Select a thin film or electrolytic capacitor for the power conversion circuit

With its low equivalent series resistance (ESR), it achieves good ripple current handling and high surge voltage ratings and self-healing properties. Film capacitors are a strong candidate for many power conditioning tasks in critical applications such as electric vehicles. Renewable energy and industrial drives. They are particularly suitable for situations where there is no need to maintain (or traverse), such as high ripple and high loss in the event of an interruption or peak in line frequency fluctuations, as well as the need to acquire or absorb a large number of high frequencies.

Film capacitors are also well suited for applications that operate high DC bus voltages to minimize ohmic losses. Since aluminum electrolytic capacitors are rated at voltages up to approximately 550 V, applications operating at higher voltages require multiple devices connected in series. It is then necessary to prevent voltage imbalance by selecting capacitors with matching values, which is expensive and time consuming, or to increase voltage balancing resistors, which can result in additional energy losses and BOM costs.

On the other hand, aluminum electrolysis is still a strong choice when pure energy storage density (Joules/cm 3 ) is the primary consideration. An example is a commercial off-line power supply that requires cost-effective, high-capacity energy storage to maintain DC output voltage during a power outage without the need for a backup battery. A suitable derating can alleviate the life and reliability issues typically associated with aluminum electrolysis.

However, aluminum electrolytic capacitors can only withstand an overvoltage of about 20% before damage occurs, and film capacitors can withstand voltages up to twice their rated value for a short period of time. Self-healing ensures a safer response to accidental stress, which is often encountered in practical applications.

In addition, film capacitors provide easier connection and mounting options and are non-polarized and therefore unaffected by reverse connection errors. They are usually packaged in an insulated, volume-efficient rectangular "box" enclosure. Various electrical connection types are available, such as screw terminals, lugs, "fast" or busbars.

Polyester types are used at low voltages, while polypropylene typically has the lowest loss and highest reliability under stress due to its low loss factor (DF) and high dielectric breakdown per unit thickness. DF is also relatively stable in terms of temperature and frequency. Segmented, highly crystalline metallized polypropylene can also be provided and provides an energy density comparable to that of aluminum electrolytes.