The Effect of AC or Ripple on DC Inductors

The effect of AC or ripple flux can be significant in many DC inductor applications. The DC energy storage curves provided on each materials page are based on a peak AC flux density of 10 gauss (1 mT) which will typically represent less than 1% ripple current. When significantly greater AC flux density is present, it becomes necessary to consider its effect on both core loss and permeability (inductance).

The interpretation of core loss in DC chokes is covered in Core Loss. The core loss curves shown on each material page also include Et/N (volt-microsecond per turn) ratings for various core sizes at a number of frequencies for a 15C° temperature rise due to core loss.

The -26 Material is a commonly used core material for DC output chokes. However, as switching frequencies increase, the lower core loss characteristics of -8, -18, and -52 Materials also make them good choices. The -8 Material will gain an additional advantage due to its lower permeability.

The temperature rise that will result from a given core loss per unit volume (mW/cm3) is dependent on the core´s effective surface area available to dissipate the heat. Since volume is a cubed function and surface area is a squared function, a core´s capacity to dissipate heat per unit volume varies inversely with size. Large cores can dissipate less heat per unit volume than small cores for the same temperature rise. The Single Layer Winding Table and "Full Winding" Table contain information on surface area and power dissipation for temperature rises of 10C°, 25C°, and 40C°.

Most DC output chokes operate with a peak AC flux density of less than 1000 gauss (100 mT); with a level of 200 gauss (20 mT) being more typical. The various iron powder material are affected by peak AC flux density as shown by the graph Percent Initial Permeability vs. Peak AC Flux Density. The percent initial permeability increases for all materials as the peak AC flux density is increased from 10 gauss (1 mT) to 1000 gauss (100 mT). The -26, -40, and -52 Materials have the most pronounced response to elevated AC flux density.

The -26 Material responds to the combined effects of AC and DC magnetization as shown by the graph "Percent Permeability vs. Total Magnetizing Force at Function of Ripple". The responses of -40 and -52 Materials are very similar.

View Energy Storage Curves which take into account both the core loss and permeability characteristics for -26 Material with 10% ripple and -26 Material with 25% ripple. Fewer ampere-turns are required for the same energy storage than when <1% ripple is present. However, with high ripple at high frequency this material will be able to store less energy due to core loss limitations.

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