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TPS544C20: TPS544C20 EMI question

user1932012
Intellectual 750 points
Part Number: TPS544C20
Other Parts Discussed in Thread: TPS544B20

TPS544C20 EMI question , Just test the radiation, not the conduction.The attachment is SCH, PCB and test data,Please confirm where I can modify it.CDPDY 原理图.PDFCDPDY2 2019.8.23.PDFCDPDY2 PCB截图.docx白屏1# FCC测试报告.Pdf

  • 0
    Intellectual 750 points

    Do you have any suggestions?

  • 0 in reply to user1932012
    TI__Genius 17905 points

    Hi Mark,

    I see you recently posted a similar request for the TPS544B20: https://e2e.ti.com/support/power-management/f/196/t/829472

    John asked some questions there that will be helpful here as well, so paraphrasing them here:

    Can you provide a switching node waveform to see if there is any ringing that corresponds to the worst frequencies in the spectrum plot?

    Can you provide a plot of the noise floor in your test environment as well as a sweep that shows the fundamental operating frequencies in the range above 100 kHz?

    I will look at your design in more detail and provide more feedback, but just a couple quick things I saw in your layout:

    1. Ideally, your smallest value input capacitors for high frequency bypassing will be as close as possible to the IC Vin and Gnd pins. I see that C17 and C21 are farther away than C16 and I do not see any vias nearby to stitch the C17/21 ground. I would place those capacitors where C16 is and shift the others as needed.
    2. Can inductor be moved closer to the SW node pins? 
    3. How did the customer choose the value and number of output capacitors? They are using a high number of 22uF caps (if I read the schematic right).

  • 0 in reply to Kris F
    TI__Genius 17905 points

    Mark,

    Here is some additional feedback for your design:

    1. Looking at the EMI spectrum further, the <40MHz spurs/noise may be due to instability of the converter. Does the output look stable? Can you please probe the switch node and the output showing several switching periods and share the waveforms? If unstable, you can try temporarily short the secondary filter L3 see if the 2 stage output filter is causing stability issues.
    2. The frequency content around 200MHz may be due to switch node ringing, which you will be able to see when you probe the switch node as I previously requested. The better input bypass cap placement as I recommended in my last reply may help with this, but if you see significant ringing, I would also recommend adding an R-C snubber at the switch node. You can take a look at Section 9.2.2.8 in the TPS544C20 datasheet, which also refers to a useful application note with more details.