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Demonstrating SEPIC capacitor overvoltage?

Discussion in 'General Electronics Discussion' started by eem2am, Apr 7, 2013.

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  1. eem2am

    eem2am

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    Aug 3, 2009
    Hello,

    Please assist in measuring short term overvoltages on my SEPIC capacitor......

    SEPIC capacitor is 22uF, X7R, 1812, 16V

    I have done a SEPIC converter.
    Max output power = 5W.
    Input voltage = 5 to 8.5V
    Output voltage = 5V or 40V (depending on luminaire)
    Switching frequency = 40KHz to 110KHz
    Constant off time control

    The SEPIC coupled inductor is coilcraft MSD1583-473
    http://www.coilcraft.com/pdf_viewer/...re:msd1583.pdf

    When i looked for overvoltages across the sepic capacitor on the Scope (200MHz bandwidth scope) , i merely placed the scope probe across the SEPIC capacitor.

    However, the scope probe ground will have an associated capacitance to the ground, and so i fear that i may have unwittingly filtered any overvoltage spikes away?
    -Must i use a differential probe to look for the overvoltages on the sepic capacitor?
    I am wondering if there will be overvoltages at start up, due to the ringing with the leakage inductance of the coupled SEPIC inductor?
    Also, as the FET switches, i fear that there may be overvoltages on the SEPIC capacitor at these intervals also?

    Do i need the differential probe?, and what bandwidth Diff probe must i purchase?
     
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