How to Select and Use Ferrite Beads in Switching Converters
Ferrite beads are electronic components that filter out noise from digital equipment and protect analog components. Selecting appropriate beads requires analyzing dominant noise frequencies and using the best configurations to resist noise. Circuit characteristics, DC resistance and rated current are fundamental to this process.
Other Names for Ferrite Beads:- Chokes
- Blocks
- Cores
- Rings
- Electromagnetic Interference Filters
Electronic Functions: The main role that ferrite beads play in circuits is passive, as they reduce high frequency noise. They do this by blocking interference both from and to an electronic device. A bead can transmit electromagnetic energy through a conductive cable. When EMI is present, the bead can prevent the cable from picking up radio interference from other devices.
Beads are useful components in medical equipment. They can be used as or added to inductors to create a passive low-pass filter that generates impedance. Ultimately, they can block specific frequencies.
Frequency Analysis: When deciding on ferrite beads, target noise frequencies need to be considered. These frequencies should fall within the bead's frequency range in which resistive impedance surpasses reactive impedance. Since ferrite beads are placed in series within power switches, DC current flows through the bead, resulting in voltage drops that are proportional to DC resistance.
While the DC resistance of the bead is not a big factor, it's important to ensure that the steady state current flowing through the bead exceeds the rated current. Otherwise, you can damage the device. Keep in mind that rated current does not remain constant over temperature. The higher the temperature your system allows, the more careful you must be selecting ferrite beads.
It's also essential to guard against DC currents falling too low due to saturation of the ferrite material, which reduces peak impedance. Core saturation must be avoided. So make sure that the bead's rated current is 50-100 percent higher than the total current you expect.
Switching Converters: The majority of switching converters usually operates at 5 MHz or lower. Between 50 and 200MHz filter inductors commonly switch from an inductive to capacitive role. High frequency ringing currents are converted to heat through dissipation.
Allied Components International manufactures both surface mount and leaded ferrite beads available in different sizes and types.
