A question often asked by a magnetics designer is, “How do I keep a core from saturating?” The rule of thumb says that voltage saturates a transformer, current saturates an inductor. But what does this mean?
Transformers
In a transformer, the relationship between B and Voltage is derived using Faraday’s law. For a ferrite transformer operating under 20 kHz, the main concern is with keeping B max below the rated value for the material. Ferrites have a rated B max, saturation flux density, ranging from 4.2 – 5.8 kGauss at 25 degrees C. depending on the material. Above 20 kHz, the factors that influence core saturation include frequency, core losses and ambient temperature. As frequency of operation increases, operating flux density of the core must be decreased. In power operations, a ferrite core can be operated at 2 kGauss at 20 kHz, but as operating frequency increases the flux density must be decreased proportionately: 50 kHz -- 1.3 kGauss, 100 kHz – 900 Gauss, 250 kHz – 700 Gauss and so on.
Temperature affects ferrite operation. Every ferrite has a Curie temperature. Just below Curie temperature, the permeability of the material is at a maximum. Above Curie temperature the permeability of the material sharply disappears—the material saturates and no longer exhibits magnetic properties. Power ferrites maximum temperature of operation is usually around 200 degrees C.; high temperature ferrites can reach 300 degrees C. Filter ferrite materials Curie temperatures are lower, 125 – 150 degrees C.
Strip wound cores are often the best choice for environments exposed to elevated temperature or intense vibration. Advantages of these cores include high B max, perm and squareness. Frequency of operation is an important consideration in determining both the core material and the material thickness for tape and bobbin cores. Eddy currents in the material that generate losses are reduced in thinner materials. Permalloy has significantly lower losses than Orthonol and should be used in higher frequency applications. The chart below gives a guideline for frequency of operation of the various strip wound materials.
Tape cores are less sensitive to temperature changes than ferrites. If the temperature change to which the core is exposed is extreme, check the curves for the material being used and reduce the operating B max of the material accordingly.
For saturating devices, B max is used with a small reduction to compensate for the temperature of operation. If the transformer is the non-saturating type reduce the flux density to at least 90% of the rated flux density. To limit core losses select a flux density that will limit core losses to between 5 and 25 watts per pound.
Inductors
In an inductor, current, I, and magnetizing force, H are related by Ampere’s Law. The choice of powder cores and gapped ferrites that are used in inductor applications is governed by the perm vs. DC Bias curve. When a ferrite approaches saturation on the DC Bias curve the change in inductance is abrupt. A ferrite must be operated in the flat area of the curve, well back from the edge of the region where the curve falls dramatically. Powder cores, with their inherent internal gap, exhibit soft saturation and are designed to be operated in the 50 – 80% region of the DC Bias curve. The combination of the high Curie temperature and the distributed gap causes the change in saturation characteristics over temperature to be minimal—only a few percent across a wide temperature range. To keep the core out of saturation do not go over the rated B max for the material. If the core is being operated above the 80% region, usually a smaller core could be selected. The perm vs DC Bias curves are usually presented on semi-log graph paper. The graph illustrates the effects of DC bias from low to high values of magnetizing force, in Oersteds or Amp/turns.
In ferrite inductors a gap is introduced in the material to shear over the BH loop. This has the effect of pushing the B/H loop to the right to allow for increasing the Magnetization current, allowing more Oersteds to be applied before saturation. It also drops the value of B remnance farther down the Y axis toward the origin reducing residual flux levels which lowers the potential for flux doubling when the inductor is switched on or when it is operated at high frequencies. In flyback transformers the air gap is essential to increase the magnetization current to store the energy needed for operation.
A ferrite DC bias curve of mu effective vs. H in Oersteds and Amp-turns/cm is provided to show the limit where the effective perm remains constant. Drive levels in the areas under the curve are suitable; above the curve inductance drops sharply.
