How to Select Ferrite Bead for Filtering in Buck Boost Converter

This presentation from Würth Elektronik by Lorandt Fölkel provides selection guide of use of ferrite beads for filtering application with practical case study in a 100W buck-boost converter as a noise source.

The presentation provides details about 3 different design concepts using almost the same BOM (bill of material) but different layout to achieve the EMC requirements as well conducted and radiated emissions.

Introduction

The last cars without electronic have been built in the sixties, since that time the electronic devices invade the cars. Started from a simple AM/FM Radio until today where autonomous driving and talking to the driver becomes standard for next generation cars. However already the in the early sixties the manufacturers was facing EMI (electro-magnetic-interferences) generated from the ignition coils and interfere with radio reception.

The “magic” solution to cancel the noise was to use a rod core inductor to filter those sparks generated RF harmonics. Later in the nineties, the cell phones boom was the one who made interferences with the audio systems generating unwanted sound. Today the noise sources is almost impossible to avoid and should be considered already in the early design stage. Therefore, is need to know the harmonics generated and to be filtered close as possible to the noise source. As well, the signals must be immune to interferences to avoid fails at the final EMC testing of the car.

Electromagnetic interference (EMI) suppression is a critical design consideration in modern electronic systems. Ferrite beads, also known as EMI suppression beads, are passive components that provide frequency-dependent impedance to attenuate unwanted high-frequency noise while allowing low-frequency signals and DC to pass. Selecting the correct ferrite bead requires balancing electrical performance, package constraints, and system-level requirements.

Chapter 1: Fundamentals of Ferrite Beads

Ferrite beads are constructed from ferrite material, a ceramic compound with high magnetic permeability. At low frequencies, the bead behaves like a simple inductor. At higher frequencies, however, the ferrite material introduces losses, converting high-frequency noise into heat. This makes ferrite beads particularly effective for EMI suppression in power supply lines and signal traces.

Chapter 2: Key Electrical Parameters

When selecting a ferrite bead, engineers must evaluate several parameters:

Parameter	Description	Design Impact
Impedance (Z)	Frequency-dependent resistance to AC signals	Defines noise attenuation capability
DC Resistance (DCR)	Resistance at DC	Affects power loss and efficiency
Rated Current	Maximum continuous current without overheating	Limits applicability in power circuits
Self-Resonant Frequency	Frequency where inductive and capacitive reactances cancel	Defines effective operating range

Chapter 3: Application-Specific Considerations

Ferrite beads are not one-size-fits-all components. Their effectiveness depends heavily on the application domain, the type of noise present, and the system’s operating environment. Below are expanded considerations for three major categories of use.

3.1 Digital Circuits

In high-speed digital systems, such as microcontrollers, FPGAs, and SoCs, ferrite beads are commonly placed on power supply rails to suppress switching noise. The rapid rise and fall times of digital signals generate harmonics that extend well into the hundreds of MHz. A ferrite bead with a resistive impedance peak in this frequency range is ideal. Designers often combine beads with decoupling capacitors to form a low-pass filter network, ensuring stable supply rails for sensitive ICs.

Use Case	Noise Source	Ferrite Bead Requirement
Microcontroller power rails	Clock harmonics (50–300 MHz)	Bead with 100–600 Ω impedance at 100 MHz
Mixed-signal ICs	Cross-coupling between analog and digital domains	Bead with low DCR, moderate impedance, paired with bypass capacitors

3.2 Automotive Electronics

Automotive environments impose stringent requirements due to wide temperature ranges, high transient currents, and strict EMC regulations. Ferrite beads used in vehicles must comply with AEC-Q200 standards and withstand temperatures from -55 °C to +150 °C. Applications include infotainment systems, battery management, and powertrain control modules. In these cases, beads must handle higher currents (up to several amperes) while maintaining stable impedance across a broad frequency spectrum.

• High-current capability (≥ 3 A continuous)
• Low DCR to minimize power loss in 12 V and 48 V systems
• Robust thermal stability for under-hood environments
• Compliance with automotive EMC standards (CISPR 25, ISO 11452)

3.3 RF and Communication Systems

In RF circuits, ferrite beads are used more selectively. While they suppress unwanted noise, they can also distort desired signals if not carefully chosen. For example, in antenna feed lines or RF front-ends, a bead with too high an impedance may attenuate the wanted signal. Therefore, RF applications often require simulation and measurement to ensure that the bead’s impedance profile aligns with the system’s frequency plan.

Typical RF use cases include:

1. Preventing digital noise from coupling into RF transceivers.
2. Suppressing spurious emissions in wireless modules (Bluetooth, Wi-Fi, LTE).
3. Filtering supply lines of low-noise amplifiers (LNAs) without degrading gain.

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Chapter 4: Practical Selection Workflow

Selecting the right ferrite bead is a multi-step process that combines theoretical analysis, datasheet evaluation, and empirical validation. Below is a detailed workflow that engineers can follow.

4.1 Define Noise Characteristics

Identify the frequency range of the unwanted noise. This can be done through spectrum analysis or by estimating harmonic content from switching frequencies. For example, a DC-DC converter switching at 2 MHz may generate harmonics up to 200 MHz.

4.2 Match Impedance Profile

Select a bead whose impedance curve peaks in the noise frequency band. The goal is to ensure that the bead presents a predominantly resistive impedance at the target frequencies, thereby dissipating noise energy rather than reflecting it.

Noise Frequency	Recommended Impedance	Typical Applications
10–50 MHz	30–100 Ω	Switching regulators, USB 2.0
50–200 MHz	100–600 Ω	Microcontrollers, digital buses
200 MHz–1 GHz	600–1000 Ω	RF modules, wireless systems

4.3 Verify Current and Thermal Ratings

Ensure that the bead can handle the maximum continuous current without excessive heating. Power loss is calculated as P = I² × DCR. For high-current applications, select beads with low DCR to minimize efficiency loss.

4.4 Evaluate in Circuit

Simulation tools (e.g., SPICE models) and real-world measurements (e.g., using a spectrum analyzer or VNA) are essential to validate performance. Engineers should measure insertion loss and confirm that the bead does not introduce unwanted resonances with nearby capacitors.

4.5 Compliance and Reliability Testing

Finally, validate the design against EMC standards relevant to the application (e.g., CISPR 32 for consumer electronics, CISPR 25 for automotive). Long-term reliability testing under thermal cycling and vibration is also recommended for mission-critical systems.

By following this structured workflow, engineers can move beyond trial-and-error selection and achieve predictable, repeatable EMI suppression results.

Conclusion

Ferrite beads are indispensable components for EMI suppression in modern electronics. By carefully analyzing impedance characteristics, current handling, and application-specific constraints, engineers can ensure robust noise filtering without compromising system performance. A structured selection workflow, supported by simulation and measurement, provides the most reliable results.