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SEPIC converter is too inefficient

Discussion in 'General Electronics Discussion' started by eem2am, Jan 31, 2013.

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  1. eem2am

    eem2am

    422
    0
    Aug 3, 2009
    Hello,

    My 5W sepic converter (LED driver) is only 67% efficient

    Vin=6V,
    Vout = 31V
    Iout = 0.13A
    F(sw) = 51KHz
    SEPIC inductor = MSD1583-223 (22uH)

    Schematic:
    http://i45.tinypic.com/2qx9cfr.jpg

    All the capacitors are ceramic.

    I suspect that due to the ceramic capacitors, there is ringing between the SEPIC inductor’s leakage inductance and the input ceramic capacitors.

    How can I snub out this ringing?…..is it by putting a series damping resistor in the path of the SEPIC “primary”?

    SEPIC inductor = MSD1583-223
    http://www.coilcraft.com/pdf_viewer/showpdf.cfm?f=pdf_store:msd1583.pdf

    (The SEPIC inductor feels hottest, of the power components..the FET and Schottky diode feel barely warm)

    Peak FET current is 3.2A and the SEPIC is fairly deeply discontinuous
    *************************************************************************************************************
    Days later....

    ....OK, new findings, my sepic inductor (coupled) is 22uH, and the sepic capacitor is 2u2, so these have a resonant frequency of 23KHz, and this is too close to my switching frequency, and is giving rise to the resonating currents that are overheating the coupled sepic inductor.

    Coming away from the hardware and using the simulator, using a much bigger faradic capacitor value gives far less rms current in the coupled sepic inductor coils.

    So here is a secret of the industry.
    "If you are doing a sepic converter with a coupled inductor, then use the biggest faradic value of sepic capacitor that you can, in order to reduce ringing and losses due to the ringing currents"

    ....This is a "gem" of information...a real golden nugget, please feast at your leisure, as you will not find this written in any article anywhere concerning the sepic converter with coupled inductor.

    (just to mote, when doing a sepic converter with separate inductors (not coupled), you should always the lowest faradic value of capacitor that you can.)

    Anyway, what is the maximum voltage seen by a sepic capacitor in a sepic converter with a coupled sepic inductor.?
     
    Last edited: Feb 2, 2013
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