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Soft start time calculation

Discussion in 'Electronic Design' started by futrtrubl, May 22, 2007.

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  1. futrtrubl

    futrtrubl Guest

    I have the below buck converter..

    http://www.edowner.net/soft-start.gif

    Now I'm trying to figure out how quickly or slowly I should soft start
    it.
    The only way I could think of doing it is to see how long it would
    take to charge the capacitor with my "excess" current. At full blast
    my load will require 3.5Amps and my power supply can supply 4.5Amps
    giving me 1Amp to use in charging the capacitor.
    Total charge Qon fully charged capacitor = C*V = 1.8e-3 * 24 = .0432
    Coulombs.
    Q= I * t, t= Q / I, so t= .0432 / 1 = 0.0432 seconds
    So I was thinking of ramping up to 100% duty linearly over that 43
    milliseconds ie. about 2%/ms.

    Is there a better way to calculate soft start time for my application?

    Info not in schematic, power supply is rated at 24V 4.5A, tiger power
    model SYM04-3 (btw, I'm still looking for more information or
    datasheet on this if anyone knows where), FET is rated at 16A
    continuous, inductor at 8 Arms and capacitor at 3.7Amps ripple. The
    load is resistive (a peltier cooler) with an R of around 6.9 Ohms.
    Switching frequency not yet set, could use some recommendations.

    Edward
     
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