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LM53635-Q1: LM53635-Q1 EMC Test

kura
Genius 4840 points
Part Number: LM53635-Q1

Our customer consider to use LM53635LQRNLTQ1(5V fix version).

They check EMC test (CISPR25 2002) on their board

Result is:

 PK:30dBuV

 AV:26dBuV

Limit is 18dBuV (So this is 12dB over)

This case their output L,C is  Inductor:2.2uH, Capacitor:22uF x 3pcs

They would like to minimize ripple current about 18dB(include margin) to meet EMC requirement.

But this case is that  Inductor value need  22uH.This part can use inductor from 1.5uH to 10uH in datasheet.

If you have solution or idea to meet their EMC requirement using with  LM53635LQRNLTQ1,please let us know it.

regards,

 

  • 0
    TI__Expert 4100 points

    Hello Kura,

    We will need more details regarding customer design to be able to help you;

    1. what type of measurement are you talking about and what frequency range (some  more details regarding test conditions)
    2. customer schematic
    3. is customer using input EMI filter or not
    4. board layout

    Looking forward to your reply

    Best regards

    Brani

  • 0 in reply to Brani Dubocanin
    Genius 4840 points
    Brani-san

    Their concerning frequency range is 0.53～2MHz.
    This part is 2.1Mhz Sw part. When SW frequency decrease to 1.85Mhz,SW frequency is in concerning frequency range .
    We will sent Materials of No2 and No3 of your request to you thru your local support team.

    We are asking to get No4 material we need to customer .
    Regards,
  • 0 in reply to kura
    TI__Expert 4100 points

    Hello Kura-san,

    I received and reviewed customer schematic. In regard to schematic everything looks good.

    It appears that all EMI testing has been performed without any EMI input filter. Please see attached EVM user guide below regarding EMI input filter design.

    In order to pass stringent CISPR25 class 5 requirements in most cases input filtering is needed.

    Please check with customer if reported test results were obtained with or without input filter?

    Also, board layout can have very significant impact on part and EMI performance.

    Please let me know if any questions.

    Best regards

    Brani

  • 0 in reply to Brani Dubocanin
    Intellectual 670 points

    Hello Kura-san, Brani-san,

    I have also been experimenting with the LM53635 EVM and have an interesting way you might be able to improve the EMI a little...

    The idea of these symmetrical pin-out parts (e.g. LT8640, LM53635, TPS56C215, MPQ8636, etc) is that current flows in balanced loops to cancel out the generated EMI:

    This is a good idea, but there’s only one inductor so it can’t be symmetrical and cancel out. So try with 2 inductors…

    I modified the LM53635 board with 2x 4.7uH inductors in parallel. Placed them with “pin 1” in opposite directions so that flux will cancel; e.g. with current flowing from left to right, one inductor will have flux going into board and the other will have flux coming out of board.

    LM53635 board: Original single 2.2uH:

    LM53635 board: Modified with 2x 4.7uH, pin 1 opposite directions:

     

    Test with Cleverscope CS328 oscilloscope and near field loop probe:

    Frequency display: Dark trace 2.2uH original inductor, light trace 2x 4.7uH (peak about 6dB better):

     

    Time display: Loop centred over inductor(s). Dark trace with 2x 4.7uH, light trace with original 2.2uH. For the 2 inductor case the flux is cancelled directly above the centre of them.

     

    Using dual inductors seems to improve the peak radiated noise (at least for near field) by about 6dB, and directly above the inductors the flux is cancelled. This also has the advantage that the current is half in each inductor, so flux density and RMS losses will lower.

    We will implement 2x inductors in our design.

    It would be very interesting to see if you also get an improvement for your EMI testing with this idea...

    Best regards, Ken