Capacitor Values: Don't Believe the Label

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  • PeteS in CA
    Badcaps Legend
    • Aug 2005
    • 3576
    • USA, Unsure of Planet

    #1

    Capacitor Values: Don't Believe the Label

    Capacitor Values: Don't Believe the Label
    May 1, 2007 12:00 PM
    By Steven M. Sandler, Program Engineer, Acme Aerospace Division and Actuant Co., Tempe, Ariz.
    Power Electronics
    Surprisingly, more often than not, a ceramic capacitor does not exhibit its specified value of capacitance in an actual power-supply application. So the 22-uF capacitor you've selected won't look like 22 uF once its designed into your circuit. The underlying problem is parasitics. These nonideal characteristics are a chronic driver of performance and can be painful to deal with, especially when you don't realize they are impacting your circuit operation. A high-frequency dc-dc converter provides a design example that illustrates the impact of various parasitics on capacitors when modeling the converter's performance in SPICE.
    ...
    At this point, we requested and were provided with the manufacturer's capacitor datasheet. It quickly became clear that there was a severe dc-bias effect (Fig. 2). The manufacturer's data indicated that at 1.8 V (the converter's output voltage) the capacitance decreased by 25%, which made the capacitance 16.5 uF.
    PeteS in CA

    Power Supplies should be boring: No loud noises, no bright flashes, and no bad smells.
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  • Harvey
    Badcaps Veteran
    • Apr 2007
    • 264

    #2
    Re: Capacitor Values: Don't Believe the Label

    Ceramic capacitors have always had a huge tolerance, its one of the reasons they were never used in critical applications, and something that only circuit simulators struggle to handle.

    But to be honest - if the circuit couldn't stand a 25% reduction in capacitance its time to go back to the drawing board.....

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