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LM5161-Q1: Noise suppression

Part Number: LM5161-Q1


We got a question from customer about LM5161-Q1.
Could you help us?

They are trying to reduce radiated noise on thier board. They put the small capasitor on primary and secondary transformer, then the noise is reduced. Therefore they plan to add the small capacitor. Are there any concerns on this usage?

Best Regards,

  • Hello Tateo,

    Placement of such capacitor is not recommended. Recommend placing RC snubber across (in parallel) the rectifying diode. Start with a value of 5.1 ohm and 100pF and increase capacitance from there to say 150pF to further reduce noise. Please note as you increase capacitance, losses in the resistor will increase.

    Also you can try placing a Y capacitor from the secondary grounds to the primary ground. Try a 1nF for each Ground.

    Hope this helps?

  • Hi, Thank you for your support. Let me confirm one thing. What kind of bad effect may the capacitors cause?

    Best Regards,

  • Hello Teteo,

    The node that the capacitor is connected to, is a high DV/DT node. Placing a capacitor here will cause higher instantaneous peak currents drawn as Vsw (sec) transitions, these currents will also reflect on to the primary side and may effect load regulation performance. Also placing a capacitor here will cause excessive Vsw (on the secondary side) ringing. placing a snubber is a better approach in that it will dampen the ringing which will reduce the noise without the effects mentioned above.

    Hope this helps?