26 SWITCHED-MODE POWER SUPPLIES www.rutronik.com must be tested with regard to these standards. Robustness of film capacitors Those who choose film capacitors should check if the application requires an increased temperature-humidity-biased (THB) class. This ensures that the capacitors are sufficiently robust against moisture – and therefore corrosion – to ensure the desired service life of the application. The temperature-humidity bias test is a recognized standard for accelerated life testing. It involves accelerating the aging process of capacitors and measuring in two different tests whether they maintain their capacitance, dissipation factor, and isolation resistance at a given temperature, relative humidity, and nominal voltage over a defined period of time. Three levels (grades) are distinguishe). Because of the relatively small capacitance values usually required, ceramic capacitors are mainly used as Y capacitors in a value range of between 10 pF and 4.7 nF. However, they are available with a maximum value of 22 nF. In addition to the classifications mentioned so far, capacitors are also differentiated according to their target application (commercial, industrial, or automotive) and according to their design. In terms of design, radial types as single layers are the most common and best known. These are ceramic single disks with spacing of 5 and 7.5 mm for X#Y2 and 10 and 12.5 mm for X#Y1 versions (Fig. 3). In addition, many SMD types are now available as X2, Y2, or X1Y2 versions as multilayer ceramic capacitors (MLCC) and Y1 or X1Y1 versions as single-layer plasticTable 1: Classification into (sub-)classes for capacitors in switched-mode power supplies Figure 2: X-Y capacitors usually carry one or more test marks as shown in the photos. Figure 3: Structure of a radial single-layer capacitor molded leadframes for surface mounting. Compared with the leaded, radial versions, these offer advantages above all in terms of smaller volume and lower overall height as well as higher suppression levels with the same capacitance values (Fig. 4). In switched-mode power supplies, a high-voltage electrolytic capacitor is typically used as a buffer downstream of the AC filter on the input side and the first rectifier. Models with a low ESR and a long service life are recommended for this purpose. On the secondary side, too, low capacitor ESR is also a priority. This allows high output currents and minimizes the residual ripple of the output voltage as much as possible. Low ESR electrolytic capacitors are usually used for this purpose. Additional parallel ceramic Issue 3 2024 Power Electronics Europe www.power-mag.com
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