Film capacitors: Characteristics and uses in power applications

Article By : Rudy Ramos

Learn about the different types of capacitors that might be considered for use in power electronics applications.

Engineers designing power electronics find that capacitors are needed for several functions, from energy storage to filters and decoupling. Different capacitor types are available, that at first sight might seem equivalent in their headline ratings of capacitance and voltage, but would not perform equally. Incorrect selection can lead to, at best, an expensive ‘over-engineered’ solution and, at worst, an unreliable or unsafe product.

This article describes the different types of capacitors that might be considered for use in power electronics applications. Particularly, electrolytic and film types are compared showing how and when each has a role. The variety of film types and their construction are described in more detail and preferred types identified. Specifications for capacitance, ripple current rating, transient voltage immunity, and safety rating, along with other characteristics are examined in detail. The phenomenon of ‘self-healing’ after voltage stress is discussed, explaining its physical mechanism and the value it confers in typical circuits. The main applications for film capacitors in power electronics are identified and guidance given on how to select appropriate film capacitor types. Detailed calculations are then given for some example circuits showing how particular capacitors and their ratings are selected. Calculations are generalized to enable engineers to use them as a basis for their designs.

It is hard to imagine any modern electronics that do not include capacitors of some type. They may be vanishingly small surface-mount types in cell phones for example, but they are still there. In power electronics, the function is filtering, energy processing, and transfer, and in contrast, capacitor volumes can be measured in cubic inches. In this application, there is sometimes a seeming choice between aluminum (Al) electrolytic and film types but in terms of stored energy density, Al-electrolytics are in some ways ahead. The only comparable film types are exotic and expensive such as ‘segmented high-crystalline metallized propylene’ which even then, do not maintain their ripple current rating well at high temperatures. Al-electrolytics have a relatively poor reputation for life and reliability, but this only applies if they are worked hard. Suitably derated with voltage, ripple current, and temperature, they can last many years. Their low cost for a given capacity-voltage (CV) rating is, of course, a significant factor. This means that they are the practical solution for high-volume energy storage applications such as on the internal high-voltage DC bus of commodity AC-DC power supplies.

Film capacitors have their place in power electronics
Film capacitor types certainly do have some advantages over their Al-electrolytic cousins; they can have much lower Equivalent Series Resistance (ESR) for the same CV rating, which gives them typically much better ripple current ratings. They are also relatively more tolerant of voltage over-stress and significantly, can in some cases ‘self-heal’ after a degree of breakdown, enhancing system reliability and lifetime. When localised breakdown does happen, a short circuit forms in the body of a film capacitor but a plasma arc occurs which acts to clear the short. This works only within stress limits though; catastrophic failure can still occur due to carbon deposition and collateral damage to the dielectric insulation. In practice, Al-electrolytics can only withstand typically 20% voltage overstress while the figure for film types can be 100% for a limited time. The difference in failure mode is significant as well; Al-electrolytics will often go short after overstress with explosive results causing discharge of liquid electrolyte and damage to other components.

It is true that theoretical failure rates for Al-electrolytic and film types can be comparable with correct derating, but in real-life applications with occasional voltage stress from, for example, inductive loads or lightning strikes, system reliability can be entirely different between the two technologies. Degradation due to humidity is an issue for film capacitors but this is in common with other components so should be controlled for best reliability.

When energy storage is not the headline parameter, large value film capacitors can be a high-performance solution. An example would be on a battery-backed DC bus such as you see in electric vehicles, alternative energy systems, and uninterruptible power supplies. In these applications, the primary function of the capacitor is to source and sink high-frequency ripple current that could be measured in hundreds or thousands of amps where low capacitor ESR is vital to achieving low losses and low ripple voltage.

The move to higher bus voltages also favors film capacitor types; the same energy is stored with smaller CV ratings at high voltage (due to the ‘squared’ in E=CV2/2) so less capacity is needed, and film types are available with kV ratings as required. Al-electrolytics are limited by their technology to about 550V and although they can be stacked for higher voltage, they have inherent high and variable leakage current requiring parallel balancing resistors with their associated cost and losses. We discussed the short-circuit failure mode of Al-electrolytics; when in series, one failing this way will then place high voltage across the others, with an avalanche of consequential damage.

A practical difference between film and Al-electrolytic capacitors is their mounting options. Film is available in volumetrically-efficient rectangular box formats with a choice of wire, screw, lug, push-on connector or even bus-bar terminations. For Al-electrolytics, a round metal can is the only standard option although with a similar range of terminations available. Unlike Al-electrolytics, film types are non-polar, they can operate happily with either polarity of voltage applied making them reverse-proof. This also means that they are ideal for applications where AC voltage is applied such as in inverter output filtering.

We have talked about ‘film’ capacitors in general but there are many sub-types with differing performance and applications. Figure 1 [1] gives a summary of some types that might be used in power electronics with their main characteristics.

Figure 1 Film capacitor characteristics

Amongst the performance data, polypropylene is a good contender for power applications, with its wide voltage and capacitance ranges and good self-healing performance. The particularly low figure for dissipation factor (DF) at all frequencies is important as well; DF is the ratio of ESR to capacitive reactance ZC = 1/2πfC. A low figure implies lower heating effects compared with other dielectrics and is a way of comparing losses per microfarad of capacity across capacitor types. Generally, DF varies a little with temperature and frequency, but polypropylene performs best in the comparison, see Figure 2 for plots.

Figure 2 Typical dissipation factor variation with temperature and frequency for polypropylene film (Source: Cornell Dubilier)

For less critical applications in power, polyester can be an excellent low-cost choice with its high specific capacitance (CV per volume) and wide temperature range.

[Continue reading on EDN US: Polypropylene capacitor construction]

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