Three-terminal capacitor filters (also called FeedThru capacitors) are being widely accepted in designs ranging from satellites and cell phones to automobiles and SMART grid controllers. The list of viable applications is growing rapidly as these capacitors evolve into smaller packages that can handle more current at higher temperatures.
FeedThru capacitor filters appeal to design engineers not only because of their small size and light weight, but also because of their high reliability, simplicity of implementation, and low cost. The filter can be a single three-terminal device or a four-element array, 10-terminal one. These miniature devices act as broadband filters and allow designers an exceptionally wide filter response across frequency spectrums — typically 300MHz wide. FeedThru capacitors can be chosen for filtering frequencies as low as ~75MHz or as high as many gigahertz.
FeedThru capacitor filters can replace the various value capacitors placed in parallel on Vcc lines. Designers sometimes use multiple capacitors with different values that combine frequency responses to create a broadband filtering effect. The problem with this approach is that the effective parallel inductance of the various capacitors is still quite high relative to the parallel inductance of FeedThru capacitor filters. Also, series inductances resulting from the additional traces and pads tend to affect the combined frequency of all the capacitors.
It is not uncommon, as part of a redesign, to replace at least two of the parallel capacitors (possibly three, depending on the frequency range) with a FeedThru capacitor filter. Replacing three parts with one improves reliability; simplifies layout and routing; decreases weight, size, and assembly cost; and can increase electrical efficiency.
It is also possible to replace some discrete LC T filters with a single FeedThru. In this case, the capacitor still offers the previously stated physical and electrical advantages, along with a frequency response that tends to be more repeatable than LC T filters and quite stable over temperature.
Recently, AEC Q200-qualified FeedThru capacitors were introduced to address strict electromagenetic interference concerns in drive-by-wire automotive systems. As auto use expands, so will the need for 150°C-rated FeedThrus. Additionally, high-reliability FeedThru capacitors are used in a variety of spacecraft, because of the dramatic size and weight reduction they provide on a systems level.
FeedThru capacitor filters are a simple, low-cost way to provide broadband filtering with a single component. The trend is for larger values of parallel capacitance to be integrated to make FeedThru capacitors more applicable to low-frequency filtering. The current rating of FeedThru capacitor filters is also increasing, both within each case size and with the introduction of larger case sizes.