last highlighted date: 2024-10-17
Highlights
- Ferrite beads are passive components, but they’re often misunderstood as acting like active ones.
- Some experts advise to NEVER use them.
- Note: ferrite
- A bead works to reduce High Frequency interference in two ways:
- Dissipating the unwanted HF energy to heat (R)
- Isolation of the HF source through a high impedance connection (L + R)
- Typical bead impedance looks like this:
- At higher frequencies (typically around 100+ MHz), the impedance is mainly resistive, the inductive properties rapidly fade away.
- The resistive peak where ferrites perform best, depends heavily on current bias.
- That is to say, datasheet impedance curves are valid for no or very little current (often not specified at all). When a DC current is applied, core saturation occurs and the resistive peak shifts to higher frequencies.
- Using ferrites to reduce 1 MHz switching harmonics of a power converter is therefore a bad idea: • The resistive region of ferrite beads usually peaks around 100 MHz or higher. (little attenuation in the first 80 (!) harmonics …) • The resistive damping regions shifts to even higher frequencies when a DC load current is applied.