Ferrite Bead DC Bias Derating & Filter Attenuation Calculator
Calculate effective ferrite bead high-frequency impedance under DC bias current saturation, filter noise attenuation (dB), and compute the required parallel damping resistor (Rd) to suppress dangerous LC resonant peaking with decoupling capacitors.
Ferrite Bead & Circuit Loading
Ω
Manufacturer nominal datasheet rating (e.g. BLM18 series).
A
Datasheet maximum continuous DC current.
A
Actual steady-state DC current flowing through bead.
MHz
Switching harmonic or RF interference frequency.
Ceramic MLCC capacitor placed after ferrite bead.
Ω
Internal winding copper resistance.
Ferrite Saturation & Resonance
DERATED 25%
Effective Impedance (Zeff)
450 Ω
-25.0% from 600Ω nominal
Filter Attenuation (IL)
14.8 dB
Noise suppression @ 100MHz
Equivalent Inductance (L)
716 nH
At specified target frequency
LC Antiresonant Freq
59.5 kHz
Tank resonance with Cdec
Required Resonance Damping (Rd)
PREVENT RINGING
Parallel Damping Resistor:
0.13 Ω
DC Voltage Drop:
25.0 mV
Placing a low-value damping resistor across the bead (or using a lossy electrolytic cap in parallel) dampens Q-peaking spikes on step transients.
Ferrite Saturation & Damping Formulas:
• DC Derated Impedance: (Z_{ ext{eff}} approx Z_{ ext{nom}} cdot left[1 - (I_{ ext{dc}} / I_{ ext{rated}})^2 ight])
• Equivalent Inductance: (L approx Z_{ ext{eff}} / (2pi cdot f))
• Antiresonance: (f_{ ext{res}} = rac{1}{2pi sqrt{L cdot C_{ ext{dec}}}}), Damping: (R_d approx rac{1}{2} sqrt{L / C_{ ext{dec}}})