Inductors biased with permanent magnets

IT HAS LONG been known that when a highly unsymmetrical (Imin ≠ Imax) current magnetizes a closed iron core in an inductor or transformer, the optimum design includes an air gap in the core to reduce saturation. The presence of the air gap has two opposing effects: (1) it increases the reluctance of the core to both the biasing (d-c) magnetization and the incremental (a-c) magnetization, and (2) the shift of magnetic operating point due to (1) reduces the reluctance of the iron to the incremental magnetization. If a permanent-magnet slab is used as a counterbiasing means instead of an air gap, the magnetic operating point may be shifted toward the maximum incremental permeability point with a gap much shorter than the optimum air gap. The direct increase in reluctance is then much smaller than in the air-gap case, and the inductance is much larger. A given core size can supply a larger inductance or, conversely, a smaller permanent-magnet-gapped core can supply the same inductance as a larger air-gapped core.