Design of LM 2678 circuit

Discussion in 'Power Electronics' started by jonfair, Mar 19, 2019.

  1. jonfair

    Thread Starter New Member

    Dec 1, 2015
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    0
    Good morning.
    I have put together a circuit as in the schematic below-- a 12 Volt to five volt converter. SCH_SPS2_0.jpg The circuit more or less conforms to the TI spec sheet. I am trying to understand the circuit behaviour.
    1. I'm running the circuit with a 12 ohm load, which means the five volt output is delivering 416 mA and delivering 2.08 watts. The input is 13.9 volts and I'm measuring 300 mA input, i.e: 4.17 Watts. TI says the LM 2678 is 95 per cent efficient so I'd expect the input to be less than 200 mA. Am I doing something wrong? (I am currently running my project trace_sps2_0.JPG SCH_SPS2_0.jpg with a 7805 driving an NPN transistor, to deliver a bit less than an amp. It works fine but its obviously very inefficient)
    2. The output looks like this:

    [​IMG]
    Firstly, presumably the artifacts are due to the switching of the LM 2678. Can I suppress them further? I'm using the output to drive some CMOS circuitry and a Rasberry Pi SBC. I'm concerned these spikes might cause me problems
    Second: the artifact seems to be occuring at 142 kHz, but the spec sheet suggests the LM 2678 switches as 270 kHz. Why this difference?
     
  2. crutschow

    Expert

    Mar 14, 2008
    21,713
    6,262
    SMPS are not very tolerant of less.
    For example, if the inductor saturates, it can seriously reduce the efficiency.
    How does it "more or less conform"?

    Also the circuit layout is critical.
    It should be on a board with a ground-plane.
    Did you build it on a plug-in breadboard?
     
  3. jonfair

    Thread Starter New Member

    Dec 1, 2015
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    Yes, it is on a breadboard, although I have tried to put C4 and C5 as physically close to the relevant pins as possible. The TI spec requires a 22uH inductor and two by 180uF capacitors at C6 and C7
     
  4. jonfair

    Thread Starter New Member

    Dec 1, 2015
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    Is 2 X 180uF the same as, say, 1 by 360uF or 20 x 18uF, in this context?
     
  5. crutschow

    Expert

    Mar 14, 2008
    21,713
    6,262
    That is likely part of the reason for the high spike voltages you are seeing.
    That should be reduced with a proper circuit board layout.
    What value and current rating inductor did you use?
    Yes.
     
  6. eetech00

    Senior Member

    Jun 8, 2013
    1,472
    289
    Hi

    The use of multiple capacitors is to adjust the total ESR. So while the total capacitance is the same, the ESR is not.

    eT
     
  7. jonfair

    Thread Starter New Member

    Dec 1, 2015
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    0
    Hi. That's interesting. I had not thought about ESR at all. So, if I have a spiky output, do I want to increase or decrease ESR? It's an issue because TI specifies 2 by 180u caps on the output. I don't have access to a 180uF cap. The best I can do is 100, 220 or 330.
     
  8. eetech00

    Senior Member

    Jun 8, 2013
    1,472
    289
    Hi

    You want as small of ESR as practical.

    Take a look at this document:
     
  9. jonfair

    Thread Starter New Member

    Dec 1, 2015
    8
    0
    Hi eetech00

    Thanks. That's what I wanted. My take on the problem at the moment is that the inductor is saturating. (It gets hot; plus it is rated at 416 mA, which is pretty close to the load current) Also, I am substituting lowESR caps.
     
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