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EMC

Power Supply Ripple Filter

Calculate LC filter attenuation and output ripple voltage for power supply EMC filtering. Find the resonant frequency and ripple rejection.

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Formula

f₀ = 1/(2π√LC), A = −40·log₁₀(f/f₀) dB

How It Works

Power supply switching noise (ripple) is a major source of conducted emissions. A single-stage LC low-pass filter provides −40 dB/decade attenuation above its corner frequency f₀ = 1/(2π√LC). The required corner frequency to achieve attenuation A (dB) at ripple frequency f is f₀ = f / 10^(A/40). Output ripple voltage is V_out = V_in × 10^(A/20). Designing f₀ well below the switching frequency (typically f₀ < f_sw/10) ensures adequate attenuation. Damping resistors may be required to prevent resonant peaking that can amplify noise near f₀. The characteristic impedance Z₀ = √(L/C) should roughly match the load impedance for best damping.

Worked Example

Problem
A 100 kHz SMPS produces 100 mV input ripple. Design an LC filter to reduce this to below 1 mV using equal L and C values with a 50 Ω load.
Solution
1. Required attenuation: A = 20·log₁₀(100/1) = 40 dB 2. Required f₀: f₀ = 100,000 / 10^(40/40) = 100,000 / 10 = 10 kHz 3. LC product: LC = 1/(2π × 10,000)² = 1/(3.948×10⁹) = 2.53×10⁻¹⁰ s² 4. For Z₀ = 50 Ω: L/C = 2500; L = √(2500 × 2.53×10⁻¹⁰) = √(6.33×10⁻⁷) = 795 μH; C = LC/L = 3.18×10⁻⁷/795×10⁻⁶ = 0.4 μF Result: A 795 μH inductor and 0.4 μF capacitor gives a 10 kHz corner frequency and 40 dB attenuation at 100 kHz, reducing ripple from 100 mV to ≈1 mV.

Practical Tips

  • ✓Choose a ferrite-core inductor for the EMI filter (not an air-core) for better high-frequency attenuation and lower core saturation risk at DC current.
  • ✓Use a π-filter (capacitor–inductor–capacitor) to achieve −60 dB/decade for difficult cases where a single LC stage is insufficient.
  • ✓Place the output filter capacitor physically close to the load, not close to the inductor, to minimise high-frequency parasitic inductance in the decoupling path.

Common Mistakes

  • ✗Using electrolytic capacitors — at 100 kHz+ their high ESR dramatically reduces attenuation; use low-ESR film or ceramic capacitors for the LC filter.
  • ✗Ignoring filter resonance — a lightly-damped LC filter amplifies noise at f₀; add a small damping resistor in series with a larger capacitor across the main filter capacitor.
  • ✗Neglecting the inductor's DC resistance — high DCR causes voltage drop under load; balance DCR against inductance for the required ripple attenuation.

Frequently Asked Questions

A ripple filter targets the switching frequency and its harmonics on the output DC rail. A conducted emissions filter (line filter) is placed on the AC input side to suppress noise coupled back to the mains. Both are LC filters but designed for different impedances and frequency ranges.
The inductor must handle the maximum load current plus the peak ripple current (typically 20–30% of load current for well-designed SMPS). Under-rating causes saturation and loss of inductance, which removes the filtering effect.
A single capacitor only provides −20 dB/decade attenuation, not −40 dB/decade. For tight ripple requirements (< 10 mV output) at high switching frequencies, an LC or even π-filter is usually necessary.

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