Accounting for resistivity and permittivity in high frequency permeability measurements. Application to MnZn ferrites.

A careful look to methods used by manufacturers to measure the permeability of soft magnetic material let appears that these data are not reliable when induced currents flow in the magnetic material leading to a screening effect This is obviously the case of bulk cast iron as soon as eddy currents flows inside. It is also applicable to MnZn ferrites because, beyond 100 kHz, they are gone through by both conduction currents due to their finite resistivity and displacement currents related to their very high permittivity. In both cases, flux density is not uniform on the core cross section. In this paper we present an original method which allows, at every frequency, simultaneous evaluation of both complex permeability and permittivity (which accounts for resistivity). Completing the traditional inductance measurement by a capacitance one, enough experimental data are acquired, at every frequency, to deduce the two complex parameters. The material is assumed to behave linearly but, owing to special experimental care, no other hypothesis is needed. This method has proven to be reliable up to 10 MHz and, above some 100 kHz, obtained permeability shows significant differences with that measured traditionally. Contrary to traditional specifications, obtained parameters suit to design new core shapes intended to work in the 100 kHz-10 MHz range.