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Power rail troubleshooting

Discussion in 'General Electronics Discussion' started by MChapman, Mar 30, 2015.

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  1. MChapman


    Sep 9, 2013
    Good morning,
    I designed a battery powered buck-boost with a 3.3v, 1.2A output.

    Capture.JPG Capture1.JPG Capture2.JPG

    The battery pack is a custom by Tadiran made of 4 D-Cell 19A-h lithium batteries with a single Tadiran power plus capacitor.
    I have tested the battery down to -40 C and even at that cold of a temp it measures 3.43v and I have had sufficient current output for the small bursts needed.
    In fact all my early tests using "first off" boards from the board manufacturer tested great.
    I reordered the boards but even though I was assured I was getting the same boards a quick visual inspection tells me they are not.
    What is expected is a constant 3.3v output from the supply from 0A to 1.2A. This was true of my "first off" boards.

    With the new boards I tested three simulated resistive loads requiring 6mA, 300mA, and 800mA. 800mA being my typical peak requirement. Measuring at the load requiring 6mA I had the same voltage output as no load. At 300mA I had dropped 0.1v and by 800mA I had dropped by 0.2v.

    Measuring directly from the supply I did not have this problem. Nor did I have this problem anywhere on the system of the "first off" boards.

    I tested for resistance of the trace and found approximately 0.04 milli ohm. The same for my "first off" boards.

    Could the increased voltage drop at higher loads be due to parasitics and if so, why would it affect one set of boards and not another?

    My assumption was parasitic inductance at the output via but my switching frequency of the supply is only 1.25MHz.
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