Planar inductor structure with variable flux distribution —A benefit or impediment?

The planar inductor with lateral flux pattern has been successfully demonstrated as a substrate for the high density 3D integrated point-of-load (POL) module. By decoupling the flux distribution from the thickness of the core, both an ultrathin core structure and a high power density can be achieved simultaneously. However, the flux distribution in the lateral flux core is very non-uniform, which is totally against the conventional sense of the inductor design, namely the flux should be as uniform as possible. This paper reveals the DC flux and AC flux counterbalance phenomenon, by which the AC flux and core loss in the saturated core are essentially limited. The comparison between the core with variable flux and the core with relatively constant flux is presented, based on a specific high current POL application. The inductance density and the performance factor are proposed as the criteria to evaluate the utilization of the core for different magnetic structures. The variable flux core with multiple DC bias safely extended the operating points into saturation region, which gives better utilization of the core. The FEA simulation and measurement also prove that the core temperature is almost uniform for this planar core with variable flux, despite of 2~3 times difference in core loss density distribution. Because of the better thermal management capability, the planar core can be pushed to higher core loss density level than the core with relatively uniform flux, to realize lower total core loss and smaller core volume. In addition, the planar core saves more core loss at light load, due to the AC flux and core loss density redistributions.