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Accurate EMI Filter Modeling

Including the parasitic elements yield more accurate electromagnetic compatibility prediction.

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Realistic Component Models ProvideRealistic EMC Analysis Results

Predicted Conducted Emissions,Ideal Components

Predicted Conducted Emissions,Realistic Components

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Better Models Provide Better Predictions• EMI filters are a rudimentary tool for controlling electromagnetic

interference. EMC testing success often hinges on EMI filtering.• Selecting or designing a suitable filter seems like a straight-forward

task. However, because EMC testing covers a broad range of frequencies, coming up with a good filter is not always easy. Filter components have parasitic elements that fundamentally affect performance. The effect of parasitics can be dramatic.

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Ideal versus RealFilter insertion loss using ideal component models

Filter insertion loss using realistic component models

At very low frequencies, ideal models may be adequate. However, over most of the frequency range using ideal components leads to performance predictions that are very different than the real filter.

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A good computer model that includes parasitic elements is essential when synthesizing a

custom filter design or predicting how an off-the-shelf filter will

perform in your circuit.

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Ideal Components• In order to understand the importance of parasitics, let’s first look at

ideal components. Ideal components are simplified models of real components that include only the core element, but ignore the parasitics.• Most EMI filters are constructed using passive components:

capacitors, inductors, and resistors; all are linear components. Their impedance changes linearly with frequency. When graphed on a log-log scale, which is convenient when evaluating frequencies that span several decades, their impedance is a straight line.

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Ideal Capacitor

Ideal Capacitor ModelIdeal Capacitor Impedance

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Ideal Inductor

Ideal Inductor Model Ideal Inductor Impedance

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Ideal Resistor

Ideal Resistor ModelIdeal Resistor Impedance

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Realistic Components• Models of real components are constructed using ideal component

elements in various combinations. • Parasitic elements may be in series with or parallel to the core

element. • The addition of the parasitic elements allows the component to be

more accurately modeled over range of frequencies to better simulate real component behavior.

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Real Capacitors• Real capacitors have inductance and resistance in series with the

capacitance, and leakage resistance in parallel. Values for the equivalent series inductance (ESL), equivalent series resistance (ESR), and leakage resistance vary by capacitor type and value.• 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.

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Real Capacitor

Ideal Capacitor Model Non-Ideal Capacitor Impedance

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Real Inductors• Real inductors have resistance in series with the inductance, and stray

capacitance in parallel. Values for these parasitics vary with wire gauge and inductor construction.• At the frequency where the inductive reactance and capacitive

reactance are equal, the inductor self-resonates. Below the self-resonant frequency, inductance dominates and impedance increases with increasing frequency. Above the self-resonant frequency, capacitance dominates and impedance decreases.

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Real Inductor

Non-Ideal Inductor Model Non-Ideal Inductor Impedance

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Real Resistors• Real resistors have inductance in series with the resistance, and stray

capacitance in parallel. Values for these parasitics vary with resistor type and size.• Except for very small value resistors, resistor ESL is usually negligible.

At low frequencies, resistance dominates, but at high frequencies the impedance of the resistor rolls off due to the stray capacitance. The frequency at which the capacitive reactance equals the resistance is the corner frequency. At the corner frequency the resistor impedance is one-half its resistance value.

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Real Resistor

Non-Ideal Resistor Model Non-Ideal Resistor Impedance

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Filter Insertion Loss• Insertion loss is a measure of how well an EMI

filter attenuates a signal as it passes through the filter. Normally expressed in decibels, filter insertion loss is the ratio of the input signal to the output signal.

• EMI filters are measured by connecting a signal

source across the filter input terminals and then measuring the signal amplitude across the output terminals. Normally source and load impedance is 50 . Differential mode insertion loss and common mode insertion loss are measured separately. The figure at right shows the measurement setup for differential mode insertion loss.

𝐼𝑛𝑠𝑒𝑟𝑡𝑖𝑜𝑛𝐿𝑜𝑠𝑠=20 log10𝐹𝑖𝑙𝑡𝑒𝑟 𝑂𝑢𝑡𝑝𝑢𝑡𝑉𝑜𝑙𝑡𝑎𝑔𝑒𝐹𝑖𝑙𝑡𝑒𝑟 𝐼𝑛𝑝𝑢𝑡𝑉𝑜𝑙𝑡𝑎𝑔𝑒

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Filter with Ideal ComponentsInsertion loss for a low-pass filter modeled with ideal components steadily increases as frequency increases. To illustrate, the simple L-C filter below has a 500 kHz corner frequency and two-pole roll-off of 40 dB per decade.

Using ideal components yields an unrealistic prediction of filter performance. At high frequencies, predicted attenuation may be orders of magnitude greater than for the actual filter.

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Filter with Real ComponentsUsing component models that include parasitic elements results in a more useful prediction of filter performance. Consider the following adjustments to make the components more realistic.

Insertion loss predicted using realistic models for the components provides a more realistic prediction of how the filter will behave over the frequency range we need it to operate.

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Real is Better Than Ideal• It takes a little longer to dig up the information needed to model a filter using realistic

components, but the difference is worth the effort. • Compare the realistic prediction to the ideal prediction above. At 100 MHz the real

filter model reveals that the filter has just 20 dB attenuation, whereas the ideal filter model predicts 90 dB. That is a 70 dB error if ideal components are used.

• One of the easiest ways to improve EMC testing results is to select or design appropriate filtering for end circuits and power lines.

• Use real component models when building filter simulation models and your designs will more closely match your actual circuits.

• For more information, check out these other helpful links:• EMI Filter Insertion Loss: How Circuit Impedance Affects EMI Filter Performance• EMC Analysis: How to Calculate Filter Insertion Loss