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Power supply return path question

Discussion in 'Power Electronics' started by MChapman, Jul 13, 2015.

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  1. MChapman

    MChapman

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    Sep 9, 2013
    Attached are the datasheet, schematics and two design options for a switching supply boosting 7.2v to 12v,20mA. My question is intended to be general but I wanted to include my current project as an example. The first design is intended to have a return path shorter than the return path of the second design. I believe I understand the role switching frequency plays on the behaviour of the return path:
    1. >MHz frequency and the return path is dictated by inductance.
    2. <MHz frequency and the return path is influenced by resistance of the copper plane.

    I've seen designs on forums before where a responder would jump on the design for having too long of a return path while it didn't look very long at all. Should I expect behavioural differences in the two layouts here? How would I determine how long is too long of a return path?



    1615-1.JPG First Design.JPG Second Design.JPG
     

    Attached Files:

  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Your generalizations of the characteristics of return paths vs. frequency are too broad. Yes, inductive reactance increases with frequency, but it always is a performance factor. Also, area and thickness are factors that contribute to the effectiveness of a return path design as much as length. For example, reducing the thickness of a trace decreases the inductance but increases the resistance. I'm not familiar with your rule of thumb for return paths, but I have one: less is more.

    ak
     
    Arouse1973 likes this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Another factor is the loop area enclosed by the high-frequency conducting traces. Keep the loop area as small as possible to minimize radiated emissions.
     
    Arouse1973 likes this.
  4. MChapman

    MChapman

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    Sep 9, 2013
    Looking for specifics. I would only adjust one variable at a time here. Frequency. Once a board is designed I cannot influence copper pour therefore resistance due to copper is constant through the scenarios.
     
  5. Arouse1973

    Arouse1973 Adam

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    1,087
    Dec 18, 2013
    What are you interested in? EMC related problems or operability. Signal integrity in many cases is a design specific environment, and it's down to you to decide if it's acceptable. Radiated, conducted emissions and immunity is a requirement by certain standards to be below a certain level.

    Signal integrity and EMC often go hand in hand and if you have good signal integrity then you would have good EMC performance and far as emissions is concerned.

    So tell us your design requirements and we can give a bit more advice.


    Cheers
    Adam
     
  6. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    This is how I would do it for a single sided PCB. You could easily machine this on a PCB milling machine. I would normally use more than one layer if I was doing it for my work but it gives you an idea of the kind of layout that should work.
    Adam

    LT1615SCH.PNG

    LT1615.PNG
     
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