Ferrite Out Better Core Materials For Your POL Design

Oct 1, 2006 12:00 PM
By Donna Schaefer, Engineer, BI Technologies Magnetic Components Division, Fullerton, Calif.

For power module designs, the inductor's saturation level is critical. Saturation level is typically defined as the dc current level at which the device's inductance declines to 75% to 80% of its nominal inductance. The dc current can saturate the inductor quickly if an air gap is not introduced into the core's magnetic path. The powdered iron materials have an inherent air gap that is distributed throughout the core, which gives them a soft saturation curve. Ferrite material must have an air gap physically inserted or ground between the mating surfaces of the core halves. The saturation curve is steeper and saturation is abrupt. Typical material saturation levels, or maximum flux density levels, are shown in Table 3.

If the most critical design parameter is current saturation level and the customer's pc board size has either height- or board-area constraints, a high-flux material or a powdered-alloy is a good choice. Ferrite-core materials have typically struggled in this area because their maximum flux density levels are about one-half to one-third the level of powdered-iron materials. In addition, the ferrite material's saturation level will decrease at higher operating temperatures, while the powdered-iron-alloy saturation level does not decrease at higher operating temperatures. This consideration is often overlooked in the customer's product specification. If ambient temperatures are 70°C maximum and the inductor has a maximum temperature rise of 40°C, then the inductor must be able to operate above 100°C. Many inductor datasheets list saturation current ratings at room temperature conditions only; often the customer must specifically request current saturation level rating information.

A New Ferrite Material

Within the last five years, manufacturers have introduced a new type of ferrite material that has increased the maximum flux density level from a previous high of 5000 G to 5800 G at 25°C, and from 3800 G to 4800 G at 100°C. The material is often referred to as a high-B_SAT material. An example of how this material can help is shown in Fig. 1.

Two devices were tested for inductance versus current at 130°C. Both inductors were constructed with an EIR12 core and a four-turn coil with a nominal inductance of 1.2 µH. The only difference between the parts tested was the core material. One used a high B_SAT material and the other used a standard ferrite-powder material. The difference in saturation levels is approximately 4 A, which represents a significant improvement in performance.

Comparing the high-B_SAT ferrite material to the high-flux material, the latter material's saturation performance is still much better. Given 1.2-µH inductor designs with an EIR9.5 core size, the high B_SAT ferrite-core material will saturate at 18 A versus the high-flux material saturating at 22 A. Fig. 2 shows inductance versus current at 25°C.

The equivalent saturation performance in a ferrite part would require the core package to be 50% larger than the high-flux design. At 100°C, the difference in saturation levels is even more pronounced.

High-performance voltage regulators require a fast transient response, which has increased switching frequencies to 350 kHz and higher. Following this trend, the inductor's inductance value has decreased to 0.50 µH to 1.5 µH, with peak currents pushing to 30 A and ripple currents rising to 30% and higher. Under these conditions, core losses are becoming a significant factor in the selection of the inductor. Most magnetics design guides recommend that the core-to-copper loss distribution be at least 50-50, but 20-80 is actually preferred. It is easier to dissipate the heat from the winding versus the core material, as copper has a higher thermal conductivity than either ferrite or powdered iron. Core losses are a function of frequency and flux swing, and are defined in the following formula:

P_V = k × (ƒ^x) × (B^y),

where P_V is the core loss density (kW/m³), f is the frequency (Hz), B is the change in flux density in Gauss (G), and k, x and y are constants derived from actual test data. Ferrite materials have a higher resistivity than powdered-iron materials, which permits them to operate more efficiently at higher frequencies. Core loss will vary with the swing in flux density, frequency and temperature. No two ferrite materials or powdered iron materials are alike when it comes to core-loss performance. In general, the less expensive the material, the higher the core losses. Most empirical core-loss data is derived using a sinusoidal waveform, which makes the testing setup straightforward. Fig. 3 shows a comparison of core losses per volume for various materials.