This article introduces different types of capacitors that can be considered for power electronics applications. In particular, electrolytic and film types are compared, showing how and when each plays a role. Various film types and their structures are described in more detail, and preferred types are identified. Specifications for capacitance, ripple current ratings, transient voltage immunity, and safety ratings, as well as other characteristics, are examined in detail. The “self-healing” phenomenon after voltage stress is discussed, explaining its physical mechanism and its value in typical circuits. The main applications of film capacitors in power electronics are identified, and guidance on how to select the appropriate film capacitor type is provided. Detailed calculations for several example circuits are then given, showing how to select specific capacitors and their ratings. The calculations are generalized to enable engineers to use them as a basis for designs.
It's hard to imagine any modern electronics product that doesn't include some type of capacitor. For example, they might be the very small surface-mount type found in mobile phones, but they still exist. In power electronics, the functions are filtering, energy handling, and transmission, while the volume of a capacitor can be measured in cubic inches. In this application, it sometimes seems like a choice between aluminum (Al) electrolyte and thin-film types, but in terms of stored energy density, aluminum electrolytes have the edge in some respects. The only comparable thin-film types are exotic and expensive, such as "segmented highly crystalline metallized propylene," and even then, they don't maintain their ripple current ratings very well at high temperatures. Aluminum electrolytes have a relatively poor reputation for lifespan and reliability, but this only applies if they work hard. With proper reductions in voltage, ripple current, and temperature, they can last for many years. Their low cost is certainly an important factor for a given capacity-voltage (CV) rating. This means they are a practical solution for high-capacity energy storage applications, such as the internal high-voltage DC bus of commercial AC-DC power supplies.
Film capacitors hold a place in the field of power electronics.
Film capacitors do have some advantages over their aluminum electrolytic counterparts. For the same CV rating, they can have a much lower equivalent series resistance (ESR), which generally gives them a better ripple current rating. They are also relatively more resistant to voltage overstress and can, in some cases, “self-heal” after a degree of breakdown, thus improving system reliability and lifespan. When partial breakdown does occur, a short circuit forms in the body of a film capacitor, but a plasma arc clears it. However, this is only effective within the stress range; catastrophic failures can still occur due to carbon deposition and collateral damage to the dielectric insulation. In practice, aluminum electrolytic capacitors typically withstand only 20% voltage overstress, while film-type capacitors can reach 100% for a limited time. The difference in failure modes is also significant; aluminum electrolytes typically short-circuit after overstress, leading to explosive results, causing the liquid electrolyte to discharge and damage other components.
Indeed, the theoretical failure rate of aluminum electrolytic and film-type capacitors can be comparable to that of properly derating, but in practical applications, occasional voltage stress from inductive loads or lightning strikes can drastically alter system reliability. Both technologies suffer from humidity-induced degradation, which is an issue for film capacitors, but like other components, it should be controlled for optimal reliability.
When energy storage is not a primary parameter, high-capacity film capacitors can be a high-performance solution. One example is battery-powered DC buses, such as electric vehicles, alternative energy systems, and uninterruptible power supplies (UPS). In these applications, the primary function of the capacitor is to provide and absorb high-frequency ripple currents that can be measured in hundreds or thousands of amperes, where low capacitor ESR is crucial for achieving low losses and low ripple voltage.
The shift to higher bus voltages also benefits film capacitor types. The same energy is stored at higher voltages with a smaller CV rating (due to the "square" in E=CV²/2), thus requiring less capacitance, and film types with kV ratings can be supplied as needed. Aluminum electrolytic capacitors are technically limited to around 550V; while they can be stacked to achieve higher voltages, they have inherently high and variable leakage currents, requiring parallel balancing resistors, along with associated cost and losses. We discuss the short-circuit failure mode of aluminum electrolytic capacitors; when connected in series, one failing in this way will exert a high voltage between the other two, leading to avalanche damage.
The real difference between film capacitors and aluminum electrolytic capacitors lies in their mounting options. Film capacitors are supplied in volumetrically efficient rectangular boxes with options for wire, screw, lug, push-in connector, and even bus terminations. For aluminum electrolytic capacitors, round metal cans are the only standard option, although a similar range of terminations can be used. Unlike aluminum electrolytic capacitors, film capacitors are non-polar, meaning they can operate happily with voltages of either polarity applied, thus protecting them from reverse polarity. This also means they are well-suited for applications using AC voltages, such as inverter output filtering.
We have discussed general "film" capacitors, but there are many subtypes with different properties and applications.
