Film capacitors shine in a wide range of applications.
By Daniel West, Lead Technical Applications Engineer at KYOCERA AVX.
When it comes to high-power applications like electric vehicles (don’t forget air, marine, and rail), solar plant inverters, power stations, and some high-energy storage/ discharge applications, you’re very likely to run into film capacitors on the bill of materials (BOM). This is because—at these power levels—it’s tough to beat the inherent safety and reliability of their unique material set.
The self-healing mechanism of film capacitors is one of the biggest differentiators to other capacitor technologies. They’re ideal in DC Link, power factor correction (PFC), and Snubber circuits to name a few. It’s worthwhile knowing why this should be. It’s also worthwhile comparing them to other capacitor technologies to understand why film capacitors aren’t as feasiblefor all power applications.
Film capacitors have polyester- or polypropylene- based dielectrics giving them their “plastic film” or “polymer film” capacitor monikers. Polyester-based dielectrics are more cost efficient, while polypropylene-based dielectrics have lower losses, meaning lower equivalent series resistance (ESR), which results in higher power handling.
The film can also have varying thicknesses that have a positive correlation to its final DC voltage rating, which can range from tens to thousands of volts. However, as the voltage range goes up (thicker dielectric layer), the maximum capacitance for a given volume goes down. This trade-off between capacitance and voltage rating is universal to all capacitor technologies.
As a note of interest, although they have limited maximum voltage ratings, tantalum capacitors are the most volumetrically efficient technology with dielectric thicknesses measured in angstroms, making them ideal bulk capacitors for high density applications (they can even pack enough capacitance in a small form factor for energy hold-up on solid state drives). Film capacitors, though, have that mysterious self-healing mechanism that we’ll get into next.
The film capacitor’s self- healing mechanism is enabled largely by the metallization process of the film that serves as the electrodes. It’s important to note that film capacitors manufactured with discrete layers of metal foil as the electrode will not have this effect. Metallized film is what we’re looking
for when it comes to high power applications. The metal (typically Aluminum or Zinc) is deposited on the film in a vacuum process and is very thin, causing the metallized film to feel like a bag of potato chips (or crisps); i.e., crinkly and flexible.
The X-Ray image shows a top-down view of an electrode layer. The black portion of the image is one metallization layer while the white portion is the film layer. Starting from the bottom ofthe image, we see columns of electrode segments that are isolated from one another going up to the termination. These electrode segments connect to the termination at the top via fuse elements. These fusing segments are essentially very thin metallization geometries designed to evaporate at a specific temperature.
A pair of the fusing segments that are intact and connected to an electrode segment is highlighted in a red circle on the figure. You’ll notice that
in most of the other electrode segments (e.g., segment “B”) the fusing segments are gone. Please know this image was taken after the capacitor had gone through harsh electrical stress. A fresh capacitor would have all electrode and fusing segments intact. You’ll also notice white dots scattered across the electrode segments; these are the hotspots that evaporated the metallization before the fusing segments (e.g., segment “A”).
Allowing holes in the electrode segment before the fusing segment evaporates is expected and intentional. This is to maintain as much capacitance for a longer period. As you can see, once the fusing elements go, the whole electrode segment is lost. The fusing segment will only evaporate when the whole electrode segment is getting too lossy, as opposed to being the weakest link of the electrode.
The long column pattern of electrode segments is just one example, these segmented patterns can become very complex to extend the lifetime of the capacitor, but this reduces the surface area of the metallization resulting in a lower rated capacitance value. For very tough applications, the film capacitors are impregnated with rapeseed oil to help cool the electrodes, slowing the loss of capacitance.
The equivalent circuit of this electrode layout is a very large array of parallel capacitors. When there is a hotspot due to a surge or prolonged electrical stress, the capacitors are effectively removed in an open circuit fashion. The drawback is that—over time—the accumulation of lost capacitance will reach a value that no longer maintains proper operation of the circuit. Having said this, if you have a large library of experimental data and accurately characterized materials, the lifetime of film capacitors is very predictable, which we’ll investigate next.
A quick comparison before we move on, though: Niobium Oxide (NbO) capacitors are the only other technology to have a self-healing mechanism. The NbO— which is actually the anode of these devices, not the dielectric—creates a highly resistive material at fault sites between the dielectric and NbO preserving capacitance and preventing a short. These are extremely small with high capacitance density but have very low voltage ratings. They are ideal for when you need guaranteed capacitance in critical safety applications like airbag deployment.
The metallized film is wound into a bobbin, sputtered with end terminations, and packaged in a wide variety of housings that are back- filled with epoxy. Many applications are often size- and weight-constrained, and obtaining an overdesigned capacitor off the shelf is not ideal. Kyocera-AVX has been manufacturing film capacitors for nearly half a century while analyzing field data and characterizing raw materials to meet the need of designing an ideal capacitor for a host of use conditions.
The thermal image is taken from the thermal modeling software for a capacitor that is meant to be mounted directly on an insulated-gate bipolar transistor (IGBT) module. With customer feedback on environmental, mechanical, and electrical profiles, the lifetime of a film capacitor is accurately estimated. In addition to thermal models, inductive models are also available for high frequency applications like the tank circuit in resonant converters. Analyzing these models is used primarily to determine hotspots. These locations of high current densities will be where most of the segmented electrodes will isolate themselves from the circuit. Once this is accounted for, design adjustments can be made to meet a minimum capacitance value throughout the full project lifetime, reducing manufacturing and testing costs, and—best of all—reducing lead times.
Film capacitors are versatile, they are available in small surface-mount technology (SMT) form factors to be used in audio and sample-and-hold circuits for their stability and low dielectric absorption, up to the size of a refrigerator capable of achieving 75kV and 150kJ. Although not as volumetrically efficient as electrolytic capacitors, and while having higher ESR than ceramic capacitors, film capacitors really shine in high power applications from motor drives to HVDC stations, and in discharge applications like the particle accelerator in CERN where their power handling and open circuit failure mode are unmatched.