Components
Non-Ideal Behavior of Passive Components
By Don MacArthur
Parasitics refers to undesirable characteristics and unwanted effects that deviate from ideal behavior in electronic components and circuits. These characteristics are often modeled using equivalent lumped elements, which include Resistance, Capacitance, and Inductance.

It is crucial to account for their non-ideal, parasitic characteristics when using passive components to mitigate electromagnetic interference (EMI). You might encounter situations where you initially attempt to employ a component to suppress an unwanted signal, only to discover that it does not yield the expected results. This discrepancy often arises due to the component’s non-ideal behavior.

For instance:

  • Beyond their self-resonant frequency, capacitors cease to function purely as capacitors and start behaving more like inductors.
  • Conversely, inductors may exhibit capacitive behavior above their self-resonant frequency.

In your exploration of system components for EMI management, understanding how each behaves beyond its self-resonant frequencies is essential. This knowledge ensures that you recognize when a component no longer adheres strictly to its ideal characteristics as a capacitor, inductor, or resistor.

The following outlines the parasitic behavior exhibited by a select few passive components:

Wires
Wires, often underestimated, wield substantial influence over circuit performance. The internal impedance of a long cylindrical conductor—such as a wire—hinges on factors like radius, permittivity, permeability, and conductivity. When scrutinizing these conductors, we observe deviations from ideal models due to material properties and construction techniques. These natural deviations occur beyond the scope of commonly accepted approximations.

At radio frequency (RF) levels, the skin effect becomes significant. High-frequency AC currents predominantly flow on the outer layer (skin) of wires, increasing AC resistance. Remember that this phenomenon also manifests in other components constructed with wires, including inductors, transformers, and common mode chokes.

Transformers
While ideal transformers are a theoretical concept, real-world transformers exhibit parasitic resistances, inductances, and capacitances.

These parasitic elements arise due to the physical construction of transformers and their materials.

Here are some key aspects of the non-ideal behavior of transformers:

  • Resistance (Rp and Rs): The winding resistance in both primary (Rp) and secondary (Rs) coils contributes to power loss and affects efficiency.
  • Leakage Inductance (Llk): Some magnetic flux does not link both windings directly, leading to energy losses.
  • Magnetizing Inductance (Lm): This inductance is essential for energy transfer but can also introduce non-ideal effects.
  • Core Loss (Rc): The magnetic core material experiences hysteresis and eddy current losses.
  • Self-Capacitance (Cp and Cs): Capacitance between windings and within each winding affects high-frequency performance.
  • Primary-to-Secondary Capacitance (Cm): Inter-winding capacitance influences frequency response.
  • Core Materials: The choice of magnetic core material greatly impacts transformer performance. Materials like powdered metals, ferrite ceramics, and air allow optimization for various applications but introduce non-ideal effects
Due to their non-ideal characteristics, transformers operate within a restricted bandwidth, exhibit insertion loss, adhere to a maximum power rating, and manifest other frequency-, temperature-, and power‑dependent behaviors.
close up of two component parts
Capacitors
No discussion of the non-ideal parasitic behavior of passive components would be comprehensive without acknowledging capacitors. These components play a pivotal role in low-pass filters, the most widely employed filter type for electromagnetic interference (EMI) mitigation.
  • Resistance (ESR): Real-world capacitors possess a small amount of equivalent series resistance (ESR). This resistance emerges from imperfections within the capacitor’s material, leading to energy dissipation. Essentially, ESR impacts the capacitor’s overall performance.
  • Inductance (ESL): Equivalent Series Inductance (ESL) arises from the physical construction of capacitors, encompassing factors like leads and internal connections. As frequencies escalate, ESL becomes increasingly significant, impacting the overall performance of the capacitor.
  • Self-Resonance: Capacitors possess a self-resonant frequency where their inductive and capacitive behaviors reach equilibrium. Beyond this frequency, capacitors cease to function purely as capacitors and instead exhibit inductive characteristics.
  • Lead Inductance: Lead inductance, arising from the connections between traces, can profoundly affect the frequency response of capacitors in real‑world circuits.
Common Mode (CM) Chokes

Another powerful player in EMI suppression is the Common Mode (CM) choke. These chokes find application in mitigating electromagnetic interference (EMI) from switched-mode power supplies (SMPS) and other circuits where CM noise suppression is essential. By incorporating CM chokes, designers ensure compliance with electromagnetic compatibility standards.

However, there is a caveat: parasitic capacitances associated with CM chokes can detract from their high‑frequency filtering performance. If this limitation goes unnoticed, it can lead to extended design cycles and escalated filter costs. To address this, modeling their non-ideal behavior involves considering parameters like equivalent series inductor (ESL) and equivalent series resistor (ESR), both stemming from parasitic effects. Understanding these nuances is critical for effective EMI management and optimal system performance.

Other Components Impacted by Parasitic Effects
Resistors
  • Real-world resistors have a small amount of inductance due to their physical construction. At high frequencies, this inductance becomes significant, affecting impedance.
  • 1/f Noise: Resistor noise increases with frequency, impacting performance.
Inductors
  • Inductors have inherent (parasitic) capacitance due to winding geometry. This affects their high-frequency response.
  • At high currents or frequencies, inductors may saturate, altering their behavior.
Summary

This article provides a succinct overview of the non-ideal behavior exhibited by passive components. However, I recommend delving into specialized articles and book chapters that go much deeper into this captivating subject for a more in-depth exploration.

Resources
  1. “Dealing with non-ideal transformers — basic RF transformer theory of operation,” Power Electronics Tips, May 27, 2020.
  2. “Demystifying RF Transformers: Part 1: A Primer on the Theory, Technologies and Applications,” MCDI & Mini-Circuits.
  3. “Non-Ideal Capacitor SPICE Model: Explained,” EMA Design Automation, April 2, 2024.
  4. Smith, D.C., High Frequency Measurements and Noise in Electronic Circuits, 3rd Edition, Springer, 1993.
  5. Paul, C.R., Scully, R.C., Steffka, M.A., Introduction to Electromagnetic Compatibility, 3rd Edition, John Wiley & Sons, 2023.
  6. Hu, R., PCB Design and Fundamentals for EMC, RANDSpace Technology LLC, 2019.
  7. Ott, H., Electromagnetic Compatibility Engineering, Wiley, 2009.
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