Sound comes from vibration. Audible noise (20 Hz – 20 kHz) from MLCCs means the capacitor vibrates under AC voltage (typically < 1 nm displacement).
Sound comes from vibration. Audible noise (20 Hz – 20 kHz) from MLCCs means the capacitor vibrates under AC voltage (typically < 1 nm displacement).
Why Do MLCCs Vibrate?
All materials exhibit
electrostriction (expansion/contraction under electric field).
For high‑dielectric ferroelectric materials, this effect is strong and called the
piezoelectric effect.
1. Direct Piezoelectric Effect
Mechanical pressure → generates electric charge on the material surface.
2. Inverse Piezoelectric Effect (root cause of squealing)
Voltage applied → material deforms mechanically.
BaTiO3 unit cell structure showing piezoelectric behavior
Direct piezoelectric effect diagram: pressure → charge
Inverse piezoelectric effect diagram: voltage → deformation
Do All MLCCs Whistle?
No — it depends on the dielectric material.
1. Paraelectric (Class I)
- Materials: SrZrO3, MgTiO3
- Examples: NPO/COG
- No whistling — very small electrostrictive deformation.
2. Ferroelectric (Class II)
- Materials: BaTiO3, BaSrTiO3
- Examples: X7R, X5R
- Whistles loudly under large AC voltage due to strong piezoelectricity.
When AC voltage is applied, the ceramic expands/contracts, transferring vibration to the PCB. If the frequency falls in 20 Hz – 20 kHz, you hear a squeal.
Vibration mechanism of X7R MLCC causing PCB resonance and noise
Common Applications Where Squealing Occurs
- Switching power supplies
- AC‑DC converters
- High‑frequency power circuits
Solutions to Reduce Capacitor Whistling
All methods add cost but eliminate audible noise.
-
Replace with non‑piezoelectric capacitors
Use film capacitors, tantalum capacitors, or Class I ceramic capacitors.
-
Modify the circuit
Reduce large AC voltage across the capacitor, or shift frequency out of the audible band (especially 1 kHz – 3 kHz).
-
Improve PCB design
Optimize stackup, layout, and rigidity to reduce resonance.
-
Use low‑noise / silent MLCCs
Low‑Noise MLCC Design Methods
①
Thickened bottom protection layer
Reduces deformation transfer to the PCB.
Cross‑section showing thickened bottom dielectric layer
②
Metal frame / standoff structure
Isolates the capacitor body from the PCB to absorb vibration.
Metal bracket structure suspending MLCC
③
Weak‑piezo dielectric materials
Doped BaTiO3 with reduced piezoelectric effect (slightly lower dielectric constant).
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