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LM5181-Q1: Failed conducted emission (Current probe method) and radiated emission

Saivin (RDL3-PU)
Prodigy 10 points
Part Number: LM5181-Q1
Other Parts Discussed in Thread: LM5181, LM5180EVM-S05

Tool/software:

Flyback

Input filter

Transformer parameters

Hello Team,

I am using LM5181 for fuel cell monitoring application

Output voltage  - 20V/40mA

Input voltage - 24V (6V-32V)

14 Number of similar fly-back circuit is used in 1 PCB. The input for all the fly-back are taken from the input filler(Figure-2).

The conducted emission in voltage method looks fine (150K-108MHz) as per CISPER-25 (Class-5) limits. But radiated emission and conducted emission by current probe method is getting failed. The plots from measurements are below.

CP Test

RE test

Can you suggest some steps to identify the route cause?

Is TI done EMI test on evaluation board as per CISPER25?

  • 0
    TI__Guru 72565 points

    Hi Saivin,

    Thank you for posting.  First, we did EMI test on the LM5180EVM-S05 board per CISPR-25, and the results are presented in the EVM user's guide.

    I see the failing is mainly in the high frequency range. Could you take a scope picture of the SW to GND voltage, and zoom in to show a couple of switching cycles, under the same operating condition as you did the EMI test?

    I also see you are using common mode chokes, but there is no Y caps to the system GND.  Could try to add Y-caps such that the CM choke can be more effective?

    Thanks,

    Youhao

  • 0
    Prodigy 10 points

    Hi Xi,

    Can you tell me which page of EVM user guide contains the EMC test data. I am not able find any.

  • 0 in reply to Youhao Xi
    Prodigy 10 points

    Hi Xi,

    If i add a Y-Cap to supply and ground, isn't like we are bypassing the CM choke?

  • 0 in reply to Youhao Xi
    Prodigy 10 points

    Primary SW waveform

    .

    Secondary waveform

  • 0 in reply to Saivin (RDL3-PU)
    TI__Guru 72565 points

    Hi Saivin,

    Sorry for the delay due to holidays.  Thanks for the waveforms. The switch nodes do not look bad. 

    Regarding the Y-caps, they would provide the return path to the source, here it is the secondary side, such that it would be confined inside the circuit and do not emit to external.  

    Note that it does not short out the CM if you can imagine the parasitic impedances along the GND path: this is about high frequency noise.

    I see you also exceed RE limits, and this may need to be more treated with layout to make any ac current loop areas minimized, and shielding may also help.  However, lets first treat the CM first. 

    Thanks,

    Youhao

  • 0 in reply to Youhao Xi
    Prodigy 10 points

    Hi Xi,

    Do you mean the Y-Cap between primary ground and secondary ground?

    If yes, we already have it.

    Or you mean something like below?

  • 0 in reply to Saivin (RDL3-PU)
    TI__Guru 72565 points

    Hi Saivin,

    I meant the 2nd configuration.  I saw you already have capacitors across the input/output isolation boundary.  For the input common mode filter,  please try to ground the Y-cap closer to the primary side of the isolation boundary, and see if the emission gets improved.

    Thanks,

    Youhao