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TIDA-00901: Nominal capacitance vs. effective capacitance

Part Number: TIDA-00901

Hello,

      I have a question on which capacitance was used in simulation, nominal capacitance or effective capacitance, for example, in Figure 14 and Figure 20. For ceramic capacitor used in the design such as C28 or C15, the effective capacitance will be about half under the DC Bias. Different capacitance could lead to different simulation results.

      Thanks,

      John

  • John,

    Nominal capacitance value was used in the simulations.

    Clark

  • Hi Clark,

       Thanks. Could you please explain why the effective capacitance was not used? Was it difficult to estimate the effective capacitance or the results would not be much different?

       Thanks and best regards,

       John

  • John,

    The simulations in the design guide use nominal values to show typical results; if more accurate simulations are desired, one should use more complete models of the components.  The variation in capacitance value, for example, will be affected by the DC bias (operating point), as you suggest.  However, the capacitance will also be affected by the initial tolerance value, aging effects, temperature effects, etc.  In order to consider all these effects, a designer might consider running a worst-case analysis, or multiple Monte Carlo simulations.   

    Focusing on effective capacitance variation with DC bias, there is a good discussion here: https://www.murata.com/en-us/support/faqs/products/capacitor/mlcc/char/0005

    For the specific example of C28, for instance, this component has part number GRM188R71C222KA01D.  Looking at page 270 of the datasheet, and considering a bias point of less than 3V for this 16V-rated component, the change in capacitance due to DC bias  would be negligible.

  • Hi Clark,

          Thank you very much for the detailed explanation. This is very helpful.

          Best regards,

          John