At a foundational level, the ability to store electric charge and more easily pass higher-frequency AC currents are two of the most identifiable properties of capacitors. However, at very high frequencies, the ideal behavior of a capacitor can be compromised. In those situations, the parasitic, resistive and inductive components of a capacitor have an outsized influence on its behavior.
Ideal capacitor math describes a scenario where impedance approaches zero as frequency increases. Real-world testing reveals an application- and component-specific frequency boundary for impedance. At that boundary, the equivalent series inductance (ESL) of the capacitor forms an LC resonance circuit with itself. This is referred to as self-resonance. Up to its self-resonant frequency, a capacitor acts like it’s supposed to—like a capacitor. Beyond this frequency, it starts to act like an inductor, which impedes AC current.
Figure 1. Behavior of a capacitor before and after meeting its self-resonant frequency
It’s crucial to note that the Q factor of a capacitor typically reaches a minimum at its self-resonant frequency. Q factor, defined as the ratio of a capacitor’s reactance to its equivalent series resistance (ESR), serves as a measure of efficiency, particularly in terms of energy loss. For optimal performance, operations should remain below this frequency.
Resonant capacitors are able to store and discharge energy to achieve specific circuit behaviors that can improve power conversion efficiency, reduce losses, and minimize switching stress.
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