Energy Does Not Come Free

Every time the voltage across a capacitor changes, a tiny bit of energy turns into heat inside the dielectric. This is dielectric loss. At low frequencies you barely notice it. At high frequencies or high ripple currents, it becomes a real problem.

Where the Loss Comes From

Leakage (DC Conduction)

No dielectric is a perfect insulator. A small DC current always leaks through — typically nanoamps to microamps in aluminum electrolytics. In a DC circuit, this is just slow discharge. In AC, it contributes to overall power loss.

Dipole Friction (AC Polarization Loss)

The bigger contributor. When AC voltage alternates polarity, dielectric molecules try to realign with each field reversal. This molecular movement generates friction — heat. At 60 Hz the molecules keep up easily. At 100 kHz they lag behind, wasting more energy. This is why dissipation factor (DF) increases with frequency.

Dissipation Factor: The Number That Matters

DF measures what fraction of reactive power becomes heat:

• Film (polypropylene): DF = 0.001 — nearly lossless
• NP0/C0G ceramic: DF = 0.001 — excellent
• X7R ceramic: DF = 0.025 — good for decoupling
• Aluminum electrolytic: DF = 0.10-0.25 — the highest of common types

A DF of 0.1 means 10% of reactive power becomes heat. In a 100W PFC stage, that is 10W to dissipate.

When It Bites You

• Ripple current heating: Heat from DF plus ESR raises core temperature. Every 10 C rise cuts life in half
• Audio distortion: High-DF capacitors in the signal path introduce nonlinear distortion. Audiophiles use polypropylene film for a reason
• RF circuits: At GHz frequencies, dielectric loss dominates. PCB substrate matters as much as capacitor choice
• Thermal runaway: Heat increases leakage, which creates more heat — a positive feedback loop that can destroy electrolytics

How to Work Around It

1. Stay within ripple current rating: The single most important rule for electrolytic reliability
2. Parallel a film cap with your electrolytic: Film handles high-frequency ripple with low DF. Electrolytic handles bulk capacitance
3. Use polymer electrolytics: Lower ESR and lower DF than wet types
4. For precision analog: Film or NP0. DF below 0.01 is worth the extra cost and size

Bottom Line

Dielectric loss is not a datasheet curiosity. It determines whether your capacitor runs cool or cooks itself. For power supply filtering, DF of 0.1-0.2 is fine. For anything precision or high-frequency, look for DF below 0.01.