EMI Bites: The Hidden EMI Risk in Your Decoupling Strategy
Decoupling capacitors are meant to help, but beyond a certain point, they can become part of the EMI problem.
Why?
Because a capacitor only provides low impedance up to its self-resonant frequency.
After that, it starts behaving like an inductor.
Why this causes problems:
- Above resonance, capacitor impedance rises—making it ineffective at high frequencies.
- Low-frequency conduction current forms large loops, creating common-mode voltage sources—a major EMI emission risk.
- High-frequency return current becomes a displacement current, flowing through the dielectric between PCB layers.
- That dielectric has impedance too, which causes voltage drops between layers, spreading noise across the board.
Key Insight:
When current spreads through your stackup, it creates voltage gradients that radiate.
How to reduce this risk:
- Use a range of capacitor values to spread resonances and cover a wider frequency band.
- Minimize current loop areas to reduce common-mode generation.
- Ensure tight coupling between power and ground planes to keep displacement paths short.
- Design your stackup to support low-impedance vertical paths for return currents.
—Dario
P.S. Want more EMI control strategies to pass EMC?