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Advantages of film, aluminium electrolytic caps

Posted: 09 Dec 2013 ?? ?Print Version ?Bookmark and Share

Keywords:capacitor? ESR? DC / DC converter? UL810? DC / AC inverter?

Storage elements that buffer energy of the most diverse forms, and make it available for subsequent conversion processes, are significant components in modern power supplies and frequency converters. A great deal of attention is paid to smoothing and the uniformity of the energy source, and a capacitor's ability to store large electrical charges plays a special role in this.

In general, the larger a capacitor's facing plates, the larger its capacity. However, the realisation of high capacities in a small volume is crucial for the limited installation space in modern equipment. In addition, the special demands of electrical engineering place great significance on dielectric strength. Film and electrolyte capacitors are typical examples of devices suited to these applications.

Film capacitors, especially metalized film capacitors, are based on a winding of two layers of metalized polypropylene. The thickness of the polypropylene foil (insulator) determines the rated voltage, which can approach several kV. A special characteristic of polypropylene capacitors is their self-healing ability. Due to the very thin foils generally used, this ability is extremely important in avoiding short circuits after flash-overs. Further design-related properties include low ESR, ESL, and a relatively wide working temperature range.

Aluminium electrolyte capacitors consist of two layers of aluminium foil separated by one or two layers of paper, and impregnated with a conducting fluid (the electrolyte). Their operating voltage is limited to approximately 500 V due to the thickness of the oxide layer of the first aluminium foil and the properties of the electrolyte. Important device properties include a very high charge storage capacity and small dimensions relative to the capacity.

However, since the electrolyte capacitor is polarized, it can only be used to a limited degree in an alternating current environment. And while the aluminium electrolyte capacitor has a higher capacitance per unit volume, the capacitance values vary with temperature and frequency due to its specific construction. Ohmic and frequency-dependent losses cause heating during charging / discharging, which limits possible ripple current. Furthermore, electrical properties change over time due to chemical processes, which can lead to an increased failure rate after the end of the specified service life.

Ceramic capacitors are resistant to extremely high voltages due to the ceramic materials used for their insulation. Very finely ground paraelectric / ferroelectric basic materials are sintered at high temperatures to a capacitive element, which as a dielectric serves as the electrode support. Ceramic capacitors can only store small magnitude charges, and are generally used for filter purposes at high-frequency voltages. In these applications, the phase and neutral conductors are short-circuited to ground via the capacitors. High-voltage capacitors, available in today's market, can process over-voltages in the range of several kilovolts.

Modern power supplies and converters reach increasingly high power densities up to the megawatt range. Modern semiconductors enable switching of high loads at constantly increasing frequencies, making possible high-power converters in compact designs at acceptable costs. However, with increasing power densities, the requirements on capacitors rise.

Generally, a converter input circuit, which is structured more or less extensively, is distinguished by the energy source. Especially in the case of solar converters, the input value depends on the sunlight intensity and is therefore subject to large variations, making the arrangement of an optimal operating point difficult. Therefore, a DC energy storage device must be provided at the input. The input circuit capacitor is implemented with electrolyte capacitors due to the high-DC-voltage component, the required high storage capacity, and their ability to be correspondingly over-sized. The capacitor is barely stressed, since very high alternating components cannot be expected.

Requirements for an intermediate circuit capacitor, also known as a DC-link capacitor, are significantly more complex. They function as energy storage devices between the DC / DC converter and the DC / AC inverter, and their input currents contain very high alternating components (ripple). The output-side voltage must be smoothed well, so that a stable DC voltage supply to the inverter is ensured. Typical examples of low-capacity converters are metalized polypropylene capacitors from the MKP1848 series, while capacitors from the HDMKP series are suitable for larger converters.

If the available space is too small, or if more energy must be stored, aluminium capacitors offer a suitable alternative. For applications in the load range of 100 kW and more, an intermediate circuit capacitoroften with larger aluminium capacitors can be set up.

When it comes to component cost, aluminium capacitors provide a clear benefit; a 470?F / 450 V aluminium capacitor costs only one fifth that of a comparable film capacitor. However, foil capacitors require little protection circuitry to restrict the effects of failures. High switching frequencies and steep switching flanks require the use of damping capacitors (snubbers). The task of the snubber MKP386M is to reduce or eliminate voltage and current spikes and switching losses. Noise emissions (EMI) are reduced by the suppression of the voltage and current overshoot caused by the switching of semiconductors.

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