Capacitor Impedance and Frequency
- The relationship between capacitor impedance (Z) and frequency (f) is fundamental. Impedance refers to the opposition a capacitor offers to the flow of alternating current (AC).
- As frequency changes, so does the impedance of a capacitor. This behavior is depicted in Figure 1.
- Keep in mind that impedance is not just about the electrostatic capacitance (denoted as C). Other factors or components come into play.
Additional Components
Beyond the basic electrostatic capacitance, there are three key components:
- ESR (Equivalent Series Resistance): This is the resistive component that exists in series with the electrostatic capacitance. ESR accounts for energy losses due to internal resistance within the capacitor.
- ESL (Equivalent Series Inductance): ESL represents the inductive component also in series with the capacitance. It arises from the physical layout of the capacitor and the leads.
- EPR (Equivalent Parallel Resistance): EPR is a parallel resistance that exists alongside the electrostatic capacitance. It’s related to insulating resistance (IR) between the capacitor’s electrodes or any leakage current.
- When you combine the capacitance (C) and the inductance (ESL), you get a series resonance circuit.
- Up to the resonance frequency, the capacitor behaves primarily as a capacitive element, and its impedance decreases.
- The exact impedance at resonance depends on the ESR.
- However, beyond the resonance frequency, the impedance characteristic shifts to an inductive behavior. As frequency increases further, impedance rises due to the inductance effect.
Pro Tip: Choosing capacitors with lower ESR and ESL values results in lower noise reduction.
Capacitors can act as filters to attenuate specific frequency components. For example:
Low-Pass Filters: Use capacitors in conjunction with resistors to create low-pass filters. These filters allow low-frequency signals (such as DC or slow-changing signals) to pass while attenuating high-frequency noise.
Other Filter Types: Although not often used in EMC work, capacitors are also used as high-pass filters (allow high-frequency signals to pass while blocking low-frequency noise) and band-pass filters (allow a specific range of frequencies to pass through).
Different types of capacitors have varying characteristics. Here are a few considerations: Ceramic Capacitors, Tantalum Capacitors, Electrolytic Capacitors, and Film Capacitors. See reference 2 for more information on capacitor types.
In summary, understanding a capacitor’s frequency-dependent characteristics helps engineers design effective circuits and manage noise issues. It’s like knowing the dance moves of a capacitor—when to waltz (capacitive behavior) and when to tango (inductive behavior)!
- EMC Mitigation: Capitalizing on Capacitors, ADI EngineerZone, EZ Blogs, EngineerZone Spotlight, June 20, 2023.
- “Capacitor Technologies Used in Filtering,” In Compliance Magazine, November 2023.
- “Let’s Talk About Why Filters Fail,” In Compliance Magazine, November 2019.
- “What Every Electronics Engineer Needs to Know About Filters,” In Compliance Magazine, November 2018.