Load regulation problems with switching regulator

Discussion in 'The Projects Forum' started by henken, Apr 1, 2013.

  1. henken

    Thread Starter New Member

    Apr 1, 2013
    2
    0
    Hello,

    I am trying to set up a switching regulator to supply 5V from a varying input source, but I am having some issues. The regulator IC I am using is the LM2672 (datasheet)

    Under a small load (for example a microcontroller), the circuit regulates the voltage very nicely throughout the range of input voltages. However, as I try to apply a larger load (~250 mA at 5V with some resistors), the circuit only regulates the voltage decently with an input of ~6.8V. As I increase the input voltage from there, the output voltage drops down to 2-3V.

    What could be going wrong?

    I have tried to follow the datasheet, and I have tried various combinations of components as given by the datasheet. I have tried using a 47μH inductor as well as a 68μH inductor. The catch diode is a 1N5817, and the output capacitor is a low ESR 100μF. At the input end I have a low ESR 330μF capacitor.

    This setup should handle significantly higher output currents than I'm trying to use. Is there something from the datasheet that I have overlooked, or some other mistake I might have made?

    Thanks in advance
     
  2. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    It would be helpful to see a picture of the coil.
     
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    ...or, a schematic....


    I had a similar experience when I first got into switching regulators and used an inappropriately sized inductor, which would regulate for small loads and drop to near 0V for large ones, the inductor had fried and seemed to make contact when there wan't much heat generated in the coil and was open when it heated up.

    To be short, is your inductor properly sized?
     
  4. henken

    Thread Starter New Member

    Apr 1, 2013
    2
    0
    Here are the inductors I have tried:

    https://www.elfa.se/elfa3~se_en/elfa/init.do?item=58-114-57&toc=20341
    https://www.elfa.se/elfa3~se_en/elfa/init.do?item=58-113-72&toc=20341

    The pictures at the links seem to be fairly generic but seem representative. The 47 μH inductor is rated 3A and has an outer diameter of 24 mm. The 68 μH one is rated for 2A and has an outer diameter of 14 mm.

    The schematic is just like the example circuit of the adjustable version of the IC (page 10 of the datasheet, figure 23), with a few resistors totalling 22 Ω set up as a test load to draw 0.23A at the regulated 5V. I don't have any better dummy load a the moment.

    The adjustment resistors R1 and R2 are 1.5kΩ and 4.7kΩ for a 5V output.

    The whole setup is made on a breadboard, but I've tried to keep the relevant connections as short as possible. The power is supplied by a bench power supply.
     
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