Capacitor Values: Don't Believe the Label

by Steven M. Sandler, Program Engineer, Acme Aerospace Division, an Actuant Co., Tempe, Ariz., and Charles E. Hymowitz, Managing Director, AEi Systems, Los Angeles, Power Electronics Technology, May 01 2007

Focus:
A number of second-order effects can cause capacitance values to vary from the specified nominal. DC bias voltage, ac voltage, frequency, temperature, and aging can all cause variations in capacitance values and some of these variables also affect ESR. This article discusses how to account for these effects when modeling power supply circuits in Spice, using a Spice model and application circuit for the LM2831 stepdown regulator as a design example to illustrate the impact of dc bias on phase margin. The discussion second-order effects mainly as they relate to ceramic capacitors, but also touches on tantalum, wet tantalum, niobium-oxide, and aluminum electrolytic capacitors. Other issues addressed here include the impact of extreme cold (-55 degrees C) on capacitance and ESR; aging rates for different ceramic dielectrics; and the impact of ESR variation on power supply phase margin and output ripple and on EMI filter performance.

What you’ll learn:

• How to account for capacitance variation versus dc bias, ac voltage, frequency, temperature, and aging when modeling power supply circuits in Spice
• How to account for discrepancies between Spice simulation results and measured performance for power supply circuits

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