Which capacitor equivalent circuit should I use in an AC PSPICE sim?

Question

I am trying to simulate the frequency response of a buck converter with the specific ceramic caps that I will be using on my board. The goal is to evaluate stability (phase margin).

The OEM does not provide SPICE models for these parts, but they do provide:

• S-Parameters
• Equivalent circuits for:
  • Impedance
  • ESR

While MatLab has a nice generateSPICE function that can be used to convert the S-parameters to a SPICE model, it's only available in their expensive RFToolbox package. So that's a no-go for me.

So I am left with the equivalent circuits, which I can either include as parts in the SCH or use to make a custom subckt.

Question is - which should I use? Ideally, the model would represent both the capacitor's impedance and it's ESR since both of those things are frequency dependent.

Answer 1

For analysis of a buck converter, a simple series RLC model of a capacitor is typically adequate.

More important than the issues you raised, though, is the nonlinearity of the capacitor. As the bias voltage of the capacitor increases, its capacitance value decreases. The capacitance value also can vary quite strongly with temperature. Top tier capacitor vendors (Murata, Taiyo Yuden, Kemet, AVX, and probably others) provide some on-line tools to estimate the capacitance value at the bias point where you will operate it and at different temperatures.

Confirming stability with the capacitor at bias and over the appropriate range of operating temperatures is more important than worrying about the changes in ESR over frequency.