CLC Pi Filter Calculator
Estimate resonance, corner frequency, and ripple attenuation for a practical CLC (capacitor-inductor-capacitor) low-pass filter.
Note: This is an idealized small-signal estimate. Real ESR, DCR, and capacitor tolerance can materially affect final performance.
What is a CLC Pi Filter?
A CLC Pi filter is a three-element low-pass filter made from a capacitor, an inductor, and another capacitor, arranged in a shape that resembles the Greek letter π (pi). It is widely used in power supply design to reduce ripple and high-frequency noise before power reaches sensitive circuits.
Typical use cases include:
- Smoothing rectifier ripple in linear supplies
- Reducing switching noise in DC/DC converter outputs
- Improving analog performance by lowering supply-borne noise
- Filtering motor driver and mixed-signal rails
How this calculator works
This calculator combines practical frequency-domain estimates for a CLC network with finite source and load resistance. It calculates:
- Equivalent series capacitance: Ceq = (C1×C2)/(C1+C2)
- Pi-section resonance estimate: fres = 1/(2π√(L·Ceq))
- Output LC corner estimate: fLC2 = 1/(2π√(L·C2))
- Ripple attenuation at your selected frequency
- Estimated output ripple (Vpp)
Why source and load resistance matter
Ideal LC networks can show unrealistic peaking. In real designs, resistances in the source, inductor winding (DCR), capacitor ESR, and load all introduce damping. Including source and load resistance in your estimate gives a more realistic preview of attenuation and potential resonance behavior.
Design tips for better CLC performance
- Choose low ESR capacitors for better high-frequency attenuation.
- Watch inductor saturation current so performance does not collapse under load.
- Keep layout tight: short return loops and clean ground routing reduce parasitic inductance.
- Target ripple frequency intentionally: for 50/60 Hz rectifier supplies, ripple is often 100/120 Hz; switching regulators may be in the 50 kHz to 2 MHz range.
- Validate with measurement: always verify with an oscilloscope and proper probing techniques.
Example workflow
Suppose you have C1 = 470 µF, L = 10 mH, and C2 = 470 µF. You are feeding a 50 Ω load from a source with 0.5 Ω impedance, and you want to estimate 120 Hz ripple reduction. Enter those values and run the calculator. If attenuation is not strong enough, increase inductance, increase C2, or both. If peaking appears near resonance, add damping (or account for ESR/DCR in the real build).
Practical notes and limitations
Use this tool for first-pass design, not as a complete replacement for simulation and bench testing. For production work, follow up with SPICE modeling including parasitics, worst-case tolerance analysis, thermal behavior, and startup transients.
Still, for quick sizing and intuition, this CLC pi filter calculator gives a fast, useful estimate for ripple attenuation and expected output ripple.