Winding Resistance Measurement in Power Inductors - Understanding the Impact of the Winding Mutual Resistance

Inductors are cornerstone components in power electronics converters. Since winding loss is the dominant loss mechanism in these components, its accurate measurement is fundamental for the validation of the inductor’s operation and design. The techniques for the winding resistance
$R_{w}$
measurement in power inductors can be classified into two groups, indirect and direct. Both techniques use coupled inductors to separate winding and core power losses. If coupled inductors with non-zero winding mutual resistances
$R_{w,m}$
are used, invalid results are obtained with these techniques. Understanding the meaning of
$R_{w,m}$
in coupled inductors is complex. In this paper, the impact of
$R_{w,m}$
on the inductor
$R_{w}$
measurement techniques is demonstrated and practical guidelines for the design of the zero
$R_{w,m}$
coupled inductors are given. Particularly, the location of the auxiliary winding for the direct technique is investigated. In order to compare the
$R_{w}$
measurement techniques and to validate the coupled inductor’s
$R_{w,m}$
impact, two different inductors are built and tested. The results are compared with the values for
$R_{w}$
calculated by FEA simulation. It is found that only the direct technique with an auxiliary winding carefully designed and located following the guidelines given in this paper makes the accurate measurement of
$R_{w}$
in power inductors possible.