Selecting Ferrite Chokes and Clamps to Minimize RFI and Resistance
When thinking of ferrites, exploration and technological progress are probably the last words that come to mind. Yet, the discovery of hard ferrites or magnet materials gave ancient navigators the “lodestones” they needed to locate magnetic north. The properties associated with hard ferrites triggered the curiosity that led to the early research into electromagnetism by Oersted, Faraday, Maxwell, and Hertz. During the 1930s and 1940s, more research led to the commercial production of soft ferrite passive components that would result in ferrite chokes. In this article, we will take a look at how ferrite chokes and clamps minimize RFI and resistance.
The Physics at the Core of Ferrites
Today’s power applications are on a continual quest to suppress electromagnetic interference; their success depends on the use of highly permeable soft ferrites containing ceramics blended from a wide range of metal oxides that form a magnetic core. The operating characteristics of ferrites depend on the type and ratios of metal oxides. Most ferrites consist of either a manganese-zinc blend (Mn-Zn) or a nickel-zinc (Ni-Zn) mix. Because Mn-Zn ferrites have high permeability and low specific resistance, the ferrites remain limited to frequencies of 1MHz or less. Ni-Zn ferrites have low permeability and high specific resistance and work well for noise suppression.
With soft ferrites, an increase in the magnetic field results in a flux density flow, and magnetization occurs. If a magnetic field applies in the opposite direction, the ferrite goes back to its original condition. If a soft ferrite has a strong magnetic property, a small change in the magnetic field causes a large change in flux density.
One of the key properties of ferrites used for noise suppression is permeability. Ferrite materials that have higher permeability allow magnetic flux to pass more easily than if the flux traveled through the air. Permeability in ferrites increases as temperature increases. However, after the permeability reaches a maximum level at certain temperatures, ferrite loses permeability. While permeability changes with temperature, it remains constant up to a given frequency. In most instances, ferrite materials that have high permeability work best for high frequency circuits.
Working with ferrites requires an understanding of the types of circuits they work well on.
Because the impedance of a ferrite changes as the load current and voltage drop change, ferrite clamps and chokes function as non-linear components. The impedance of the devices becomes highly resistive for a thin band of frequencies. High frequencies cause the impedance to decrease as the ferrite becomes more capacitive than resistive. Increasing the frequencies beyond a particular threshold causes the capacitive impedance to decrease and the impedance becomes resistive.
Ferrite Chokes and Clamps: The Many Styles of Ferrite Components
Odds are good that somewhere within the reader’s periphery, there are multiple ferrite chokes and clamps. They are essential for dissipating noise and attenuating signals to prevent RF interference. While the basic functionality remains the same for these different devices, their package and implementation adjust to fit design needs better:
- Ferrite bead - A standard axial through-hole package, nearly identical in appearance to an axial through-hole resistor or inductor. Ferrites are highly resistive cores with low electrical conductivity, meaning that eddy currents are unlikely to form in appreciable magnitudes, reducing energy loss and rendering the ferrite bead as a low-pass filter.
- SMD ferrite bead - An SMD version of the axial through-hole ferrite bead. Essentially identical to an SMD inductor chip, but possessing ferrite material to resist noise in the signal.
- Ferrite clamping core - Takes the standard design and turns it on its head: instead of a wire wrapped around a ferrite core, the ferrite encapsulates the wire to achieve RF filtering. While there are no turns in the wire, the clamp can generate sufficient self-inductance to operate as an RF filtering device.
- Ferrite ring filter - A wire is passed through a ferrite ring, looped around a particular amount of times to establish a target inductance and exits the other side of the ring, forming a toroid.
- Ferrite core - Identical to the inner contents of the ferrite bead, the core may have a wire looped externally along the lateral surface area to form a basic ferrite device.
The naming conventions are somewhat loose, to say the least; a ferrite choke could be referred to as a ferrite bead with little pushback. Additional synonyms, such as ferrite collars, also exist. For the most part, know that the operation between the different variants is nearly identical. However, applications may differ due to different physical attributes arising from differences in the component structure.
Applications of Different Ferrite Components
Soft ferrite cores can be used to reduce radio frequency interference (RFI) in an electrical conductor. For that reason, ferrite beads can attenuate interference for switched-mode power supplies. Ferrite chokes--or beads--attenuate high-frequency EMI in a circuit by working as low-pass filters. Only low-frequency signals pass through a circuit. Unlike a traditional low-pass filter that works for a wide band of frequencies, ferrite devices only attenuate frequencies that occur within the ferrite’s resistive band. Wirewound ferrite ring filters provide more design flexibility with a high magnitude of attenuation over a wide frequency range, lower DC resistance, and higher current ratings. While chip ferrite beads offer value, the devices have a limited attenuation and frequency range.
When two halves of ferrite are placed around a conducting wire--such as a power cable, a ferrite collar is formed. The ferrite collar provides an inductive impedance for signals traveling through the cable. It’s important to note a clamp serves two simultaneous EMI purposes: it prevents the cable carrying a signal from acting as either a transmitting or receiving antenna. By Faraday’s Law, a magnetic core placed around a conductor induces a back electromagnetic force (EMF) in the presence of a high-frequency signal. Given the high permeability of ferrite, the material offers less resistance to the flow of magnetic flux in the conductor and, as a result, the ferrite absorbs noise energy, which is dissipated as heat.
Ferrite Components and Heat
Heat generally acts as a major vector for material wear and eventual failure, but here it serves a beneficial purpose. By absorbing the energy traveling through the circuit as reactive inductance, transforming it into heat, and allowing it to radiate outwards, the ferrite component prevents a signal reflection elsewhere in the circuit where it may cause irregular operations. Fortunately, heat dissipation is typically so low it is a nonissue, and it is only by significant noise absorption and eddy current formation that it becomes tangibly noticeable.
Choosing a Ferrite Choke
Selecting a ferrite component depends on the source of the EMI and the range of unwanted frequencies, as the undesired frequencies must fall within a resistive band of the collar. Along with matching the clamp to frequency requirements, the rated DC of the device must match the currents seen in the circuit. If the circuit current increases beyond the rated current, saturation occurs and the choke or clamp will become 50% less inductive and have impedance reduced by 90%. Under these conditions, the choke cannot suppress EMI.
Because ferrite beads are resistive, the devices can cause voltage drops in a circuit. The resistive properties of ferrite devices also may cause unintentional heating as a result of high-frequency energy dissipation. Always check the manufacturer’s specifications for the maximum operating DC and the DC resistance rating. Any ferrite filter must have a DC rating more than twice the value of the required current for the rail. Design teams can use PCB design software and design rules to determine the correct placement for chokes to avoid voltage drop issues.
Ferrite materials covering components can help avoid interference-related issues
The manufacturer’s specifications for ferrite chokes and clamps are worth nothing; the specifications for clamps include impedance versus load current curves that show the characteristics of the devices at specific currents. The manufacturer’s specifications for ferrite chokes also show impedance versus frequency response. In addition, most manufacturers include equivalent circuit models for ferrite beads that work for system simulations.
Because ferrite collars are inductive and capacitive, circuit designs must also account for “Q” or the reactance of the inductor divided by the AC or RF resistance plus any DC resistance found in the choke windings. Chokes that have a high Q can create unwanted resonance in power isolation circuits.
PCB design software provides the analytic tools to find the approximate value of ferrite choke inductance and to determine the resonant frequency cutoff. The suite of design and analysis tools available from Cadence can help minimize the difficulty of your manufacturing process. Utilizing OrCAD PCB Designer is a great way to give your designers the layout capacity they need to finalize designs seamlessly.
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