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PMP9175: Question for PMP9175 and EMC

na na78
Guru 16770 points
Part Number: PMP9175
Other Parts Discussed in Thread: TPS23753AEVM-004, TPS23753AEVM-001

Hi

Our customer has the following question for PMP9175 design.

1.

PMP9175 has secondary synchronous FET for high-efficiency operation.

Compared with un-synchronous operation, how is impact of EMC?

Do you have any data about EMC?

2.

Does the PMP9175 design have good EMC?

3.

Is there reason why R17, C16 and R18 has following parameter?

4.

What impact can be considered if the timing of primary-side FET and secondary-side FET is not synchronized well?

Could it be worse EMC?

BestRegards

  • 0
    Guru 16770 points
    I appreciate if you could reply.
  • 0 in reply to na na78
    TI__Intellectual 2350 points

    I do not have EMI test data for the PMP9175 design.  The TPS23753AEVM-004 is a similar design.

    www.ti.com/.../toolssoftware

    The user guide contains a preliminary measurement of the conducted EMI.  Note that it was measured with the output ground floating and not tied to earth ground.  If the end product requires the output ground tied to earth ground the emmissions will be much higher.  A common mode inductor of 2mH or larger on the input is usually required to meet emmissions for this test case.  If the end product has external connections such as a USB connector or grounded antenna, then the output ground is usually required to be tied to earth ground for EMI testing.

    The TPS23753AEVM-001 is a diode rectified design, at lower power, but has similar EMI characteristics.

    R17, R18 and C16 help to control the turn on and turn off time of the synchronous FET.  The sync FET must have very fast turn off to minimize the time that both the primary FET and sync FET are on to limit any shoot-through current.  Yes, the EMI can get worse if the shoot-through is too large.  The key is to use a sync FET with very fast turn-off time.  The capacitance of a diode rectifier causes similar turn-off votlage spikes, resulting in similar EMI.

    There are also the slua454 and slua469 application guides on EMI compliance for PoE that may be helpful.

    http://www.ti.com/product/TPS23753A/technicaldocuments