A Brief Discussion on Common Capacitor Fillers: The Trade-Off Between Performance, Cost, and Safety

A Brief Discussion on Common Capacitor Fillers: The Trade-Off Between Performance, Cost, and Safety

In the field of power electronics, metallized film capacitors are core components whose reliability and service life depend heavily on the internal filler material selected. Common fillers include silicone oil, nitrogen, black glue (solid epoxy resin), liquid epoxy resin, paraffin wax, asphalt, and castor oil. In recent years, epoxy soybean oil has also drawn increasing attention for its application in metallized film capacitors.

The primary functions of filling materials are insulation and thermal conduction. Their overall quality is governed by multiple factors, including insulation performance, thermal conductivity, chemical compatibility with the film, and stability under high temperatures and electric field stresses.

Black glue is widely adopted due to its low cost and significant price advantage. However, its solid-state nature means it only liquefies after the temperature rises to a certain level, resulting in poor heat dissipation. When internal film breakdown occurs, the generated heat and gas cannot be slowly released through the black glue. Once accumulated to a critical point, they burst outward suddenly, often causing rupture of the capacitor's top cover or housing. More troublingly, during internal faults, the external temperature rise and appearance often remain normal, with only capacitance decay as an indicator, making potential hazards difficult to detect in time.

Silicone oil offers the best heat dissipation performance among all fillers, but its high cost limits widespread use. In practice, a compromise solution using a "quartz sand + silicone oil" mixture is often adopted, leveraging quartz sand's insulating and protective properties to achieve reasonable thermal performance while reducing expenses.

Nitrogen filling is inexpensive, environmentally friendly, and chemically inert. A well-sealed nitrogen-filled capacitor maintains internal pressure with minimal variation under temperature changes. However, this approach demands extremely high sealing quality in the housing. Once nitrogen leaks and air enters, impurities in the air can cause discharge within the capacitor, accelerating capacitance decay and potentially leading to explosion-proof failure, fire, or even catastrophic explosions.

Other materials such as liquid epoxy resin, paraffin wax, asphalt, and castor oil each find their specific use cases. Meanwhile, epoxy soybean oil, as an emerging eco-friendly material, is gradually being explored in metallized film capacitor applications. Its advantages lie in renewability, good biodegradability, and moderate insulation and thermal conduction potential. However, its long-term high-temperature stability and compatibility with film materials still require further verification.

In conclusion, the selection of capacitor fillers is by no means a mere cost consideration; it demands a delicate balance among electrical performance, thermal management, process feasibility, and long-term safety and reliability. Looking ahead, as power electronic equipment moves toward higher power density and enhanced reliability, novel filler materials that combine environmental friendliness with excellent comprehensive performance will become a key focus of research and development.