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LM3481 based SEPIC convereter radiating the EMI

Other Parts Discussed in Thread: LM3481, PMP10610

HI, I am using LM3481 as a SEPIC converter in on e of the product . I have used the 475KHz as switching frequency.

The product fails in the radiated emission test with peaks in the 300-350 MHz range. I have analysed on spectrum analyser with the help of near field probes and found out that the SEPIC inductors are raditaing so much noise around 300 MHZ .

can anybody help me out to resolve this issue?

  • Hi Dileep,

    Can you please share the Schematic , layout and Emission result .

    THe SW node ringing may have parasitic coupling and can also cause emssion at higher frequency . The first thing to see is the SW node ringing and try to dampen it either using snubber or Slew rate control via gate resitance .
    Further you may use a metallic sheilding around inductor and ground it to create Faraday cage . Are you using coupled or uncoupled inductor for your design ?

    ---Ambreesh
  • Hi Ambreesh,

    Thanks for your support. Below is the schematic . I am using the coupled inductor. I do not want to use the shield . do you have any other solution.

  • Hi Dileep,

    Your Schematic didnt come through . Can you please repost the schematic  .

    Further as i mentioned earlier, our first focus should be on SW node ringing . Please probe the SW node and try to dampen it using snubber or through slew rate control using gate resistance .

    Also i hope your are using shieded coupled inductor .

    To further debug the issue  , can you please also post your Layout  as well as radiated emission test results .

    ---Ambreesh

  • Hi Ambreesh
    I am using the below inductor

    www.bourns.com/.../SRF0703.pdf

    I have checked this is already magnetically shielded.
  • Thanks Dileep for sharing the details of your design . 

    Below are some points on Schematic ,layout and the ways you can debug the radiated  EMI issue .

    Schematic:

    Can you please let me know why are you using both an uncoupled inductor L4 and a coupled inductor L5 in your design ? You can easily get 100UH 1A inductor in much smaller size and replace L5 or use coupled inductor and replace L4 and L5 .For more details , please refer to  below links:

    www.ti.com/.../pmp10610

    www.ti.com/.../slyt411.pdf

    www.ti.com/.../slyt309.pdf

    Layout :

    The hot loop in SEPIC is high frequency switch current path as indicated below by red trace 

    In your case that path is obstructed at the current sense resistance to ground to output cap ground .You could have easily connected them together .

    Further HF decoupling cap should be place closer to VIN as well as Vcc pins of LM3481 .Please follow the layout example of PMP10610 

    www.ti.com/.../pmp10610

    Radiated EMI :

    1. Remove coupled inductor L5 and place a shielded 100uH, 1A inductor . 

    2. Connect R12 ground directly to inductor ground . 

    3. Probe the Drain of the MOSFET to look for Switch node ringing . 

    4. Note the frequency you are seeing at the SW node . 

    5. Add RC snubber (MOSFET's drain to ground) to dampen the ringing .Use 10 ohm and 1nF combination to start with . Probe the SW node and see if the ringing went away . 

    6. You may also try to slow down the FET by adding 20 ohms in series with the gate of the FET and probe for the SW node ringing .

    7. Again do the radiative EMI test .

    8 .If you donot pass the test ,  use the ferrite bead pi filter at the input and at the output to arrest HF . You may use something like BLM31PG601SN1 which has impedance of 600 ohm  at 100MHz . 

    9.  if you still do not pass the test , before laying out a new board with optimized layout , you could  just add a metallic shielding outside the inductor and ground it just to evaluate the performance .

    -----Ambreesh