Key Takeaways

The unwanted capacitance offered by an inductor is called parasitic capacitance.

In equivalent circuit modeling, the parasitic capacitance of the inductor is parallel to it.

The parasitic capacitance limits the upper operating frequency of the inductor.
Parasitic resistance is in series with the inductor, whereas parasitic capacitance is parallel to the inductor
Passive elements such as resistors, inductors, and capacitors are present in almost all circuit designs. Apart from their selfproperty, such as resistance, inductance, or capacitance, passive elements exhibit other unwanted properties, which we call parasitic. Parasitic dependencies of passive elements are inevitable and must be considered when developing sensitive circuits or equivalent circuit models for analysis.
Inductors give an inductance property as well as parasitic resistance and parasitic capacitance. In the equivalent model of inductors, parasitic resistance and capacitance are included. The parasitic resistance is in series with the inductor, whereas the parasitic capacitance is parallel to it. The inductor parasitic capacitance is an important parameter to understand, as it puts limitations on the frequency of application of the inductor. In this article, we will discuss the challenges of inductor parasitic capacitance in more depth.
NonIdealities Due to Parasitics in Inductors
Inductors can be singlelayer or multilayer. An inductor is basically a coil made by winding an insulated wire around a ferrite core. The aircore inductors, and, in certain special cases, nonmagnetic core inductors are also in use. As long as windings are present in an inductor, the parasitic capacitance cannot be eliminated completely.
An ideal inductor is a myth, as realworld inductors have parasitics. The parasitic resistance is the unwanted resistance offered by the inductor and is in series to it. The unwanted capacitance offered by the inductor is called parasitic capacitance and is parallel to the inductor. The parasitic resistance (R_{p}) and parasitic capacitance (C_{p}) make the inductor nonideal. The equivalent circuit of a nonideal inductor is given in the figure below.
Equivalent circuit of a nonideal inductor
SingleLayer Inductor Parasitic Capacitance
If you analyze a singlelayer inductor piecewise, you can see two sections of wire (two windings) separated by an insulator. There are several capacitors like this between the windings in an inductor. The surface area between the windings and the gap between the windings are factors affecting the value of inductor parasitic capacitance. The potential across the parasitic capacitance varies slightly from two consecutive windings to others because of the winding inductance and resistance.
The parasitic capacitance in the inductor is a distributed parameter. However, parasitic capacitance is represented as a lumped element connected in parallel to the inductor in the equivalent model illustrated above.
MultiLayer Inductor Parasitic Capacitance
Compared to singlelayer inductors, multilayer inductors have multiple parasitic capacitances. In multilayer inductors, the total parasitic capacitance is formed by the combination of three different parasitic capacitances. They are:
 Parasitic capacitance between the turns of the same layer.
 Parasitic capacitance between the turns of the adjacent layer.
 Parasitic capacitance between the turntocore and turntoshield.
Frequency Dependency of Inductor Parasitic Capacitance
The parasitic capacitance influences the frequency characteristics as well as the performance of the inductor. The parasitic capacitance limits the upper operating frequency of the inductor. The equivalent impedance of the inductor takes into account the selfinductance of the coil, parasitic resistance, and parasitic capacitance.
There is the phenomenon of selfresonance between the combination of inductance and parasitic capacitance. Below the selfresonant frequency, the equivalent impedance of the inductor is inductive in nature. As the frequency increases, the parasitic capacitance dominates the equivalent impedance of the inductor. Above the selfresonant frequency, the equivalent impedance is predominantly capacitive and the inductor no longer behaves like an inductor, but more as a capacitor. Thus, parasitic capacitance limits the operation of the inductor at high frequency.
The inductor parasitic capacitance is a serious issue in commonmode chokes, EMI filters, and transformers working in high frequency. It can become a hindrance in realizing the power converters containing inductors and highfrequency transformers. Cadence software can help designers extract the parasitic capacitance of an inductor by identifying its selfresonance frequency.
Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.
About the Author
Visit Website More Content by Cadence System Analysis