Inductance

Inductance “rolls off” with increasing current and the way this happens depends on the material:

Iron powder, molybdenum permalloy powder (MPP), sendust and amorphous powder that employ a distributed air gap

Inductance roll off starts at very low current levels and continues in an almost linear manner as the current is increased.

The peak current could be specified at anything up to a 50% roll off with operation beyond this being possible provided the inductor is not overheated.

Ferrite material

The inductance falls sharply at the point where the whole core becomes saturated. Before this point is reached, the inductance remains almost constant.

The peak current would normally be specified for a 10% to 30% reduction in inductance from the open circuit value. Operation at higher current levels are not recommended, as the inductance would quickly fall to a low level.

Look at datasheet roll off curves when selecting inductors.

Current Rating

Inductors are normally specified with two current ratings

Continuous (Irms)

Normally specified as the dc current which produces an inductor temperature rise of 40°C (allows for an ambient temperature of 85ºC and therefore a surface temperature of 125ºC).

Peak (Isat)

The peak current which produces a specific roll off in inductance, specified as a percentage reduction from the open circuit value and can vary from 5% to 50%.

These current ratings are a guide to the inductor performance and the actual maximum operating current will depend on the application.