What is power film capacitor?
Power capacitors are mounted inside industrial equipments in three main positions: AC Filtering, Snubber and DC-Link. Every field of application prefers the usage of a particular film technology depending on two main factors: Power of the equipment and circuits layouts with different peculiarities. The windmill generators, solar converters, drivers (welders) and UPS represent the different fields where power film capacitors are utilized in the industrial market. Giving particular attention to wind power inverters, the voltage trend is continuously increasing from 450-500Vdc to higher voltages in the range of 700Vdc to 1350Vdc. High voltages have the advantage to reduce power loss in the alternator and to decrease the current values and the total capacitance value of the components. The use of film capacitor reduces the number of components and improves life reliability with excellent self-healing, long useful life and excellent overvoltage capabilities till the maximum class Temperature of the component.
What’s the application for dielectrics?
The dielectrics normally used for film capacitors are:
PP: polypropylene;
PET: polyethylene terephthalate;
PEN: polyethylene naphthalate;
PPS: polyphenylene sulphide.
The Self-healing proprieties (the dielectric ability to regenerate from internal drop of insulation resistance) of the plastic base film take a fundamental place to ensure a safe failure mode in Power applications. The main plastic material for Power film capacitors is PP because of its high self-healing properties and low and stable dissipation factor. PEN and PPS can’t meet these requirements sufficiently well and are normally excluded. Generally in industrial applications temperature > +105°C are not required and even PET is not used.
To compare the self-healing properties of different materials we have to consider its chemical composition. Naturally there are many other parameters to consider in the self-healing process and the energy content during the clearance process play a key rule: the process needs high energy to be efficient and to insulate the two metal layers. Many developments are running with the most relevant names of base film manufacturers to increase the performances of the actual dielectrics available on the market. The trend is to overcome the temperature limitations of polypropylene film maintaining its self-healing properties or to develop a new dielectric to bring the market to new generations of components.
Some applications have reached 230-250V/μm using high quality PP base film and accurate metallization profiles. The metallization process of plain film is performed by evaporation of metals; the two main materials are Aluminum and Zinc often combined together to obtain the proper performances in every zone of the film. The metallization processes take care of base film parameters and adopt different slope profiles with many physical-chemical treatments to increase the adhesion of the evaporated metals on the dielectric and to protect the thin metallization layers of Al and Zn. Free margins technologies are more and more accurate in particular if combined with some particular drawings like segmentations. These ones, together with the proper profile, are able to increase the safety margins of the final components, increase its life time decreasing the probability of short circuits. The right combination between metallization profile and segments reduces the catastrophic failures, increase the lifetime expectancies and limits the end of life failure mode to drop of capacitance.
The configurations of capacitor
The today’s increasing demand for energy savings and for alternative energy sources, has boosted the research and development in strategic inverter-driven products like Solar Converters and Wind Generators. All of them profoundly rely on a better management of the energy conversion. High reliability, withstanding of tough ambient conditions, long expected life, are just some targets of the agenda of the technical innovation initiatives today.
The strong advancement of inverter products calls for DC-Link capacitors with high mechanical flexibility with ability to be attached to the semiconductors (IGBTs and MOSFETs) in the best possible way. Technologies for producing the internal construction are available in Wound and Soft-winding constructions. These internal constructions can be packaged in a large range of configurations in order to offer, according to the application, the smallest possible size and inductance, or the highest possible voltage and ripple current, or the best possible connections.
What’s the future of power film capacitor?
In many new applications there are specifications for high rated voltage and over voltage
The most innovative applications also require high capacitance per volume that means it is necessary to use a thinner film than has been the practice in the past. The average value of film thickness decreasing and gradient of voltage increasing of the different series is moving from 200V/μm to 230-250V/μm in dry technology, this kind of density was proper of wet technology in the past years.
More energy in less space, that's the ambitious and challenging goal for new power designs involving film capacitors.
This scenario strongly contributes to creation of a new generation of metallized film capacitors for Power Electronics
applications, a typology that along with the well established characteristics of self-healing, stable performances vs. time and
high ripple current / capacitance ratio, now also can count on more reliable Polypropylene base films and on new techniques
of metallization.
Lifetime of the components
Industrial Applications require a long life time that can be accomplished by utilizing new metallization configurations of the film together with plastic or metallic housing sealed by dry resins. Manufacturing machineries and new production process are oriented to preserve and maintain the thermal treatment.
Working life time at rated voltage (Un) vs. Hot Spot Temperature for capacitors fabricated using soft-winding & round-winding technology. 100,000 hours of life can be achieved at temperatures up to 75°C and approximately 30,000 hours of life can be achieved at 85°C.
Higher number of hours could be reached in particular conditions and with a Voltage significantly lower than the rated voltage and moderate ripple content. For polypropylene film capacitors we estimated a maximum lifetime of 200khours, after that every estimation is purely theoretical.
