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.

The tolerance of this data is typically ±15%. This data is based on the core manufacturer's standard material characteristics and is taken at a flux density of 100 G at 25°C.

At switching frequencies of 300 kHz and higher, many power modules use ferrite-core inductor designs, requiring significant pc board area to accommodate the inductor. However, there are new powdered-iron materials that perform well with relatively low core loss levels up to 500 kHz and higher. The 60 perm-powdered alloy shown in Fig. 3 is close in performance to a MPP material. It is difficult to compare core losses in various types of materials because the flux-density level is inversely related to the turns and core area. These parameters can vary because the material permeability levels are so different, especially between a ferrite design and a powdered-iron design. With that said, a benchmark study was conducted to determine the difference in ac power losses (coil plus core) for several powdered-iron core inductor designs versus a high B_SAT ferrite-core design. Included in this comparison was a molded powdered-alloy inductor. The core in this inductor is pressed around the coil instead of assembling two core halves. The design parameters for the inductor were as follows: 1 µH nominal at 0 Adc, peak current equals 16 A, ripple current equals 6 A_PK-PK. The various cores were assembled with flat ribbon wire coils and the finished package sizes are listed in Table 4.

Fig. 4 shows the power losses that were measured on a Clarke-Hesse V-A-W meter. The ferrite design has the lowest power losses. However, the package size is much larger than the powdered-iron options.

If board area is critical, a powdered-iron alloy may be a good choice. Conversely, if height is critical, a molded powdered alloy may be a good choice. However, either will not significantly increase the ac power losses at switching frequencies up to 500 kHz. Finally, pricing is comparable between all of these options.

Given that most engineers designing voltage regulators are looking for smaller size in both footprint and height, as well as higher current-handling capability, the new ferrite and powdered-iron materials will provide new solutions to meet this challenge.

Table 1. Inductor core materials.

Category number	Material trade name	Composition
1	Powdered iron	Fe
2	High flux	Fe-Ni alloy
3	Powdered alloy	Fe-Si alloy
4	Sendust	Fe-Si-Al alloy
5	MPP	Ni-Fe-Mo alloy
6	Ferrite	Fe-Mn-Zn oxide

Table 2. Core material cost comparison.

Material	Cost multiplier
Powdered iron	1.0
Powdered alloy	3.4 to 4.0
Ferrite (ungapped)	3.4 to 4.3
High flux	3.1
MPP	15 to 20

Table 3. Core material saturation levels.

Material	B at 25°C (G)	B at 100°C (G)
Powdered iron	11,000 to 14,000	11,000 to 14,000
High flux	15,000	15,000
Powdered alloy	9000 to 15,000	9000 to 15,000
Sendust	10,000 to 10,500	10,500 to 10,500
MPP	7000 to 7500	7000 to 7500
Ferrite	4300 to 5800	3700 to 4800

Table 4. Package size comparison for 1-µH inductors.

	Powdered alloy	High flux	Molded powdered alloy	High-BSAT ferrite
Dimensions: L×W×H (mm)	9×10.5×5.6	9×10.5×5.6	10.5×11.5×4	12.5×13.5×6
Volume (mm3)	529	529	483	1013
Typical DCR (mΩ)	2.2	2.2	2.6	1.7