EMI Bites: Why Your Capacitors Might Be Causing Common-Mode Noise
Decoupling capacitors are essential, but they don’t behave the way you think across all frequencies.
Here’s the problem:
- A capacitor’s impedance is low only up to its self-resonant frequency.
- Beyond that point, it starts acting inductive, and that's where EMI issues begin.
Why this may cause problems:
- After resonance, capacitor impedance increases with frequency due to parasitic inductance.
- The low-frequency portion of the conduction current flowing through the capacitors often forms large loops, which generate common-mode voltage sources—a major source of radiated EMI.
- The remaining high-frequency return current flows as displacement current through the dielectric between layers, not through the capacitors.
- This dielectric has impedance, so as current flows, it causes voltage drops between layers, creating potential differences that lead to unwanted emissions.
Key Insight:
Capacitors don’t kill all noise. Beyond resonance, if used incorrectly, they actually feed EMI problems.
How to manage this:
- Don't use capacitors with different values and types to spread the resonant points; this can cause antiresonance effects.
- Minimize current loop areas to reduce common-mode noise sources and emissions from differential-mode current.
- Ensure solid, low-impedance return paths for all signal layers.
- Pay attention to dielectric behaviour, as the field spreading through the stackup can generate voltage gradients.
—Dario
P.S. Want more EMI control strategies to pass EMC?