Capacitor Values: Don't Believe the Label

[PeteS in CA]

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.
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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.

[Harvey]

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.....