The use of solar panels and electric vehicles (EVs) continues to increase. Their power system relies on DC/DC converters and DC/AC inverters, requiring capacitors to reduce low-frequency ripple, filter high-frequency components that cause electromagnetic interference (EMI), and absorb transient load currents to prevent these factors from affecting the primary side of the power supply. The capacitors used in such power applications must be reliable, compact, lightweight, have a long lifespan, and have good high-frequency performance.
Although thin film capacitors are highly suitable for these power applications, designers must understand their structure and characteristics to make the right choices.
This article will briefly introduce thin film capacitors. Then, taking Eaton Electronics Division's products as an example, discuss how to select and use thin film capacitors for power applications.
film capacitor
Like all capacitors, film capacitors consist of two conductive plates separated by an insulating medium composed of a layer of plastic film. The plastic film is usually made of polypropylene and is a low loss, high-strength medium (Figure 1). A conductive plate is a thin metal foil or a thin metal layer deposited on a dielectric. Wrap metal foil and film around the core shaft, connect the leads, and then encapsulate the capacitor in a plastic casing and seal it with epoxy resin to protect it from environmental influences.
The shown is a thin film capacitor composed of a wound core shaft containing alternating metal layers and dielectric layers
Figure 1: A thin film capacitor consists of a wound core shaft containing alternating metal and dielectric layers, sealed in a protective plastic casing. (Image source: Eaton Electronics Division, modified by Art Pini)
Although the energy density of thin film capacitors is relatively low, they have high capacitance density and other characteristics. Firstly, thin film capacitors have no polarity and are suitable for both AC and DC circuits. Compared with capacitors using liquid or semi liquid electrolytes, the dry solid dielectric of thin film capacitors has higher reliability, stable capacitance value, and excellent temperature stability. The lower equivalent series inductance (ESL) and equivalent series resistance (ESR) ensure efficient handling of high ripple currents, making thin film capacitors highly suitable for high-frequency applications. The biggest feature of thin film capacitors may be their self-healing ability. If dielectric breakdown occurs, a local hot spot will be generated, causing adjacent metals to evaporate and form a non-conductive hole, but the capacitor can still function normally, thereby extending its service life.
film capacitor
Thin film capacitors are designed for specific applications, with common types including safety, DC link, AC filter, and pulse. Safe film capacitors are used to reduce conducted radiation in AC line filtering applications. Many international safety standards specify requirements for conducting EMI. Consider equipping electric vehicles with a DC charger that is powered through a circuit. In DC fast charging stations, common mode and differential mode EMI filtering through capacitors can provide a low impedance path for noise signals to be diverted while achieving the lowest power dissipation.
EMI suppression is achieved through a line filter, which includes a thin film capacitor between the power line and the switching power supply (Figure 2).