Ferrite Components Offer Broad Functionality

Key Takeaways

• Ferrite components use ferrite metal ceramics of varying coercivities to achieve certain magnetic properties.

• The anisotropicity of ferrite components allows for optical polarization of incident signals.

• Ferrite components in RF/microwave applications can restrict signal transmission directions.

Ferrite components include inductors, transformers, and various microwave components.

Magnetism (and its inseparability from electricity) is a foundational aspect of circuit design, but magnetism is much more complex than classical E/M treatments. There are nearly ten distinct forms of magnetism depending on the interaction and materials – while circuit designers don’t necessarily have to concern themselves with the nitty-gritty of material science and quantum magnetism, it is valuable to understand the applications of the different magnetic forms vis-a-vis their materials. Ferrite components use metal ceramics due to their ability to retain a history of externally applied magnetic fields and their anisotropicity for microwave circuits.

Comparison of Ferrite Component Material

Ferrite Components’ Characteristics

Ferrite components encompass materials that combine iron oxide (Fe2O3) with other metals to form ferrimagnetic materials. A ferrimagnetic material comprises a lattice of oppositely aligned magnetic moments that do not cancel, i.e., the material possesses a nonzero magnetic moment below its Curie temperature. In the presence of an external magnetic field, the material follows a predictable path known as magnetic hysteresis: the external magnetic field aligns the initially opposing moments and reaches a maximum magnetization before an externally applied monotonically decreasing field in the opposite direction flips the alignment of the moments to a maximum negative magnetization. Removing the external magnetic field has the material retain a nonzero magnetic flux; this value persists after removal of the exterior field until the material experiences temperatures greater than its Curie temperature or exposes it to an opposite-direction magnetic field exceeding its coercivity. The material thus exhibits a “memory” of the most recently applied field, a cornerstone of early and some ongoing data storage technologies.

As a broad class of materials, circuit functions are equally vast. There’s a division of devices based on their coercivity:

• Hard ferrites have a high coercivity, making demagnetization difficult; these materials make permanent magnets like those found in motors.

• Soft ferrites have a low coercivity, rendering them conducive to demagnetization. Unlike other magnetic materials, they possess low losses at high frequencies, making them highly suitable for RF applications or switched-mode power supply cores (zinc-based ferrites are especially appropriate for this domain). Ferrites in RF applications typically operate within saturation to limit loss. 

While both ferrite types see heavy usage depending on circuit function, the latter group offers more implementations due to its role as a magnetic conductor, i.e., it reduces losses during magnetic conduction. Advantageously, ferrite materials also possess a high resistivity owing to their ceramic-like nature (we can generalize the formula for ferrites as MO·Fe2O3, where M is a divalent metallic species), reducing the loss caused by eddy currents through induction. Ferrites also possess incredibly high magnetic permeabilities with some relative permeabilities in the thousands, improving the magnetic dipole alignment response from externally applied fields.

It’s necessary to understand that many ferrite features descend from its anisotropicity or that the material’s properties differ with directionality:

• Nonreciprocal propagation - A signal traveling through a ferrite material in different directions produces distinct transmission coefficients. Left and right circularly polarized waves have different propagation constants along the direction of the external magnetic field (also known as gyromagnetic effects). These attributes allow for ferrite materials to act as optical isolators/diodes.

• Magnetic properties - The permeability of the ferrite is a second-rank tensor that requires matrix representation for a full characterization.

Common Ferrite Applications

Due to their magnetic characterizations, applications for ferrite components are vast across the electronics spectrum. Soft ferrites find use in inductors and transformers as the field storage/transfer mechanism, which is ubiquitous throughout devices, but a more specialized function is microwave circuitry. Even in this narrower context, ferrite components run the gamut:

• Circulator - A three-port device with directionality where signals must exit on the port immediately following the port which they entered. Consider it analogous to a microwave roundabout divided into mandatory exits of n-slices. General circulators contain two subcategories:

  • Junction circulator - Contains a resonator and transmission lines; due to the ferrite’s anisotropicity, polarization occurs, splitting the microwave signal into clockwise and counter-clockwise components that experience destructive interference at every output port except the intended.

  • Phase-shift circulator - The system uses rectangular waveguide components and one or more ferrite slabs (with external permanent magnets) to achieve nonreciprocal transmission.

• Antenna - Found ubiquitously in AM broadcast receivers, ferrite small rod antennas improve inefficiency at low frequencies by increasing the radiation resistance and the Q-factor. 

• Isolator - An isolator functions as the optical equivalent of a diode: it is a two-port device that restricts signal propagation in one direction. Ferrite isolators come in a variety of flavors:

  • Resonance absorption - The ferrite absorbs the microwave energy of the signal in one direction. 

  • Field displacement - Similar to the resonance absorber, the field displacement isolator conducts the energy away from the ferrite towards a resistive film, preventing thermal buildup that can inhibit dipole alignment.

Cadence Solutions for Magnetic Modeling

Ferrite components encompass a variety of basic circuit functionalities for broad PCB applications or more focused microwave devices. Incorporating this functionality requires robust E/M simulation to reflect real-world performance accurately. Cadence’s PCB Design and Analysis Software suite gives design teams a comprehensive ECAD environment with powerful modeling tools and customization options. Simulation results are then fed into OrCAD PCB Designer, ensuring DFM has never been easier or more thorough.

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