Electric (EV), hybrid (HV) and plug-in hybrid (PHV) vehicles are becoming increasingly popular for use as personal cars, buses, trams, trains, industrial trucks, forklifts and more.
Capacitors have always had a wide range of applications in automotive systems and they are finding new applications in electric, hybrid and plug-in hybrid vehicles. Typical uses of capacitors in hybrid and purely electric vehicles include applications in motor controllers, motor inverters, boost converters, onboard chargers, communications, sensing and other subsystems.
The performance characteristics of film capacitors have made them the most popular capacitive elements for EV, HV and PHV applications. Aluminum capacitors are also used for limited applications in electric and hybrid vehicles. Whereas film capacitors can be used for both high and low voltage applications, aluminum electrolytic capacitors are limited to low and medium voltage uses.
Advances in capacitor technology have significantly contributed to the realization and development of a wide variety of electronic subsystems for electric vehicles. The motor drive is one of the most important subsystems of an electric vehicle. Since a DC power source is used, a buffer between the source of power and the inverter is required. A DC link capacitor is commonly added to the circuit to act as a buffer between the two subsystems. In the case of a typical electric vehicle, a DC link capacitor is inserted between the power source or DC/DC converter and the DC/AC inverter. The DC link capacitor ensures that the DC voltage supplied to the inverter is stable. It also ensures that the inverter circuitry is protected from surges and voltage spikes.
Some electronic components are highly sensitive to ripple currents. In hybrid and purely electric vehicles, capacitors are commonly used to prevent ripple currents from flowing back to the source of power. Metallized film capacitors are widely used in the electronic circuits of EV, HV and PHV to protect the power source from potentially harmful ripple currents.
Some components within the inverter circuit tend to produce electromagnetic interference, and it can easily spread and affect the entire inverter network. A DC link capacitor helps to prevent spreading of electromagnetic interference. The DC link capacitor is usually exposed to voltage stress and high temperature, and components with high energy density, long service life and high reliability are required.
Most of the earliest electric vehicles used thyristor technology. Use of this technology has declined since manufacturers have switched to IGBTs. In hybrid and purely electric vehicles, film capacitors are commonly used to protect semiconductors such as thyristors and IGBTs.
Like conventional automotive applications, electronic subsystems for hybrid and purely electric vehicles demand components with outstanding performance characteristics. These applications require passive components with excellent reliability, high frequency performance, long service life and capability to withstand high currents.
Polypropylene film is commonly used in the manufacturing of high performance capacitors for use in EV, HV and PHV applications. Motor drive applications demand capacitors that are small in size, relatively cheap, and with a long service life. Film capacitors for use in motor drive units usually have vapor-deposited metallized film and thin dielectrics, typically several micrometers. This construction yields high reliability, excellent electrical properties and impressive levels of safety. Polypropylene film capacitors also possess a self-healing mechanism which allows them to degrade gracefully in an automotive application instead of failing short.
The volume of a typical film capacitor is nearly directly proportional to the square of the thickness of the film. Although most of today’s polypropylene films are produced with a thickness of 2.5 micrometer, advances in technology will certainly allow production of even thinner film. Such thinner films are required for film capacitors for next-generation hybrid and purely electric vehicles.
As films for capacitors become thinner, the need for electrodes with improved ability to withstand higher voltage grows. To achieve components with such performance characteristics, better vapor-deposition technologies are required. Development of better technologies for vapor deposition could also help to enhance service life and degree of safety for capacitors.
To meet the performance requirements of today’s electric vehicles, manufacturers are employing advanced technologies to optimize the structural design of capacitors. Many manufacturers are using finite element method to predict the performance of film capacitors in the early stages of development. Such approaches have helped manufacturers to significantly reduce the cost of production and produce components with optimized performance.