The Short Version
A capacitor stores charge because its dielectric material polarizes under an electric field. The dielectric constant measures how strongly a material polarizes. Higher dielectric constant means more capacitance from the same physical size.
Where Capacitance Comes From
The formula: C = dielectric constant x permittivity of free space x area / distance. Three levers increase capacitance: larger plate area, thinner dielectric, or higher dielectric constant. Aluminum electrolytics use all three — etched foil for surface area, nanometer-thin oxide, and an oxide with a dielectric constant around 9.
Tradeoffs of High Dielectric Constant
Breakdown Voltage
High dielectric constant materials tend to have lower breakdown voltage. Barium titanate (constant ~1,000-10,000) breaks down at far lower fields than aluminum oxide (constant ~9). Aluminum oxide is special because its dielectric strength is exceptionally high — 700-1,000 V per micrometer — letting electrolytics handle hundreds of volts.
Temperature Behavior
Some high-constant ceramics lose capacitance with temperature. Y5V (constant >10,000) can lose 82% of its room-temperature capacitance at 85 C. X7R (constant ~2,000-4,000) limits the change to +/-15%. NP0/C0G (constant ~20-100) changes less than 0.05% per degree.
DC Bias Effect
Class II ceramics (X7R, X5R, Y5V) lose capacitance under DC bias. A 10 uF X7R capacitor rated for 6.3V might deliver only 5 uF at 5V bias. Film and electrolytic capacitors do not have this problem.
Aging
High-constant ceramics age: capacitance decreases about 3-5% per decade-hour after soldering. Film, electrolytic, and NP0 ceramic capacitors do not age significantly.
Choosing the Right Dielectric
| If You Need | Use | Why |
|---|---|---|
| Bulk capacitance, cost-sensitive | Aluminum electrolytic | Best CV product per dollar |
| Decoupling at MHz, small | X7R MLCC | Good density/stability balance. Watch DC bias |
| Precision timing, stability | NP0/C0G or film | Near-zero aging, tight tolerance |
| High voltage, long life | Film or electrolytic | Film for kV, electrolytic for hundreds of volts |
| Smallest possible size | Tantalum or X7R | High volumetric efficiency |
Key Takeaways
- The dielectric constant is not just a formula multiplier — it comes with stability, voltage, and aging tradeoffs
- If your ceramic decoupling cap delivers half its rated capacitance, it is probably DC bias, not a bad part
- Aluminum electrolytics dominate power electronics not because of high dielectric constant (only 9) but because of combined thin dielectric, etched surface area, and cost
- When stability matters more than density, go with a lower dielectric constant: film or NP0
