How to Extract Parasitic Values from a Capacitor Datasheet
In a previous article, Improve EMC Testing Results with Realistic EMI Filter Modeling, we discussed the importance of including parasitic elements in EMI analysis models. Here we will show you how to extract those parasitic values from a component datasheet, using a capacitor as an example
It is really quite simple.
Real capacitors have inductance and resistance in series with the capacitance, and leakage inductance in parallel. Values for the equivalent series inductance (ESL), equivalent series resistance (ESR), and leakage resistance vary by capacitor type, value, and size.
When capacitive reactance and inductive reactance are equal, the capacitor self-resonates. At frequencies below its self-resonant frequency, capacitance dominates and the impedance of a real capacitor decreases with increasing frequency. Above the self-resonant frequency, the ESL dominates and the impedance increases. At the self-resonant frequency, the impedance equals the ESR.
Most capacitor datasheets provide a graph of capacitor impedance versus frequency. Often the impedance of different values capacitors is plotted on the same graph.
The table and graph below from an AVX multilayer ceramic datasheet show capacitor properties and impedance for capacitors ranging in value from 2.2 nF to 47 nF.
Let’s extract the element values for the 10 nF capacitor.
That’s easy. It is 10 nF, the red line on the graph above.
Below about 100 MHz, the impedance of the capacitor is determined primarily by the capacitance value. The other parasitic elements have little effect.
ESL – Equivalent Series Inductance
Above about 300 MHz, the capacitor impedance is dictated by its ESL. Extracting the ESL is straight-forward. From the straight line portion of the graph above the self-resonant frequency we can estimate that the impedance is 1.5 Ω at 3 GHz. ESL is given by the following equation.
ESR – Equivalent Series Resistance
At the capacitor self-resonant frequency, the low point of its impedance curve, its impedance is equal to its ESR. For the 10 nF cap, minimum impedance is at about 200 MHz, where its value is 60 mΩ. ESR is therefore 60 mΩ.
Leakage resistance is normally specified on the datasheet. Here it is spec’d as insulation resistance, 1000 MΩ.
Check the Results
EMI Analyst has built-in models for real components. The capacitor model is shown below.
Using the values obtained above, a graph of the capacitor impedance matches nicely with the datasheet, confirming that the parasitic elements extracted from the datasheet are correct.
Improved EMC Analysis
Using a realistic model of the capacitor improves the accuracy of EMC analyses, not only EMI filter calculations, but also emissions and susceptibility computations where the capacitor is part of the circuit.
For more information, check out the following helpful links:
Improve EMC Testing Results with Realistic EMI Filter Modeling
EMI Filter Insertion Loss: How Circuit Impedance Affects EMI Filter Performance
EMC Analysis: How to Calculate Filter Insertion Loss