Product Insights: Why Do Ferrites Never Seem to Work When We Need Them To?

Disclaimer

This article does not aim to criticize the use of ferrites or the suppliers who provide them. Ferrites play a crucial role in achieving electromagnetic compatibility (EMC) compliance for a wide range of devices and applications. Without ferrites, meeting EMC standards would be significantly more challenging.

Introduction

During critical moments, like when you’re racing against a tight deadline and your product fails an EMC emissions test, have you ever turned to ferrite beads or other RF-absorbing materials to suppress unwanted RF emissions? Sometimes, despite our hopes, these ferrite beads don’t seem to work as expected. In those moments, we playfully dub them ‘prayer beads,’ hoping for a miraculous solution. However, perhaps the issue lies not with the ferrite itself but with our understanding of how and when they are most effective.

Background Information

Before delving into the intricacies of why ferrites often fall short of our expectations, let’s start with a brief review of what ferrites are and how they function.

A ferrite bead, also called a ferrite choke or ferrite core, serves as a passive electronic component used for noise suppression and filtering in circuits. It achieves this by dissipating high-frequency currents within a ferrite ceramic. When installed on power pins of digital circuits, ferrite beads effectively suppress high-frequency signals.

Ferrites consist of alloys containing iron/magnesium or iron/nickel. These materials are selected for their high permeability at high frequencies and high impedance. At low frequencies, ferrites primarily exhibit inductive behavior; at high frequencies, they behave predominantly as reactive components. Conceptually, they can be considered a parallel combination of a resistor and an inductor, dissipating high-frequency energy in the form of heat.

Ferrite beads find applications in three main areas:

• External Cabling: Large ferrite beads are commonly used on external cables.
• Internal Circuits: Smaller ferrite beads are employed internally around pins of components such as transistors, connectors, and integrated circuits.
• DC Conductors: Beads can block low-level unintended radio frequency energy on wires intended for DC signals.

Reasons Why They May Not Work When We Need Them To

Ferrite beads may not always meet expectations due to several factors:

• When attempting to suppress noise using a ferrite bead, we often find ourselves in a situation where we don’t fully understand the source of the noise. In such cases, we’re essentially guessing its location.

In a recent personal experience, I encountered two different end-products that incorporated similar TFT liquid crystal displays. Both devices exhibited excessive RF emissions in the 30 to 50 MHz range. Attempts were made to suppress these emissions by placing a properly selected ferrite sleeve around the display’s I/O cable. Interestingly, the outcomes differed significantly:

1. 1. 1. 1. First Case: The ferrite effectively did its job, reducing emissions below Class B levels.

1. 1. 1. 2. Second Case: Surprisingly, nothing changed. The ferrite appeared ineffective.

Upon investigation, it became clear that the source of emissions in the second case was not the display itself, nor did they radiate from the display’s cable. Instead, they originated from another source entirely. Consequently, the ferrite’s lack of effectiveness was justified.

Side Note: In the first scenario, high-frequency probing techniques were employed to successfully pinpoint the problematic display I/O cable. However, in the second case, no efforts were made to identify the precise source of emissions.

• Circuit impedance is too high.

A ferrite bead or choke behaves like a lossy inductor. Consequently, it is effective primarily between low-impedance circuits. If placed in a high-impedance circuit or transmission line, it provides minimal attenuation.

• An application note or other outdated advice may contain errors.

Perhaps you’ve followed conventional wisdom and added a few ferrites to your design as a precaution. However, the current advice suggests refraining from adding ferrites unless you’re specifically addressing a problem. When attempting to create proper filtering in the power delivery network (PDN) on a printed circuit board (PCB), adding ferrites can inadvertently disrupt the network’s impedance. The ferrite’s impedance may resonate with the network’s capacitance, leading to significant voltage spikes. By adding the ferrite, you prevent digital circuits on the PCB from drawing power at high frequencies. Interestingly, this wasn’t a concern a decade ago where the ferrite’s gain at resonance wasn’t a problem.

Pro Tip: If you need filtering in a PDN, there are better options available:

1. 1. 1. 1. Use a π-filter with inductors and capacitors.
         2. Use an RC filter with resistors and capacitors.
         3. Use an inductor instead of a ferrite.

• The incorrect ferrite was used.

Typically, when we use ferrites to address a problem, it’s for a specific frequency range. Various materials are employed to make ferrites, each offering attenuation over specific frequencies. Haphazardly grabbing any available ferrite from the test lab and hoping for the best can lead to errors. It’s crucial to invest extra time in researching and selecting the right ferrite material tailored to your specific situation.

Summary

The era of casually slapping a ferrite onto a circuit and expecting miracles is behind us. Ferrites are intricate components that require thoughtful consideration to achieve the desired performance. Interestingly, there are cases where adding ferrites can exacerbate noise issues. However, when used appropriately, ferrites remain valuable tools that will perform as we expect.

References: Further Reading and Watching

1. Montrose, M.I., EMC and the Printed Circuit Board – Design, Theory, and Layout Made Simple, IEEE, 1999.
2. Altium Academy, Ferrites in PCB Design: What the Experts Say. Retrieved from Ferrites in PCB Design: What the Experts Say
3. Altium Academy, Ferrites as a Filtering Element – Are They Effective? Retrieved from Ferrites as a Filtering Element – Are They Effective?
4. Altium Academy, Ferrites in Power Delivery Networks – Part One. Retrieved from Ferrites in Power Delivery Networks – Part One
5. Altium Academy, Ferrites in Power Delivery Networks – Part Two. Retrieved from Ferrites in Power Delivery Networks – Part Two