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We present experimental measurement of thermal effects on magnetic hysteresis of soft ferrite cores commonly used in high-frequency inductors and transformers and propose a method to model the thermal effects. We measured the major hysteresis loops of soft ferrite core samples from various vendors at different temperatures, and found that the saturation points of the limiting hysteresis loops vary with the temperature substantially in a nonlinear manner, but within the normal operating temperature range, typically 20degC-100degC, this variation can be approximately regarded as linear with an acceptable error. We propose a simple method based on this approximation for determining the limiting hysteresis loop of a soft ferrite for a given temperature. This method is validated by the substantial agreement between the experimental measurement and simulation by the scalar Preisach hysteresis model of major hysteresis loops of the test samples at different temperatures. Since most manufacturers provide the limiting loops of their soft ferrite cores at two typical temperatures, the proposed method is very convenient for engineering applications.
Date of Publication: Nov. 2007