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Inductor Impedance and Parallel Self-Resonance Calculator

Calculate complex impedance, phase, Q, and self-resonance using a first-order series L-DCR branch in parallel with parasitic capacitance Cp.

FORMULAv1.0.0

INPUT PARAMETERS

Enter inductor and parasitic parameters

L, DCR, and parallel parasitic capacitance Cp are required. Data source and model verification limit are optional and do not affect the numeric calculation.

Calculation frequency

Inductance L

DC resistance DCR

Parallel parasitic capacitance Cp

Data sourceOptional; does not affect the calculation

Curve sweep rangeDisplay setting; does not change the current-frequency calculation

Verified parameter/model frequency limitOptional; used only to indicate model range

CALCULATION RESULT

Calculation result

Calculated locally

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Enter the parameters and run the calculation to see results and model assumptions here.

FORMULA & TRACEABILITY

Formulas and applicability

Formulas, variables, and model boundaries are published. References last reviewed on 2026-08-30.

4 formulas · 2 references

CORE EQUATIONS

Core equations

These equations directly produce the primary results and define the tool’s core model.

F01

First-order parallel self-resonant inductor impedance

Z_s=DCR+jωL, Y=1/Z_s+jωC_p, Z=1/Y

Model the winding as a series L-DCR branch in parallel with equivalent self-capacitance Cp, then calculate impedance real part, imaginary part, magnitude, and phase through complex admittance.

L: InductanceH
DCR: First-order series equivalent resistanceΩ
C_p: Equivalent winding parallel self-capacitanceF
Z: Port complex impedanceΩ

Applicability

First-order lumped model under linear time-invariant, small-signal sinusoidal steady state
L, DCR, and Cp are treated as constant across the selected sweep
Frequency-dependent core loss, skin/proximity effects, and higher-order parasitics are excluded

DERIVED & CONVERSION

Derived and conversion equations

Derive units, levels, and supporting engineering quantities from the core values.

F02

Nominal self-resonant frequency ignoring DCR

f_SRF,nom = 1/(2π√(L·C_p))

Equate the ideal-inductor and parallel self-capacitance susceptance magnitudes to obtain the nominal SRF commonly used in datasheets and first-order estimates.

f_SRF,nom: Nominal lossless self-resonant frequencyHz
L: InductanceH
C_p: Equivalent winding parallel self-capacitanceF

Applicability

Ignores the DCR-induced shift of the zero-phase point
Describes only the first parallel self-resonance

F04

Series winding-branch Q

Q_s(f)=ωL/DCR

Give the reactance-to-resistance ratio of the series L-DCR branch at the current frequency; this is not a universal quality factor for the complete device including Cp at all frequencies.

Q_s: Series winding-branch Q1
ωL: Inductive reactanceΩ
DCR: First-order series equivalent resistanceΩ

Applicability

DCR is treated as an equivalent constant across the current sweep
Does not replace a vendor frequency-dependent Q curve

BOUNDARIES & RULES

Boundary and rule equations

Check model applicability, measurement conditions, and regulatory rules.

F03

DCR-inclusive first-order zero-phase condition

ω_0²=1/(L·C_p)−(DCR/L)², |Z(ω_0)|=L/(C_p·DCR)

Derive the complete first-order zero-phase frequency from total-admittance imaginary part B=0 and give the purely resistive impedance at that point; no zero-phase crossing exists when ω₀²≤0.

ω_0: DCR-inclusive zero-phase angular frequencyrad/s
|Z(ω_0)|: Impedance magnitude at zero phaseΩ

Applicability

DCR, L, and Cp are positive
The right-hand ω₀² must be positive

REFERENCES

References

01Coilcraft · Inductor application noteMeasuring Self Resonant FrequencyOfficial technical material · Explains real-inductor parasitic resistance, self-capacitance, the impedance peak, and the first parallel self-resonant model.

02Keysight Technologies · Impedance measurement application guidanceHow to Measure Parasitic Capacitance in Test FixturesWeb edition accessed in 2026 · Explains that fixtures, connections, calibration, and parasitics change high-frequency impedance measurements; the model limit should come from vendor data or measurement.

Engineering use notice

Results use the models and assumptions shown on this page for design estimates and pre-compliance risk review. Complex structures, dispersion, near-field coupling, and test setup can cause significant deviation.

FAQ

Frequently asked questions

What is the inductor self-resonant frequency formula?

f_SRF = 1 / (2π·√(L·C_p)), where C_p is the winding parasitic capacitance. Above the SRF the inductor becomes capacitive and its impedance falls rapidly.

What is the EMC difference between ferrite beads and inductors?

Ferrite beads are lossy (resistive), converting noise energy to heat and rarely forming sharp resonances; inductors store energy and resonate with parasitic capacitance. Beads are usually preferred for high-frequency noise.

What happens to impedance near the SRF?

Impedance peaks at the SRF (nearly purely resistive), then turns capacitive and drops. Choose parts whose SRF is well above the operating band.

PRE-COMPLIANCE SUPPORT

Need accredited testing or pre-compliance support?

These tools support design-stage estimates. For accredited EMC testing, compliance decisions, and troubleshooting, contact us.