Inductor Quality Factor

Formula

Description

The quality factor of an inductor measures how effectively it stores energy relative to the energy it dissipates per cycle. Higher Q means lower losses and sharper frequency selectivity in resonant circuits. Q increases with frequency up to a point, then decreases as skin effect and core losses become significant. Air-core inductors achieve the highest Q values but require larger physical size. Ferrite-core inductors are compact but have lower Q due to core losses, especially at high frequencies.

Variables

• Q — Quality factor (dimensionless)
• f — Operating frequency (Hz)
• L — Inductance (H)
• Rdc — DC winding resistance (Ω)

Practical Notes

Typical Q values: power inductors 20-50, RF inductors 50-200, air-core inductors 100-500. At high frequencies, replace Rdc with the AC resistance including skin effect. The self-resonant frequency (SRF) of an inductor is where Q drops to zero because parasitic capacitance resonates with the inductance. Always operate well below the SRF.