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LM3409HV: LM3409HV EMI

Eric Shen42
Expert 2750 points
Part Number: LM3409HV
Other Parts Discussed in Thread: LM3409

Dear TI :
    My cusomer used LM3409HV for PWM dimming lighting design, when they do EMI test, the RE is a little over the limit.

   Customers try many methods to optimize EMI, then they found that the string beads at the Source of PMOS could reduce EMI, but the Gate waveform would be abnormal.

   Can you give some advice for optimize EMI?

   The data of EMI before string beat:

   

    The data of EMI after string beat:

    

     The gate pin waveform before string beat:

     

       The gate pin waveform after string beat:

        

     I will update the schmatic and PCB layout in the reply.

Regards!

Eric

  • 0
    Expert 2750 points

    Dear TI:

       update the schematic and PCB layout：

       

    D-DR-B01-01-A-V2.0 -Y1-BL.pdfD-DR-B01-01-A-V2.0 -Y1-TOP.pdf

  • 0 in reply to Eric Shen42
    TI__Mastermind 28655 points

    Hello Eric,

    Does the new waveform look like it is operating correct? What else has changed on the board, the off-time is different between the two oscilloscope pictures? Have you measured any of the circuit operation after the change such as average output current, efficiency, etc.?

    I would not recommend placing a bead in series with the source. If it is done correct a bead in series with the gate may work but it has to be designed correct. Placing a bead in series with D2 may help, also reducing the size (or junction capacitance) of D2 can help. Look at the drain slew rate to see what is helping with EMI. If a bead in series with the current path helps it should be on the drain side of the MOSFET, not the source. I would still look at all the waveforms to make sure it is operating correct.

    If the efficiency is lower with the bead in the source lead just try changing R15 to a higher value to see if it can pass if it is very close. Also reducing the switching frequency will help if it is related to drain slew rate.

    I forgot to ask, what are the attachments, there is more here then the schematic.  Also, the MOSFET/power loop isn't that ideal for the LM3409.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    TI__Mastermind 28655 points
    Hello Eric,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,
  • 0 in reply to Irwin Nederbragt
    Expert 2750 points
    Hi Irwin:
    The customer has been on holiday for the past four days, he will confirm the information and give feedback these two days.
    Thanks!

    Best Regards
  • 0 in reply to Eric Shen42
    TI__Mastermind 28655 points
    Hello,

    I haven’t heard back from you, I’m assuming you were able to resolve your issue.
    If not, just post a reply below (or create a new thread if the thread has locked due to time-out)

    Best Regards,