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LM3481 SEPIC high output ripple noise

Pablo Oliva
Prodigy 160 points
Other Parts Discussed in Thread: LM3481

Hi,

I am designing a SEPIC converter with an LM3481. The converter specifications are: VOUT = 12V, IOUT = 3A, VINmin = 9 and VINmax = 14. I use webench to obtain the value of the components and for the layout, I use as a guide the reference design board PMP1061. The difference between my layout (third image) and the board layout are the inductors. In the reference board, there is coupled inductors and I use 2 uncoupled inductors. The board regulates the voltage to 12V very well but there is a lot a ripple noise (first image). I notice that there is a lot of overshoot and oscillations in the signal that activates the MOSFET (second image). I don't know if this issue in the MOSFET activation signal is the problem or  I need extra capacitors in the output.
I have the evaluation board AN-2094, and that board has electrolytic capacitors in addition to the ceramic capacitors in the input and the output. As a design guide I read the application

report AN-1484 "Designing A SEPIC Converter", but there is no information about that capacitors. I also check the schematic in the PMP1061 board and there is also the extra electrolytic capacitors.

Can someone have someone help me with that ripple noise or look at my layout and give me their opinion? Thanks!!
In the signal images the coupling was AC, so the DC component does not appear.
  • 0
    TI__Intellectual 1775 points

    Hello,

    One design guideline to follow (mentioned in Section 10 for the LM3481 for the boost case) is to reduce the size of the high frequency current loop. For the Sepic that loop is highlighted in the following figure:

    The main cause of noise in your circuit is the turnON/OFF of the FET. A simple way to reduce that noise would be to add a few ohms of resistance between the DR pin and the gate of the FET. Start with one ohm and see if you need to add more.

    Placing a 1uF capacitor with low ESR on the output could also help filter out the high frequency noise.

    If you have several scope probes connected in your setup, you might also be catching ground coupling noise on the scope that is not really present on the output.

    Best Regards,

    Florent Boico

  • 0
    TI__Mastermind 32990 points

    Hello,

     

    Also, please take note how you are measuring the vout ripple.  Often long ground leads on the scope probe can present noise that really is not there due to "pick up" on the ground loop.  To verify this is not happening, please make sure you are measuring directly across the cout capacitors with a very short ground lead.  They have specialized spring clips that slide onto the barrel of the probe which will present a very short ground return for this very purpose.  I hope this makes sense and helps?