Most grade-school-age children have read about explorers such as America’s Lewis and Clark trading trinkets to Native Americans for food and safe passage. It is noted that many of these trinkets included glass beads. I’ve witnessed student after student wonder, “what would anyone want with glass beads?”
While I can’t answer for why one would want glass beads, I know I’ve faced a very similar bead-related question in regard to ferrite beads in manufacturing and electronics. While I’m of the camp that ferrite beads are truly valuable, many might come back to the same question: “Why would they want ferrite beads?”
Ferrite Beads: Our Electronic Unsung Heroes
The answer to that probing question rests within our endless quest to eliminate electromagnetic interference. This quest becomes especially important for those of us who use cell phones, televisions, DVDs, gaming systems, and computers.
When we dive deeper into technologies with high speed data rates, we find that PCB designs often require multiple—but separate—digital and analog supply rails. High-speed clock signals, data, and I/O switching rates produce frequencies and harmonics that can overwhelm power rails. As a result, a circuit can experience voltage ripple and output jitter. The use of ferrite beads between the rails allows PCB designers to simplify power designs and minimize board space while maintaining high-frequency separation between rails.
The relationship between the performance of those devices and ferrite beads becomes clearer when we examine the electromagnetic properties of ferrite. Because ferrite beads consist of a conductor inserted through a hollow cylinder of a highly permeable iron oxide ceramic material, the electromagnetic properties influence current flow. The permeability of the iron oxides contained within the ceramic material supports the formation of a magnetic field when current flows through the conductor.
You can find nickel-zinc (NiZn) or manganese-zinc (MnZn) ferrite beads. Each has different electromagnetic properties that can become enhanced through manufacturing processes. Those processes vary the impedance of the ferrite bead by changing the chemical composition and physical length of the ferrites. As a result, the beads become optimized for suppressing frequencies within certain bandwidths.
Nickel-zinc ferrite beads work best for low power, high inductance circuits operating in the 500 KHz to 100 MHz frequency range because of low permeability, high volume resistivity, good temperature stability, and high Q factors. Given a permeability range of 20 to 850 µ, NiZn beads can work for wide band transformer applications. The high resistivity of NiZn ferrite beads allows designers to use the beads at frequencies ranging from 2 MHz to 500 hundred MHz.
In contrast to the NiZn beads, manganese-zinc ferrite beads have high permeabilities that range above 800 µ, low volume resistivity, and low Q factors. MnZn ferrite cores work with switched mode power transformers that operate in the 20 KHz to 100 KHz range. Because MnZn ferrite cores can attenuate RF signals in the 2MHz to 250 MHz range, PCB designers use the devices in inductor applications that have low operating frequencies.
EMC suppression can be achieved with the help of ferrite
Wallflowers of the Electronic Component Society
When we design PCBs, ferrite beads rarely enter into the conversation. After all, they are passive devices. And…ferrite beads cannot claim to be capacitors, inductors, or resistors. However, the beauty of ferrite beads rests within their capacitive, inductive, and resistive characteristics. Ferrite beads have resistive characteristics over one-to-two frequency decades. While ferrite beads are inductive at lower frequencies, the ferrites are capacitive at higher frequencies.
In contrast to their passive cousins, ferrite beads dissipate—rather than store—high frequency energy. Rather than reflecting back into the system, the energy disperses as heat. As we have already seen, low-Q ferrite beads have no impact on a circuit at low frequencies but resist high frequency energy.
Low-Q ferrite beads offer the best solutions for power supply filtering by attenuating high frequency power supply noise in a thin band. Because inductors store energy, high frequency energy can combine with capacitance in the circuit and generate ringing. Designs for high-speed digital circuits often use low-Q ferrite beads to isolate high frequencies between shared voltage supply rails and to meet target impedances. Connecting a ferrite bead in series with a power supply rail and as part of a low-pass filter network also helps with cutting high frequency power supply noise.
High-Q ferrite beads work for high-level resonance applications such as signal filtering in telecommunications and prevent interference from transferring either from a device or to a device. The High-Q beads work well for building RF oscillator and filter circuits that require high resonant elements and low losses.
From a design perspective, you should use ferrite beads that have well-defined temperature factors and that remain stable over time. Those characteristics allow the LC combination to operate as temperature changes. When PCB traces act as RF antennas, high-Q ferrite beads help to reduce electromagnetic interference emitted from the board. The beads also prevent interference from entering the circuit through the traces.
Using Ferrite Beads in Your PCB Design
As with any electronic component, you must choose a ferrite that has the characteristics that match your application. For example, you cannot use High-Q ferrite beads in power isolation circuits because of unwanted resonance. Because the undesired high frequency noise that requires attenuation must match with the resistive band of the ferrite, you must also recognize the source of the EMI and the range of unwanted frequencies. When you perform noise filtering circuit design and analysis, you can find an approximate value for the bead inductance and determine the resonant frequency cutoff.
In addition, you can check the manufacturer’s specifications for the ferrite bead impedance versus frequency response. Most manufacturers also include equivalent circuit models for ferrite beads that work for system simulations.
Proper utility of SPICE tools can also save time, increase performance and determine accuracy when working with ferrite beads. Power decoupling, high-frequency attenuation simulation, and representing the ferrite bead’s effect with RLC is particularly easy to achieve while increasing your circuit’s security and representation.
Manufacturing checks will be made easier if you know what you’re sending to your manufacturers
If your design places a ferrite bead with a decoupling capacitor in a dc-to-dc converter, resonance peaking can occur. With the ferrite bead inductance and the high Q decoupling capacitance forming a low-pass filter network, resonance peaking can happen at the switching frequencies for the converters and create crosstalk. Adding a large capacitor with a series damping resistor in the decoupling capacitor path can dampen the resonance without impacting the attenuation characteristics of the ferrite bead.
Manufacturers rate ferrite beads according to specific currents. Any amperage above the rated current may damage the ferrite bead. When the dc bias in a power supply circuit reaches above 30-40% of the rated current, ferrite beads saturate and lose the capability to suppress electromagnetic interference. With this, the effective inductance of ferrite beads may decrease by as much as 50%. An additional increase in dc bias can drive impedance down by 90%. You can use manufacturer-provided impedance versus load current curves to find ferrite beads that handle dc currents without reaching saturation or losing impedance.
With the resistive characteristics of ferrite beads in mind, you should also plan for voltage drops in your circuit. In addition, you also need to plan for heating that occurs as the ferrite dissipates high frequency energy. Always check the manufacturer’s specifications for the maximum DC current and the DC resistance rating. The ferrite bead that you select must have a DC current rating more than twice the value of the required current for the rail.
No matter what you are designing, if you are considering using a ferrite bead make sure that you have the simulation and analysis happening with your board before you ship it off for production. Cadence’s OrCAD PSpice simulator will be sure to provide you the analysis you need before and throughout your designing process.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.
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