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LMR16020: EMI CE conduction testing shows "exceed standard" 10db

Dylan You
Intellectual 2630 points
Part Number: LMR16020
Other Parts Discussed in Thread: BQ25895

Dear teams, 

Could you help give some suggestions how to pass the EMI CE conduction test for our customers here/

Vin: 48V. Vout: 5V.

At 180KHz, it shows the EMI CE conduction testing results 10db exceed the standard. The else is PASS. 

LMR16020 switching resistor is 23.2Kohm. SW frequency is 1MHz.

The schematic diagram shows below:

Could you help give some suggestions to help customer pass the EMI CE testing?

Thanks!

Dylan

  • 0
    TI__Mastermind 35210 points

    Hello Dylan,

    I recommend customer do placement of an electrolytic capacitor for damping on the input.

    This calculator (Below) can be utilized for computing the respective damping capacitance and ESR.

    https://training.ti.com/sites/default/files/docs/InputFilterCalculator.xls

    Please reference this appnote which shows the result of damping capacitor in EMC compliance testing

    www.ti.com/.../snva859

  • 0 in reply to Marshall_Beck
    TI__Intellectual 2630 points

    Hi Beck,

    Thanks a lot for your information! Customer do refer the document your provided to design. However, they found the EMI CE conduction testing can't pass until now when they set the LMR16020 RT resistor to 23.2Kohm. The switching frequency of LMR16020 has been set to 180KHz. Then, they did further investigation, and guess maybe it is caused by the second stage charger BQ25895. LMR16020 output 5V will connect to the Vbus of BQ25895. When they disconnect the battery on the BQ25895, LM16020 EMI CE test can be passed. Once they connect the battery, they found I2C of BQ25895 does not work, and Vsys lose power. 

    Could you help give some suggestions on this case? Following picture is the schematic diagram.

    Thanks!

    Dylan

  • 0 in reply to Dylan You
    TI__Mastermind 35210 points

    Hello Dylan, the EMC of a power converter scales with output loading.

    With that said, it may not be a conclusion that the BQ device is causing the issue, but the reduced loading on the output of the converter is resulting in compliant EMC.

    I suggest for them to conclude that the power converter is "at fault," to re-conduct the EMC scan with a resistive load equivalent to the load current demand of the BQ device.

    In addition, what is the DC rating of the EMC filter capacitors? It looks like the EMC filter capacitator is a single 10uF ceramic, 50VDC?.

    If that is the case, at 48Vin the capacitance would be greatly derated. I suggest the customer add more effective capacitance to the EMC filter if that is indeed the case.