Stability of multi-winding coupled inductors in buck converters

Coupled inductors in multiphase buck are becoming more popular in point of load applications. Most interest has focused on 2-winding coupled inductors. Analysis and design of 2-winding devices is maturing. However, a generalized understanding of multi-winding coupled inductors is less understood. This paper discusses a general theory for multi-winding structures. In particular, stability is examined in detail. It is shown in this paper that for large negative coupling coefficients, the multi-winding coupled buck exhibits unstable negative inductance. This paper is the first to document such an instability.