Hervé Chazal

Improving reliability of magnetic mutual impedance measurement at high excitation level

Power electronic designers are interested in characterization of the magnetic cores permeability up to 10MHz and at high induction level. To achieve this aim, different experimental setups are used to measure mutual impedance spectra. First, impedance measurement methods are carried out on toroidal wound core of 20 μm nanocrystalline ribbons. Measurement uncertainties are estimated and a confidence factor is introduced as a useful consistency test to improve measurement reliability. Then a lumped equivalent circuit is identified to model electrostatic and magnetic frequency behavior of the device under test. It allows calculating complex permeability spectra over the resonance frequency of the device under test. Finally, we point out the limitation due to high excitation level. According to that, a flux-metric experimental setup is described and elliptical hysteresis-loops are measured. These results allow to consider magnetic linear behavior until a few 10mT and to extend complex permeability calculations to high induction level with good reliability.

Capacitive behavior of HF power transformers: Global approach to draw robust equivalent circuits and experimental characterization

This paper addresses the representation and the characterization of the electrostatic behavior of n-windings HF power transformers. A global approach, mainly based on energy considerations about linear electrostatic circuits, is introduced. It leads to equivalent circuits which include minimum numbers of elements. Besides, owing to a special approximation that is often acceptable, the general circuit can be greatly simplified and so it is for the experimental identification. This is discussed. Then, knowing the topology of the needed circuits, we show how to experimentally find the values of all involved components. This is achieved by using a wide frequency range impedance analyzer according to a well defined process. An industrial 3-winding transformer is used to illustrate this identification process and, finally, curves related to the equivalent circuit are compared to experimental ones.

Modelization of Superferromagnetism in Soft Nanocrystalline Materials Based on an Accurate Description of Magnetostatic Interactions

At high temperature, Fe-Si nanograins obtained by partial crystallization of amorphous Fe-Si-Cu-Nb-B precursor are superparamagnetic, due to the disappearance of the magnetism of the residual amorphous matrix. At a transition temperature Ttr above the amorphous Curie temperature, a spontaneous polarization JSsf appears, despite a much lower blocking temperature. Dipolar interactions, or a residual ferromagnetic coupling between grains, have been invoked to explain this effect. We investigate here the dipolar hypothesis. The Lorentz field (L.f.) model is improved the idea being that the field acting on a nanograin is screened by a surrounding soft shell. The attenuation factor is calculated, leading to a decrease of Ttr compared with the L.f. approximation. Moreover, a description of the spontaneous magnetization curve JSsf (T)/JS(T) is obtained, with a slope near Ttr much sharper than predicted by the (Ttr - T)1/2 law associated to an invariant molecular field factor. Comparison with the experiments shows good agreement.

Effect of Anisotropy and Direction of Magnetization on Complex Permeability of Ferromagnetic Rectangular Thin Slabs

Ultrasoft magnetic materials, such as amorphous, nanocrystalline, and polycrystalline alloys, have been successfully used for power electronic applications during recent years. However, enhancements are needed for the integration of power electronic features, which involves high power densities and operating frequencies up to a few megahertz. Complex permeability spectra, which are used to describe material behavior in these applications, depend mainly on domain-wall motions and coherent magnetization rotation mechanisms. In this paper, we present a model describing these mechanisms, in terms of magnetic anisotropies and domain structure of the material. To validate the model, we measured permeability spectra of polycrystalline Ni-Fe alloys under mechanical stress using a specific setup. These measurements are suitable for comparing the physical model according to different magnetoelastic anisotropies. Results are useful for correlating high-frequency magnetic behavior and the annealing process of materials.

Detailed modeling of local anisotropy and transverse Ku interplay regarding hysteresis loop in FeCuNbSiB nanocrystalline ribbons

This article focuses on the modeling of the hysteresis loop featured by Fe-Cu-Nb-Si-B nanocrystalline alloys with transverse induced anisotropy. The magnetization reversal process of a magnetic correlated volume (CV), characterized by the induced anisotropy Ku, and a deviation of the local easy magnetization direction featuring the effect of a local incoherent anisotropy Ki, is analyzed, taking account of magnetostatic interactions. Solving the equations shows that considering a unique typical kind of CV does not enable accounting for both the domain pattern and the coercivity. Actually, the classical majority CVs obeying the random anisotropy model explains well the domain pattern but considering another kind of CVs, minority, mingled with classical ones, featuring a magnitude of Ki comparable to Ku, is necessary to account for coercivity. The model has been successfully compared with experimental data.

Study of magnetic properties of NiZnCu ferrite synthesized by Pechini method and solid-state reactions

Ni-Zn-Cu ferrites are exhibiting excellent magnetic properties at large frequencies and are consequently produced as low-loss magnetic materials used for magnetic components in power electronics. Here we present an experimental study of (Ni0.31Zn0.49Cu0.20)0.979Co0.021Fe1.9-dO4 ferrites synthesized by the Pechini type sol-gel method. Their structural and magnetic properties are characterized and compared to the ones of reference ferrites synthesized by classical solid-state reactions. Optimized parameters for Pechini method allow obtaining comparable low magnetic core-losses for both synthesis methods. The cores obtained by this sol-gel method present higher static permeability and lower coercive field than the ones obtained by classical solid-state reactions. Results suggest that the microstructure of ferrites synthesized by Pechini method can be considered as a promising alternative route for the design of low-loss magnetic components.

Magnetic behavior measurements under high frequency mechanical solicitations

The magnetic behavior under applied mechanical stress at audible and inaudible frequencies is investigated. A focus is given on how remanent induction, coercive field strength and permeability values that describe hysteresis-loop shape are involved. To this aim, an experimental setup is introduced to both measure mechanical and magnetic behaviors. The mechanical resonant modes of a magnetic ring core sample are tracked to allow large longitudinal bendings. It results that the coercive field strength and remanent induction are greatly reduced under these mechanical solicitations. Those results are then set forth to show that elastic mechanical applied stress is relevant to describe these effects for both steady and transient operating conditions. The experimental results are then focused on how specific magnetic losses and complex permeability spectra depend on the applied stress amplitude. These results are finally discussed to highlight future applicative developments.

Magnetic coercive field measurement under ultrasonic mechanical excitation

The decrease of coercive magnetic field strength according to applied mechanical stress at audible and inaudible fre- quencies is investigated. To this aim, an experimental setup is introduced to both measure mechanical and magnetic behaviors. The mechanical resonant modes of a magnetic ring core sample are tracked to allow large longitudinal bendings and to reduce significantly the hysteresis loop width. Results are then set forth to show that elastic mechanical applied stress is relevant to describe the reduction of coercive field strength measured for both steady and transient states.