Know Your Magnetics
Increased performance expectations and tightened space constraints of many modern circuits have minimized design margins and necessitated additional scrutiny of component level selection, even for discrete passives such as inductors, capacitors, and resistors. With DC-DC switching power conversion occupying a critical role in most sub-systems, especially in energy optimized mobile applications, proper magnetics selection can offer significant gains in size, cost, and thermal performance.
Understanding how different constructions of inductors, chokes, and transformers each offer unique operating characteristics can make the difference between a successful product and a prototype that’s unable to hold its smoke in.
From Electrons To Permeability
The theory and practice of inductance relative to time-varying voltages and currents are well-understood and documented. Coil shape and number of turns can quickly yield an approximate inductance for an air core design. When the core is constructed from a magnetic material however, things become significantly more complicated.
Magnetism is a truly remarkable property, requiring a deep dive into quantum mechanics to properly explain. Staying at a high level, electrons carry electrical charge, and in the classical sense, they are both moving in their orbits and “spinning.” As we know from Faraday and Maxwell, such a charge in motion establishes a magnetic field. A single atom with unpaired electrons in motion can therefore act as a magnetic monopole.
Microscopically, this magnetism of a single atom is relatively insignificant. In magnetic materials, however, all of these monopoles can be aligned with the help of an applied external field, yielding a very significant cumulative effect. This is the basis for ferromagnetic inductor cores.
When the generated magnetic field aligns the domains of a ferromagnetic core, the net magnetic field can be orders of magnitude greater than a traditional air core. This can allow the designer to reduce the number of turns and the cross sectional area of the coil while achieving the same overall inductance.
The multiplicative effect of magnetic materials is quantified as “permeability” and represents how easily a material becomes magnetized. The formal definition is the ratio of the magnetic flux density (B) to the magnetizing force (H), often depicted as the BH curve.
Inductance and Energy Storage
Understanding how permeability relates to inductance and magnetic energy storage is by no means trivial. A Texas Instruments application note in 1994, “Magnetic Core Properties” by Lloyd Dixon, is a must read to really challenge the common understanding of permeability.
He presents the notion of highly permeable materials as a sort of magnetic friction, requiring energy input to progressively rotate and align the magnetic domains within. High inductance therefore, does not equate with high energy storage. More importantly, the gaps and imperfections within a magnetic core (or the air gaps intentionally added to steel cores) are critical features acting specifically to reduce permeability and increase energy storage.
This balance between inductance and stored energy is a primary driver behind the design of various core materials, shapes, and construction styles. It also directly impacts the topology choice for inductors, chokes, and transformers, where the desirable characteristics and operating conditions may be very different.
A choke, for example, often contains a DC component. Flyback transformers have specific energy storage needs. Inductors may live in a sea of high frequency AC signals. Choosing the proper topology and magnetic core is critical to a successful design.
Magnetic inductor cores come with some additional design baggage as well. Eddy current losses, due to the conductivity of certain core materials, can result in significant heat generation. Saturation of the magnetic core can occur when all of the material domains are completely aligned. In this case, increasing the applied force will no longer results in an increase in flux density. Increased temperature tends to exacerbate the saturation phenomenon as well. In designs with discrete air gaps, flux fringing is another factor that needs to be accounted for.
Metal Composite Inductors
When considering all of these effects, especially in power system applications (DC/DC conversion and filtering), a topology known as the “metal composite core” stands out as an attractive offering. These devices are built using round copper wire windings molded inside of a sintered magnetic metal powder and silicon binder.
In surface mount packages, the magnetic material and binder offer inductance ranges in the tens of micro-henries, with a highly smoothed saturation characteristics. The reduced eddy and core losses combined with low temperature sensitivity allow these tiny inductors to handle impressively large peak currents, up to tens of amps. Most importantly, they serve to illustrate that even today, advances in manufacturing techniques and materials science continue to push the envelope of miniaturization and energy conservation.
What many designers might brush off as a boring old inductor might just be the key to enabling the next generation of cutting edge designs. KEMET’s selection of metal composite inductors is a great jumping off point to get a feel for the available packages and characteristics. Similarly, the KEMET Engineering Center is a great place to start with application guides and design tools to kickoff your next successful project.