Within the last few years, designers have established a solid powertrain foundation for hybrid vehicles (HEVs) and electric vehicles (EVs). All major auto manufactures see the hybrid and electric vehicle market as one continuing to enjoy steady growth. One major driving factor to the projected increased growth is the significant technology advances taking place that directly impact the HEV and EV power electronic systems.

At the heart of HEV and EV power electronic systems is the inverter (converting the DC battery energy into ac drive power), and at the heart of the inverter is the DC link capacitor. This article discusses the enabling technology in terms of capacitor performance, size, weight and lifetime that is realizing the energy densities needed for future generations of powertrain designs.

The popularity of vehicle electrification is growing rapidly for many reasons, including high fossil fuel costs and concerns over the environmental impact of fossil fuel use and exploration - particularly with the recent Gulf Coast oil spill. The development of advanced, high-efficiency power generation technology has undoubtedly improved and is now enabling the mass production of hybrid and pure electric vehicles.

At present, “traditional” hybrid vehicles have been the most popular approach, where a gas engine is augmented by electric power for improved efficiency. Meanwhile, pure electric, or battery electric vehicles (EVs or BEVs), continue to gain market share, especially in the short-range sports or low speed vehicle (LSV) markets. Based on these two approaches, the plug-in hybrid (PHEV), a hybrid vehicle with rechargeable batteries that can be charged by connecting a plug to an external electric power source, has evolved.

Power Film Capacitors Enable Advancements in Hybrid Vehicles

Figure 1. AVX's power film capacitor product offering.

One of the major factors in hybrid inverter design is versatility for both driving and charging profiles. Current system designs include both series configuration —where the internal combustion engine (ICE) turns a generator to supply current to an electric motor linked to the drive wheels — and parallel configuration — where the electric motor is battery-powered) with the ability to operate both systems simultaneously. These translate to charge depleting (CD) and charge sustaining (CS) modes of operation. The CD mode has become increasingly important as current PHEVs are designated by their all electric (CD) range, typically achieving between 15 and 40 miles before the ICE is needed. Likewise, battery charging can be via the ICE or by plugging into an external source while parked.

Consequently, the capacitors used in these types of inverter applications will encounter a wide range of electrical loading through their lifetime as well as being required to meet the full range of mechanical and environmental specifications associated with automotive applications. As noted by the Department of Energy, the capacitors used in existing inverters occupy a significant fraction of inverter volume (~35%), weight (~23%) and cost (~25%), so the more precisely the capacitor can be tailored to the application, the more efficient the inverter design can be.

With CD mode travel ranges continuing to improve, the more important it is becoming to have a remote charging infrastructure in place to augment domestic charging options. While many EV and PHEV models are being designed to be rechargeable from a domestic wall socket, having access to remote, purpose built charging stations will bring with it the possibility to provide faster charging by supplying power at higher voltages and currents, in turn creating a requirement for higher voltage charging capacitors.

For a typical vehicle inverter, the total DC link capacitance required, either assembled into a single block or deployed in a block for each phase, is in the range 450uF to 1250uF, depending on the engine power requirement. The additional application parameters are as follows:

• 600V - 1000 V duty rating
• Low (<br>
• Low (5 to 25 nH) equivalent series inductance (ESL)
• 250A + ripple current (Irms) capability
• Low thermal impedance
• Fail-safe operation

The capacitance / voltage (CV) requirements for the volume available (volumetric efficiency) can be addressed by both film and aluminium electrolytic technologies. However, a DC link film capacitor has the advantage in terms of ESR, ESL, and ripple capability because of its internal construction. The trend is to design film capacitor modules with low ESR internal capacitor elements that are arranged with an internal busbar structure to achieve both low ESL and low thermal impedance.

The combined low ESR and low ESL provides the ripple capability while the low thermal impedance will reduce the “hot spot” temperature – the internal temperature of the capacitor above ambient. This is an important factor as ambient temperatures for operation are trending higher, as systems go toward higher power density, and higher coolant temperature (from water to water-ethylene-glycol (WEG) systems).

Finally, the fail-safe operation is a critical consideration because the capacitors are connected across the battery. A short circuit would certainly result in a “walk home” event, something to be avoided at all costs. Film capacitor technology has the advantage of allowing electrode designs to enable “controlled self-healing”, meaning the part will always remain functional as a capacitor with no short-circuit mechanism over the lifetime of the vehicle.

<H5>Controlled Self-healing</H5>
The major advantage of film capacitors is their ability to overcome internal defects that could result in short-circuit failure. The dielectric films used for DC filter capacitors are only a few microns thick and have large surface area to provide high capacitance values. These are coated with a very thin metallic layer. This metallization is applied in segmented areas over the film, essentially making a single capacitor with up to a million internal sub-elements. In the case of any defect, the metal from one of these sub-elements evaporates and therefore isolates the defect, effectively self-healing the capacitor. This feature ensures a significantly higher efficiency of usage in the installed system, and enables the capacitor to last for the vehicle’s lifetime with no need for replacement.

Although there are a number of "segmented metallized film technologies" on the market, not all achieve the same level of controlled self-healing.

Non-optimized segmentation can generate unexpected results, such as loss of controlled self-healing if under segmented or very low lifetime expectancy if over segmented.

With 30 years of experience of manufacturing controlled self-healing capacitors, AVX can claim extensive knowledge and experience in this area.

The main advantages of this technology are:

o Proven field reliability, with zero catastrophic failures even under severe usage conditions.

o Providing a competitive solution, while maintaining the highest electrical and mechanical specifications.

o Long lifetime expectancy.

A major requirement for HEV/PHEV DC link capacitors is the ability to handle ripple current. For this requirement, film capacitors have a major advantage. Using aluminium electrolytics would require banks of several capacitors being used, not because a higher capacitance value is required, but simply to handle the current. By using film capacitors the designer only needs to consider the minimum capacitance value required for the system. As a result, designs that use film technology frequently save valuable space and weight.

Other film advantages are in reverse voltage and surge voltage capability. A film is able to accommodate a reverse voltage, or over voltage higher than 1.5 times rated voltage without the need to connect a diode in parallel, improving the power efficiency.

<H5>Life expectancy</H5>
Ultimately, the main advantage of film capacitors over aluminium electrolytic is life expectancy. Our internal data shows that AVX controlled self-healing DC filtering capacitors exhibit a maximum capacitance drop (DC) of just 2% after 100,000 hours operation. When added to the fact that, compared to aluminium electrolytics, complete device failure is very unlikely to occur, it means that during the full lifetime of an installed HEV / PHEV power system it will not be necessary to change the capacitors. This represents a major maintenance saving for the user.

System voltages and ambient temperatures are continuing to increase hybrid and electric vehicle applications. As these voltage requirements have risen they have passed the 600V barrier which represents a major hurdle for aluminium electrolytics. These are limited in voltage and require connection in series to successfully address this application, which can add significant cost in terms of space, as well as being much more complex to design and install.

Film capacitors offer significant technological advantages, including superior life expectancy and environmental performance as well as the ability to handle the various types of “in-application” technical issues, such as over-voltage and reverse voltage, which can easily occur.

Whether used in the vehicle inverter or remote charger system, these capacitors are often deployed in “walk home” applications, so given the fact that they require minimal maintenance and down-time, the advantages of using of film capacitors for HEV / PHEV applications are very significant.

Since 1980, great improvements have been made in DC filter capacitor technology by a combination of different segmentation schemes of the metallization matched to specific film dielectrics. Both volume and weight have been reduced by a factor of 3 or 4 over the last few years.

Chris Reynolds is technical marketing manager at the AVX Corporation.