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

Capacitors can provide significant traversal (or hold) energy or mitigate ripple and noise in power conversion circuits. Choosing the right type can profoundly impact the overall size, cost, and performance of your system.

Film and electrolytic capacitors: basic comparison

Thin film capacitors have low equivalent series resistance (ESR) and therefore good ripple current handling capability, high surge voltage rating and self-healing performance, and are important in many important applications such as electric vehicles, renewable energy, and industrial drives. A powerful contender for power conditioning tasks. Film capacitors are particularly suitable for applications that do not need to be held (or traversed), such as between power outages or peaks in line frequency fluctuations, requiring high or high ripple currents to be supplied or absorbed with high reliability and low loss.

Film capacitors are also well suited for applications operating at high DC bus voltages to minimize resistive losses. Since aluminum electrolytic capacitors can only provide voltages up to about 550V, applications operating at higher voltages require multiple devices in series, and then it is necessary to prevent voltage imbalance by selecting capacitors with matching values. It is expensive and time consuming; or it adds voltage balancing resistors, which increases the extra energy loss and BOM cost.

On the other hand, aluminum electrolytic capacitors are still a strong choice when pure energy storage density (Joules/cm3) is the main parameter of concern. One 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. Properly reducing the rating can reduce the life and reliability of aluminum electrolytic capacitors.

However, in fact, aluminum electrolytic capacitors can only withstand an overvoltage of about 20%. If there is a higher overvoltage, damage will occur, and the film capacitor can withstand an overvoltage of up to about twice the rated voltage in a short time. As is often the case in practical applications, self-healing capabilities ensure a safer response to accidental overpressure. In addition, film capacitors allow for easier connection and installation, and because they are non-polarized products, there is no reverse connection error. They are typically packaged in an insulated, highly efficient rectangular "box" enclosure for various electrical connections such as screw terminals, lugs, "fastons" or bus bars.