Monday, November 27, 2006

Ferrite Out Better Core Materials For Your POL Design

Power architectures with nonisolated voltage regulation continue to evolve, and power inductor designs are fundamental to the success of the new product designs. The trend toward two-stage conversion with nonisolated point-of-load (POL) modules is fueling the demand for low-profile, high-power surface-mount inductors with current ratings up to 25 A. Power system design engineers have a wide variety of power inductors at 8 A and higher to choose from when working on a new design. This can be rather overwhelming for a new designer because datasheets can be confusing, and an experienced designer may not be aware of some of the newer core materials available such as powdered iron and ferrite materials. These materials can help reduce size, cost and power losses.

The Right Material for the Right Job

As power densities and current levels continue to increase and more competitors enter the market, product performance becomes critical for successful product sales. For the magnetics design engineer seeking to minimize power losses, the core material selection and physical package size are critical. The job becomes even more difficult as the size of the device decreases and the current increases.

The Right Material for the Right Job

The most common core materials are listed in Table 1. These materials have been around for a long time, but manufacturers are continuously developing new ones. The basic core material ingredient is iron, but it is the alloy blends or oxide formulations and the process controls that give materials their unique performance characteristics. Even though there are many categories, the majority of the materials are considered powdered irons. Categories 1 through 5 in Table 1 outline powdered irons.

Each material category occupies a niche where it is the best material for a given design application. If cost is the critical factor in the design application, then powdered iron is the traditional choice. Table 2 depicts a core material cost comparison using standard powdered iron as the benchmark.

The cost multiplier has been declining in recent years for the ferrite, powdered alloy and high flux materials, because more vendors are developing new materials and selling this type of product. The molypermalloy (MPP) material is cost-restrictive for most applications; however, it is often used for low-volume, height-restrictive applications where a toroidal shape is required. MPP manufacturers are starting to expand their MPP offerings into EIR core shapes, but this product will probably remain too expensive for most high-volume, high-current inductor designs.

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