (C24).

In my design a 100uF cap is used at the input of the aux. power supply with the same topology (QR flyback). My question is, what is the effect of  using such a big capacitor on the PFC stage?  In may case the PFC stage is an interleaved boost with UCC28070.

My another question is, why the aux power supply in PMP8740 has not been fed from 400V-PFC voltage? 

 Thanks

  • 0
    TI__Expert 6005 points

    Hello Mehdi,

    It is possible to supply the auxiliary Flyback directly from the 400V-PFC voltage or from the rectified VAC (like in the PMP8740). I preferred this second way because the nominal VAC is 230Vrms (325VDC), and this way the voltage stress on the FET Q6 is lower. If I select the PFC voltage, then during sudden load release (from full load to zero), an overshoot can reach 430V-440V. This high voltage, compared to the rectified 325VDC, is stressing more the FET. Furthermore, using the diode D5 and capacitor C24, does not impact so much the power factor, at least at full and mid load; It will impact it at light load, but typically PF and THD are measured at full or mid load.

    If you need to have also good THD at light load, you can feed the Flyback directly from PFC voltage; in this case, please check what is the maximum voltage on the drain of Q6 in worst case condition; if this voltage is too close to the maximum rating, just select a 700V or 800V rated FET.

    Best regards,

    Roberto

  • 0 in reply to Roberto S.
    Prodigy 160 points

    Thank you for your answer, Roberto.