In performance data, polypropylene is a strong contender for power applications, offering a wide voltage and capacitance range as well as good self-healing properties. Its particularly low dissipation factor (DF) across all frequencies is also important; DF is the ratio of ESR to capacitive reactance ZC = 1/2πfC. A lower number, compared to other dielectrics, implies lower heating effects and is one way to compare capacitance losses per microfarad for different capacitor types. Typically, DF varies slightly with temperature and frequency, but polypropylene performs best in comparison.
For less critical power applications, polyester can be an excellent low-cost option due to its high specific capacitance (CV/volume) and wide temperature range.
Polypropylene capacitor structure
Now, let's examine polypropylene capacitors in more detail. There are two basic structures—metal foil and metal deposition. In the former, a metal foil approximately 5 micrometers thick is sandwiched between dielectric layers, providing high peak current capability but lacking self-healing ability. In the metallized thin-film structure, aluminum or zinc or a zinc alloy is deposited onto a polypropylene film under vacuum at approximately 1200°C, with a thickness of approximately 20 to 50 nm. The film is kept at a low temperature during deposition, typically -25°C to -35°C. Self-healing is enabled during this process. In use, localized breakdown leads to intense heating, potentially up to 6000°C, resulting in a plasma arc. This locally evaporates the metallization layer, and the rapid expansion of the plasma extinguishes the arc, isolating the defective area within approximately 10 µs and allowing the capacitor to continue operating. Some capability is lost, but the impact is negligible.
Metallization is sometimes segmented into millions of regions on a thin film, with narrow “gates” feeding current into these sections and acting as fuses for severe overloads. While the peak current handling capability is slightly reduced due to the narrowing of the total current path, the introduced extra safety margin allows for higher capacitor voltage ratings.
In some designs, foil and metallization are combined to provide a trade-off between peak current handling and self-healing performance. Metallization can also be graded from the edges of the capacitor so that thicker material at the edges provides better current handling and stronger solder or weld terminations. The grading can be stepped or continuous.
Partial discharge effect
The polypropylene film used has a dielectric strength of approximately 650 V/µm and a thickness of approximately 2 µm, making it easy to obtain device ratings of several kV and part voltages of 100 kV. However, very high voltages can have an effect—partial discharge, or “PD.” Sometimes called “corona,” this is the breakdown of micropores in the dielectric material bulk or air gaps between insulating layers. Its effect is to insert “partial” short circuits into the insulation, effectively shortening the insulation path and locally lowering the breakdown threshold voltage. Each short circuit puts additional stress on the remaining insulation layer, which accumulates over time, reaching a critical point where complete breakdown occurs. Detection of partial discharge relies on specialized equipment that records individual breakdown events by transient additional currents flowing during high-voltage testing. While the energy in these events is measured in picocoulombs and is difficult to detect, it provides a good measure of how insulation condition changes over time. The 'Paschen curve' describes the PD effect," curve A, characterized by a little-known "minimum" in the relationship between micro-orifice size and breakdown voltage. Curves B and C are two example electric fields through an insulator—points above the Paschen curve (A) that may cause PD breakdown. PD has an "initiation" voltage for starting breakdown but a smaller "exhaustion" voltage before breakdown stops.
Oil-impregnated high-voltage capacitors help treat PD by displacing air, which has a lower breakdown threshold, from the insulation interface. Resin-filled low-voltage capacitors also help in this regard and also improve mechanical strength.
Voltage damping (PD) is an effect caused by the electric field strength (kV/mm), so thicker dielectric materials are less affected, but at the cost of larger-sized components with the same voltage coefficient (CV) rating. Capacitors can be connected in series so that they individually see lower voltage stress below the PD onset point, but balancing resistors may be required. Sometimes high-voltage capacitors consist of series elements in a single case to avoid PD.
Applications of capacitors in power electronics
We mentioned a key application: capacitors on the DC bus of power converters or inverters. The need for "through" or "hold" is a differentiating factor in choosing between aluminum electrolytic capacitors and film capacitors. It might be enlightening to see how each type fits in, for example. Consider a 1kW offline AC-DC converter with 90% efficiency and a power factor correction front end. Its internal DC bus operates nominally at 400VDC, dropping to 300VDC before the converter stops regulating.
If a 20ms walk-through time is required after a power outage, a capacitor needs to be installed on the DC bus to provide power to the converter so that it can continue to operate at 1kW output during the outage. To calculate the required capacitance (C), we equalize the energy difference when the capacitor drops from 400V (Vn) to 300V (Vd) with the energy supplied to the converter, i.e., power (Po) multiplied by time (t) divided by efficiency (η).
