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LM5185-Q1: High levels of low frequency noise EMI

S K
Prodigy 20 points
Part Number: LM5185-Q1
Other Parts Discussed in Thread: LM5180-Q1

Tool/software:

We are using the LM5185-Q1 for 24V-15V conversion. We are seeing high levels of low frequency noise on our initial electromagnetic compatibility testing. Below are the results of our unintentional radiation without any modifications:

Here are the results when a ferrite bead is attached to the power input:

And here are the results when a 2.2nF capacitor is connected across the transformer between the isolated GND and non-isolated GND (also with the ferrite bead):

Trying other capacitor values shifts the spectrum, but does not improve the results.

We have also tried:

  1. A 100Ohm/150pF snubber on the input side of the transformer (like the evaluation kit):
  2. Connecting the isolated and non-isolated ground
  3. Shorting our EMI filter's inductor
  4. Changing the value to a much higher value for the inductor
  5. Putting capacitance on the output of a 50 MHz oscillator we have on the board

None of these made any significant change.

Any suggestions?

  • 0
    TI__Mastermind 18465 points

    I have reassigned this thread to the appropriate team.

    Best regards,

    Ridge

  • 0
    TI__Genius 10956 points

    Hi SK,

    I will be back to you tomorrow.

    Thank you

  • 0
    TI__Genius 10956 points

    Hi SK,

    -Firstly, could you please share full Schematic and layout? Please, show the EMI filter and schematic used where you got initial results.

    -Is LM5180-Q1 the only DUT/noise source? Is there any other switching component that might add EMI to the readings?

    -Secondly, looking at your results, the noise in the range of 30MHz-100MHz is suspected to be CM noise dominant due to the HF range compared with the fsw. However, DM noise might be present as well.

    -Thirdly, please see bellow an app note that provides CISPR25 Class 5 CE Pass Results. Pay attention to the HF range of 30MHz-100MHz pass results.

    -The schematic and modification made to get pass results are below. Take particular attention to the following:

    a. Y-cap C1 used has 1nF capacitance value. This cap helps to attenuate common mode currents flowing through isolation barrier.

    b. T1 Flyback transformer is shielded and connected to primary ground GND (see picture in app note). Shielding transformer helps to suppress magnetic field coming from air gap from transformer that might be coupling on EMI filter.

    c. Snubber caps increased to 100pF in series with 100ohm resistor. Snubber across switching node positive input and secondary power diode help to mitigate HF harmonics for.

    f. Reducing DM inductor on EMI to 4.7uH and adding an extra 100nF DM cap on EMI pi filter might also help for DM noise. Note that extra 100nF is placed next to LISN.

    g. Finally, I also recommend adding a CMC (common mode choke) to attenuate CM noise too.

    h. Regarding the layout, avoid placing copper under the transformer and EMI filter inductor. Expand your ground planes GND and SGND to improve return connections for the switching loop.

    App note: LM5180 EMI Performance

    Thank you

  • 0 in reply to Manuel Alva Hernandez
    Prodigy 20 points

    Hello Manuel,

    I can email you the schematic and layout if you are able to email me or provide an email address. There are some modifications to the schematics and layout to note, as Youhao previously assisted in getting the power supply up and running in general.