kernesaturation

Core saturation, or kernesaturation, is a magnetic phenomenon in which the magnetic core of a transformer or inductor reaches the limit of its magnetization. As the flux density B approaches the material's saturation flux density Bsat, the incremental permeability drops, causing the inductance to fall and the magnetic circuit to lose its linearity.

Mechanism: In ferromagnetic materials, magnetic domains align with the applied field. At low flux levels, domain wall motion and rotation increase magnetization efficiently. As B rises toward Bsat, domains become fully aligned and further increases in magnetizing force H produce only small increases in B. The B-H curve flattens, and the material behaves as if its permeability has decreased dramatically.

Consequences: When saturation occurs in a transformer or inductor, the inductance becomes current-dependent and non-linear. This can lead to waveform distortion, increased harmonic content, higher copper losses, overheating, and, in power electronics, poor voltage regulation or clipping of the signal.

Occurrence and design considerations: Saturation is more likely under high peak currents, DC bias, low operating frequency, or insufficient core cross-section. Materials have different Bsat values; ferrites saturate typically around 0.3–0.5 T, while laminated steels may reach 1.5–2 T. Mitigation strategies include using larger cross-sectional area, introducing an air gap in energy-storage cores, selecting materials with higher Bsat, operating with allowable flux density, and controlling duty cycle and current.

Measurement and modeling: Core behavior is described by B-H curves and energy-storage characteristics. Designers use nonlinear inductance L(I) models and core-loss models, while finite-element methods help predict saturation under transients.