A quick rundown on permeability and impedance in a ferrite core.
Lossy material and the construction of the coil are key to EMI functionality.
Some steps to narrow down the viable ferrites necessary for a design.
Acting as an oversized inductor, ferrites provide excellent EMI suppression capabilities
In the era of high-speed design, counteracting EMI is more important than ever. Speeds are well into the gigabyte range and are only expected to increase with further developments in manufacturing technology. With faster rise and fall times, noise is becoming more pronounced, especially in the case of fast-switching power devices.
While material selection and proper layout practices can help reduce the effects of noise in a system, sometimes additional measurements need to be taken. Ferrites offer fantastic capabilities for removing noise from system signals, especially during product testing when most design revisions are more costly than ad-hoc methods of addressing EMI.
EMI suppression ferrite is specifically designed to act as a nonconductive material. Through the power of magnetic induction, EMI suppression ferrite is able to remove some of the signal energy, significantly attenuating noise that falls outside of the targeted frequency range.
Permeability and Impedance in Ferromagnetics
In ferromagnetics, EMI suppression arises from the complex permeability of the material. To properly contextualize complex permeability, the discussion must begin at B-H loops. Referenced most commonly with magnetic hysteresis, B-H loops show the relationship between the magnetic field H (sometimes referred to as the magnetic flux density) and the magnetic flux B. To reduce confusion, it is important to note that B is sometimes used to refer to the flux or the flux density, but the usage here reflects only the former designation.
When a demagnetized ferromagnetic is subjected to an increasing magnetic field, the magnetic flux and permeability increase along similar curvatures until the permeability reaches its maximum point, known as saturation. At saturation, a continued increase in the magnetic field causes the permeability to decrease asymptotically towards zero, while the magnetic flux approaches a maximum value.
But what does this have to do with EMI suppression? The magnetic permeability is related to the intrinsic impedance of free space according to the following equation:
Note that Z = impedance, μ = magnetic permeability, σ = electrical conductivity, ε = electrical permittivity, and j⍵ is the product of the imaginary unit times the radial frequency. σ is not insignificant in comparison to j⍵ε, as ferrites function as a lossy medium.
Material Selection and Coil Construction
The idea of choosing a material that encourages energy loss may seem strange, but it is in fact the mechanism by which ferrites help reduce and eliminate noise. In effect, by running the wire through ring-shaped ferrite, technicians are building a large-scale inductor with the conducting wire providing the windings about the core. As current flows through the wire, magnetic flux is built up in the core.
The central tenet of magnetic flux from a big-picture level is that it will attempt to resist any changes by converting current into magnetic energy. However, the selection of the lossy material ensures the conversion of current to magnetic energy (and vice versa) is a purposefully inefficient process. As a result, the separate currents are diminished, with the expectation that the smaller magnitude of the noise is more proportionally diminished by this energy loss.
The ferrite-coil offers additional benefits as a low-pass filter. Impedance can be resolved from a point on the complex plane as the magnitude of its real and imaginary components. As frequency increases, the impedance changes from being dominated by the reactance (imaginary term) to the resistance (real term). Because the associated “resistance” to reactance involves only a phase shift in the signal (as opposed to a change in magnitude), low-frequency signals are allowed to pass through unfettered.
Alongside the materials, the construction of the coil also influences the EMI suppression capabilities. The number of loops or windings the conductor wire wraps around the core material correlates to the inductance of the coil, with more windings increasing the inductance (and vice versa). With more windings, the average distance between each wind shrinks. If the distance between windings decreases enough that the charged conductive surfaces come into close proximity, capacitive coupling can arise. As in all things engineering, a balancing act ensues: designers can choose to emphasize the high-speed frequency noise reduction with fewer windings or target lower speed frequencies with additional windings.
EMI Suppression Ferrite Design: Questions to Consider
Armed with a basic idea of how EMI suppression ferrite functions, the next important step to realizing its use in electronics is learning how to incorporate this information into engineering design flows. These key points provide a general guide as to how selection should proceed:
Filter - What frequency range does the noise in the circuit operate in? Ferrite cores can be heavily modified via material, shape, and construction to attenuate noise.
Field conditions - Materials change function with heat, and impedance is no exception. Check the manufacturer’s datasheet to determine how the EMI suppression ferrite behaves at different temperatures.
Impedance - Designers need to be conscious of the source and load impedance they are targeting.
Attenuation - How steep does the dropoff need to be at the cutoff frequencies to properly tamp down on signal EMI?
Source - How is the EMI traveling through the design? Tackling radiated versus conducted EMI requires two very different approaches.
Location - Does the board design support the inclusion of an appropriately sized coil for proper EMI performance? Will it fit within an enclosure? Will there be any thermal issues related to the dissipation across the lossy material? Will the coil exist off-board (as is common in many designs), and will that necessitate further design decisions?
EMI ferrite suppression is a powerful tool for correcting EMI issues on a finished board. Designers can use Cadence’s powerful diagnostic PCB design and analysis software to ensure their design is operating at its highest possible level of performance.
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