Choosing a traditional high-grade capacitor from TDK's B43508 series, which is approximately 3 cubic inches (52 cm³), would yield a similar total capacitance and voltage rating from a similarly rated film capacitor in TDK's B32678 series. This would require 16 capacitors in parallel, resulting in a total volume of 98 cubic inches (1600 cm³). The difference is approximately 30 times the size, with a similar cost ratio.
In cases where crossing is not required but the capacitor is used to minimize ripple voltage on a 400VDC bus, such as in EV applications, a typical value might be 80A rms ripple current (Irms) for a downstream 20kHz converter with a maximum ripple voltage of 4V rms. The capacitor C can be approximated as:
We sometimes see a rule of thumb for capacitors at this position, namely a rating of 20mA per µF, which aligns with our results. The TDK B43508 series has a small, low-cost component rated at 180µF, 450V, but only a 3.5A rms ripple current rating at 60°C, including its frequency correction factor. Therefore, for 80A ripple, we would need 23 in parallel, resulting in an unnecessarily large total capacitance of 4140µF and a volume of approximately 38 cubic inches or 621 cm³, considering the poor package factor. Each capacitor has an ESR of approximately 1 ohm, so at 3.5A rms, each capacitor would dissipate approximately 10W. If we look at film capacitors again, also from the TDK B32678 series, there are only four in parallel, totaling 160µF, providing 132A rms capability at 450V, with a volume of 24.5 cubic inches or 402cm³. The capacitance ESR is 2.5 milliohms, and each capacitor dissipates only 1W. The tables have turned; film capacitors are the right choice, offering lower dissipation, better overvoltage tolerance, optimal capacitance, and significantly less surge energy than in the case of 4140µF. Film capacitors are easy-to-terminate leadbox capacitors, requiring only four.
The deciding factor in choosing a capacitor is likely cost rather than physical size and dissipation, so we can take the same two TDK series capacitors and compare the energy storage per joule and the ripple current rating per ampere. Using data from a high-service distributor [4], the calculated cost for an aluminum electrolytic type is approximately $0.47/joule for a rating of approximately 180uF 450V, while the energy storage for a film type is approximately $3/joule. For ripple current, the cost is $2.68/ampere for aluminum electrolytic and $0.42/ampere for film. This shows how a near 6:1 cost advantage can be reversed, depending on the application requirements. At higher volumes, the absolute cost will be lower, but the ratio may remain similar.
Thin film capacitors as buffers
Another high-value application of capacitors in power converters is "buffering," which involves intentionally slowing down switching waveforms to reduce EMI and semiconductor stress. Here, an important consideration is the capacitor's ability to withstand high dV/dt or applied voltage changes, which can push high RMS currents into the component. Polypropylene is again a good choice, especially when the metallization is double-sided and manufactured with metal foil to withstand high currents. Capacitors used in this application typically also have very low inductive terminations for low impedance to AC, and high voltage margins to handle sometimes unpredictable peak voltages.
Thin film capacitors as power filters
Although filtering is often considered a signal level function, in inverters and motor drives, output capacitors carry high ripple currents to prevent stress and EMI caused by high dV/dt levels on the cables. When AC is delivered to the load, the capacitors must be non-polarized, excluding the use of aluminum electrolytic capacitors. The application environments are often harsh, requiring the robustness, ripple rating, and volumetric efficiency of polypropylene capacitors.
EMI filter
Film capacitors are widely used in power line EMI filters, not so much for their ripple current ratings, but for their self-healing properties in response to voltage transients. Institutionally safe polypropylene capacitors are typically rated "X1" or "X2" for 4kV and 2.5kV respectively on the line, and can reach values of several µF to meet EMI standards. Line-to-ground capacitors for attenuating common-mode emissions are of the "Y1" and "Y2" types with rated voltages of 8kV and 5kV, but are limited by line leakage current considerations. In these EMI filtering applications, the low self-inductance of typical film capacitors is an advantage, maintaining a high self-resonance.
Film capacitors are widely used in power electronics, performing exceptionally well when high ripple current ratings are required or when environmental overvoltage stress is applied; polypropylene types are particularly valuable. A more in-depth analysis comparing the CV ratings of film and aluminum electrolytic capacitors reveals that while aluminum electrolytic types outperform those with simple energy storage considerations, practical component selection must take into account ripple current ratings and reliability considerations when film is generally the better choice.
