Column: EMC
it to the RF reference point via a long trace. It is worth knowing that the ESL of an
MLCC is generally much lower than the connection inductances on the board. So the connection inductance remains the single most important parameter affecting the high-frequency impedance of a ceramic capacitor.
Film capacititors A film capacitor can lose its value significantly over years of service, especially if used in a humid environment. This happens when damp air leaks into the capacitor, between the wires and the epoxy potting compound. The aluminium metallisation then oxidizes, which reduces the capacitance, since aluminium oxide is a good electrical insulator. To combat this degradation, use the
best film capacitors with a thicker film, which makes them larger than others. Thinner film makes capacitors less resistant to overload (voltage, current or temperature) and also less likely to self-heal. If the film capacitor is not
permanently connected to the supply, there is less concern. For instance, for a product with a hard switch between the mains and the capacitor, size is probably more important than lifetime, so you can then choose a thinner film capacitor.
Figure 1: Simulation of an electrolytic capacitor’s damping - circuit (above) and response (below)
However, if the capacitor is permanently connected to the supply, then it must be highly reliable. Oxidation of capacitors is not
inevitable. If the capacitor epoxy material is of good quality and the capacitor is not routinely exposed to temperature extremes, value degradation will be minimal.
Capacitor layout rules Since capacitors store and provide energy per load demand, the general rule is that they should be located close
to the switch and load, limiting the ‘hot loop’ area. If possible, it is recommended to use
multiple capacitors in parallel rather than a single large one, since parallel capacitors effectively reduce ESL and ESR. In this way, energy can be supplied to the load more quickly. Finally, with ceramic capacitors, it is
a good idea to use several sizes, so the energy is available over a wide frequency spectrum. It is even better to locate the smaller-value capacitors near the active components requiring decoupling.
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